View source for Water collected between towers

{{task}}

;Task:
In a two-dimensional world, we begin with any bar-chart (or row of close-packed 'towers', each of unit width), and then it rains, 
completely filling all convex enclosures in the chart with water. 

                                                        
<pre>9               ██           9               ██    
8               ██           8               ██    
7     ██        ██           7     ██≈≈≈≈≈≈≈≈██    
6     ██  ██    ██           6     ██≈≈██≈≈≈≈██    
5 ██  ██  ██  ████           5 ██≈≈██≈≈██≈≈████    
4 ██  ██  ████████           4 ██≈≈██≈≈████████    
3 ██████  ████████           3 ██████≈≈████████    
2 ████████████████  ██       2 ████████████████≈≈██
1 ████████████████████       1 ████████████████████</pre>
                                                        

In the example above, a bar chart representing the values [5, 3, 7, 2, 6, 4, 5, 9, 1, 2] has filled, collecting 14 units of water.

Write a function, in your language, from a given array of heights, to the number of water units that can be held in this way, by a corresponding bar chart.

Calculate the number of water units that could be collected by bar charts representing each of the following seven series:

<pre>   [[1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]]</pre>


See, also:

* [https://youtu.be/ftcIcn8AmSY?t=536 Four Solutions to a Trivial Problem] – a Google Tech Talk by Guy Steele
* [http://stackoverflow.com/questions/24414700/amazon-water-collected-between-towers/ Water collected between towers] on Stack Overflow, from which the example above is taken)
* [https://gist.github.com/paf31/9d84ecf6a6a9b69cdb597a390f25764d An interesting Haskell solution], using the Tardis monad, by [https://gist.github.com/paf31 Phil Freeman] in a [https://gist.github.com/paf31/9d84ecf6a6a9b69cdb597a390f25764d Github gist].


<br>

=={{header|11l}}==
{{trans|Python}}

<syntaxhighlight lang="11l">F water_collected(tower)
   V l = tower.len
   V highest_left = [0] [+] (1 .< l).map(n -> max(@tower[0 .< n]))
   V highest_right = (1 .< l).map(n -> max(@tower[n .< @l])) [+] [0]
   V water_level = (0 .< l).map(n -> max(min(@highest_left[n], @highest_right[n]) - @tower[n], 0))
   print(‘highest_left:   ’highest_left)
   print(‘highest_right:  ’highest_right)
   print(‘water_level:    ’water_level)
   print(‘tower_level:    ’tower)
   print(‘total_water:    ’sum(water_level))
   print(‘’)
   R sum(water_level)

V towers = [
   [1, 5, 3, 7, 2],
   [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
   [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
   [5, 5, 5, 5],
   [5, 6, 7, 8],
   [8, 7, 7, 6],
   [6, 7, 10, 7, 6]]

print(towers.map(tower -> water_collected(tower)))</syntaxhighlight>

{{out}}
<pre>
highest_left:   [0, 1, 5, 5, 7]
highest_right:  [7, 7, 7, 2, 0]
water_level:    [0, 0, 2, 0, 0]
tower_level:    [1, 5, 3, 7, 2]
total_water:    2

highest_left:   [0, 5, 5, 7, 7, 7, 7, 7, 9, 9]
highest_right:  [9, 9, 9, 9, 9, 9, 9, 2, 2, 0]
water_level:    [0, 2, 0, 5, 1, 3, 2, 0, 1, 0]
tower_level:    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
total_water:    14

...

highest_left:   [0, 6, 7, 10, 10]
highest_right:  [10, 10, 7, 6, 0]
water_level:    [0, 0, 0, 0, 0]
tower_level:    [6, 7, 10, 7, 6]
total_water:    0

[2, 14, 35, 0, 0, 0, 0]
</pre>

=={{header|8080 Assembly}}==
<syntaxhighlight lang="8080asm">	org	100h
	jmp	demo
	;;;	Calculate the amount of water a row of towers will hold
	;;;	Note: this will destroy the input array.
	;;;	Input: DE = tower array, BC = length of array
	;;;	Output: A = amount of water 
water:	xra	a	; Start with no water
	sta	w_out+1
wscanr:	mov	h,d	; HL = right edge
	mov	l,e
	dad	b
wscrlp:	dcx	h
	call	cmp16	; Reached beginning?
	jnc	w_out	; Then stop
	mov	a,m	; Otherwise, if current tower is zero
	ora	a
	jz	wscrlp	; Then keep scanning
	push	b	; Keep length
	push	d	; Keep array begin
	mvi	b,0	; No blocks yet
	xchg		; HL = left scanning edge, DE = right
wscanl:	mov	a,m	; Get current column
	ora	a	; Is zero?
	jz	wunit	; Then see if an unit of water must be added
	dcr	m	; Otherwise, decrease column
	inr	b	; Increase blocks
	jmp	wnext
wunit:	mov	a,b	; Any blocks?
	ora	a
	jz	wnext
	lda	w_out+1	; If so, add water
	inr	a
	sta	w_out+1
wnext:	inx	h	; Next column
	call	cmp16
	jnc	wscanl	; Until right edge reached
	mov	a,b
	cmc		; Check if more than 1 block left
	rar
	ora	a
	pop 	d	; Restore array begin
	pop	b	; and length
	jnz	wscanr	; If more than 1 block, keep scanning
w_out:	mvi	a,0	; Load water into A
	ret 	
	;;;	16-bit compare DE to HL 
cmp16:	mov	a,d
	cmp	h
	rnz
	mov	a,e
	cmp 	l
	ret
	;;;	Calculate and print the amount of water for each input
demo:	lxi	h,series
load:	mov	e,m	; Load pointer
	inx	h
	mov	d,m
	inx	h
	mov	c,m	; Load length
	inx	h
	mov	b,m
	inx	h
	mov	a,d	; If pointer is zero,
	ora	e
	rz		; stop.
	push 	h	; Otherwise, save the series pointer
	call	water	; Calculate amount of water
	call	printa	; Output amount of water
	pop	h	; Restore series pointer
	jmp	load	; Load next example 
	;;;	Print A as integer value
printa:	lxi	d,num	; Pointer to number string
	mvi	c,10	; Divisor
digit:	mvi	b,-1	; Quotient
dloop:	inr	b	; Divide (by trial subtraction)
	sub	c
	jnc	dloop
	adi	'0'+10	; ASCII digit from remainder
	dcx	d	; Store ASCII digit
	stax	d
	mov	a,b	; Continue with quotient
	ana	a	; If not zero
	jnz	digit
	mvi	c,9	; 9 = CP/M print string syscall
	jmp	5	; Print number string
	db	'***'	; Output number placeholder
num:	db	' $'
	;;;	Series
t1:	db	1,5,3,7,2
t2:	db	5,3,7,2,6,4,5,9,1,2
t3:	db	2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1
t4:    	db	5,5,5,5
t5:	db	5,6,7,8
t6:    	db	8,7,7,6
t7:	db    	6,7,10,7,6
t_end:	equ	$
	;;;	Lengths and pointers
series:	dw	t1,t2-t1
	dw	t2,t3-t2 
	dw	t3,t4-t3 
	dw	t4,t5-t4
	dw	t5,t6-t5
	dw	t6,t7-t6
	dw	t7,t_end-t7
	dw	0</syntaxhighlight>

{{out}}

<pre>2 14 35 0 0 0 0</pre>

=={{header|8086 Assembly}}==
<syntaxhighlight lang="asm">	cpu	8086
	org	100h
section	.text
	jmp	demo
	;;;	Calculate the amount of water a row of towers will hold
	;;;	Note: this will destroy the input array.
	;;;	Input: DX = tower array, CX = length of array
	;;;	Output: AX = amount of water	
water:	xor	ax,ax		; Amount of water starts at zero	
	xor	bx,bx		; BH = zero, BL = block count
.scanr:	mov	di,dx		; DI = right edge of towers
	add	di,cx
.rloop:	dec	di
	cmp	di,dx		; Reached beginning?
	jl	.out		; Then calculation is done.
	cmp	bh,[di]		; Otherwise, if the tower is zero,
	je	.rloop		; Keep scanning
	xor	bl,bl		; Set block count to zero
	mov	si,dx		; SI = left scanning edge
.scanl:	cmp	bh,[si]		; Is the column empty?
	je	.unit		; Then see whether to add an unit of water
	dec	byte [si]	; Otherwise, remove block from tower
	inc	bx		; And count it
	jmp	.next
.unit:	test	bl,bl		; Any blocks?
	jz	.next		
	inc	ax		; If so, add unit of water
.next:	inc	si		; Scan rightward
	cmp	si,di		; Reached the right edge?
	jbe	.scanl		; If not, keep going
	shr	bl,1		; If more than 1 block,
	jnz	.scanr		; Keep going
.out:	ret
	;;;	Calculate and print the amount of water for each input
demo:	mov	si,series
.loop:	lodsw			; Load pointer
	test	ax,ax		; If 0,
	jz	.done		; we're done.
	xchg	ax,dx
	lodsw			; Load length
	xchg	ax,cx
	push	si		; Keep array pointer
	call	water		; Calculate amount of water
	call	prax		; Print AX 
	pop	si		; Restore array pointer
	jmp 	.loop
.done:	ret
	;;;	Print AX as number
prax:	mov	bx,num		; Pointer to end of number string
	mov	cx,10 		; Divisor
.dgt:	xor	dx,dx		; Divide by 10 
	div	cx
	add	dl,'0'		; Add ASCII 0 to remainder
	dec	bx		; Store digit
	mov	[bx],dl
	test	ax,ax		; If number not zero yet
	jnz	.dgt		; Find rest of digits
	mov	dx,bx		; Print number string
	mov	ah,9
	int	21h
	ret
section	.data
	db	'*****'		; Output number placeholder
num:	db	' $'
	;;;	Series
t1:	db	1,5,3,7,2
t2:	db	5,3,7,2,6,4,5,9,1,2
t3:	db	2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1
t4:    	db	5,5,5,5
t5:	db	5,6,7,8
t6:    	db	8,7,7,6
t7:	db    	6,7,10,7,6
t_end:	equ	$
	;;;	Lengths and pointers
series:	dw	t1,t2-t1
	dw	t2,t3-t2 
	dw	t3,t4-t3 
	dw	t4,t5-t4
	dw	t5,t6-t5
	dw	t6,t7-t6
	dw	t7,t_end-t7
	dw	0</syntaxhighlight>

{{out}}

<pre>2 14 35 0 0 0 0</pre>

=={{header|Action!}}==
<syntaxhighlight lang="action!">PROC PrintArray(BYTE ARRAY a BYTE len)
  BYTE i

  Put('[)
  FOR i=0 TO len-1
  DO
    IF i>0 THEN
      Put(32)
    FI
    PrintB(a(i))
  OD
  Put('])
RETURN

BYTE FUNC Max(BYTE ARRAY a BYTE start,stop)
  BYTE i,res

  res=0
  FOR i=start TO stop
  DO
    IF a(i)>res THEN
      res=a(i)
    FI
  OD
RETURN (res)

BYTE FUNC CalcWater(BYTE ARRAY a BYTE len)
  BYTE water,i,maxL,maxR,lev

  IF len<3 THEN
    RETURN (0)
  FI
  water=0
  FOR i=1 TO len-2
  DO
    maxL=Max(a,0,i-1)
    maxR=Max(a,i+1,len-1)
    IF maxL<maxR THEN
      lev=maxL
    ELSE
      lev=maxR
    FI
    IF a(i)<lev THEN
      water==+lev-a(i)
    FI
  OD
RETURN (water)

PROC Test(BYTE ARRAY a BYTE len)
  BYTE water

  water=CalcWater(a,len)
  PrintArray(a,len)
  PrintF(" holds %B water units%E%E",water)
RETURN

PROC Main()
  DEFINE COUNT="7"
  BYTE ARRAY
    a1=[1 5 3 7 2],
    a2=[5 3 7 2 6 4 5 9 1 2],
    a3=[2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1],
    a4=[5 5 5 5],
    a5=[5 6 7 8],
    a6=[8 7 7 6],
    a7=[6 7 10 7 6]

  Test(a1,5)
  Test(a2,10)
  Test(a3,16)
  Test(a4,4)
  Test(a5,4)
  Test(a6,4)
  Test(a7,5)
RETURN</syntaxhighlight>
{{out}}
[https://gitlab.com/amarok8bit/action-rosetta-code/-/raw/master/images/Water_collected_between_towers.png Screenshot from Atari 8-bit computer]
<pre>
[1 5 3 7 2] holds 2 water units

[5 3 7 2 6 4 5 9 1 2] holds 14 water units

[2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1] holds 35 water units

[5 5 5 5] holds 0 water units

[5 6 7 8] holds 0 water units

[8 7 7 6] holds 0 water units

[6 7 10 7 6] holds 0 water units
</pre>

=={{header|Ada}}==

<syntaxhighlight lang="ada">with Ada.Text_IO;

procedure Water_Collected is

   type Bar_Index     is new Positive;
   type Natural_Array is array (Bar_Index range <>) of Natural;

   subtype Bar_Array   is Natural_Array;
   subtype Water_Array is Natural_Array;

   function Flood (Bars : Bar_Array; Forward : Boolean) return Water_Array is
      R : Water_Array (Bars'Range);
      H : Natural := 0;
   begin
      if Forward then
         for A in R'Range loop
            H     := Natural'Max (H, Bars (A));
            R (A) := H - Bars (A);
         end loop;
      else
         for A in reverse R'Range loop
            H     := Natural'Max (H, Bars (A));
            R (A) := H - Bars (A);
         end loop;
      end if;
      return R;
   end Flood;

   function Fold (Left, Right : Water_Array) return Water_Array is
      R : Water_Array (Left'Range);
   begin
      for A in R'Range loop
         R (A) := Natural'Min (Left (A), Right (A));
      end loop;
      return R;
   end Fold;

   function Fill (Bars : Bar_Array) return Water_Array
   is (Fold (Flood (Bars, Forward => True),
             Flood (Bars, Forward => False)));

   function Sum_Of (Bars : Natural_Array) return Natural is
      Sum : Natural := 0;
   begin
      for Bar of Bars loop
         Sum := Sum + Bar;
      end loop;
      return Sum;
   end Sum_Of;

   procedure Show (Bars : Bar_Array) is
      use Ada.Text_IO;
      Water : constant Water_Array := Fill (Bars);
   begin
      Put ("The series: [");
      for Bar of Bars loop
         Put (Bar'Image);
         Put (" ");
      end loop;
      Put ("] holds ");
      Put (Sum_Of (Water)'Image);
      Put (" units of water.");
      New_Line;
   end Show;

begin
   Show ((1, 5, 3, 7, 2));
   Show ((5, 3, 7, 2, 6, 4, 5, 9, 1, 2));
   Show ((2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1));
   Show ((5, 5, 5, 5));
   Show ((5, 6, 7, 8));
   Show ((8, 7, 7, 6));
   Show ((6, 7, 10, 7, 6));
end Water_Collected;</syntaxhighlight>

{{out}}
<pre>
The series: [ 1  5  3  7  2 ] holds  2 units of water.
The series: [ 5  3  7  2  6  4  5  9  1  2 ] holds  14 units of water.
The series: [ 2  6  3  5  2  8  1  4  2  2  5  3  5  7  4  1 ] holds  35 units of water.
The series: [ 5  5  5  5 ] holds  0 units of water.
The series: [ 5  6  7  8 ] holds  0 units of water.
The series: [ 8  7  7  6 ] holds  0 units of water.
The series: [ 6  7  10  7  6 ] holds  0 units of water.
</pre>

=={{header|AppleScript}}==
{{Trans|JavaScript}}

<syntaxhighlight lang="applescript">--------------- WATER COLLECTED BETWEEN TOWERS -------------

-- waterCollected :: [Int] -> Int
on waterCollected(xs)
    set leftWalls to scanl1(my max, xs)
    set rightWalls to scanr1(my max, xs)
    
    set waterLevels to zipWith(my min, leftWalls, rightWalls)
    
    -- positive :: Num a => a -> Bool
    script positive
        on |λ|(x)
            x > 0
        end |λ|
    end script
    
    -- minus :: Num a => a -> a -> a
    script minus
        on |λ|(a, b)
            a - b
        end |λ|
    end script
    
    sum(filter(positive, zipWith(minus, waterLevels, xs)))
end waterCollected


---------------------------- TEST --------------------------
on run
    map(waterCollected, ¬
        [[1, 5, 3, 7, 2], ¬
            [5, 3, 7, 2, 6, 4, 5, 9, 1, 2], ¬
            [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1], ¬
            [5, 5, 5, 5], ¬
            [5, 6, 7, 8], ¬
            [8, 7, 7, 6], ¬
            [6, 7, 10, 7, 6]])
    
    --> {2, 14, 35, 0, 0, 0, 0}
end run


--------------------- GENERIC FUNCTIONS --------------------

-- filter :: (a -> Bool) -> [a] -> [a]
on filter(f, xs)
    tell mReturn(f)
        set lst to {}
        set lng to length of xs
        repeat with i from 1 to lng
            set v to item i of xs
            if |λ|(v, i, xs) then set end of lst to v
        end repeat
        return lst
    end tell
end filter

-- foldl :: (a -> b -> a) -> a -> [b] -> a
on foldl(f, startValue, xs)
    tell mReturn(f)
        set v to startValue
        set lng to length of xs
        repeat with i from 1 to lng
            set v to |λ|(v, item i of xs, i, xs)
        end repeat
        return v
    end tell
end foldl

-- init :: [a] -> [a]
on init(xs)
    if length of xs > 1 then
        items 1 thru -2 of xs
    else
        {}
    end if
end init

-- map :: (a -> b) -> [a] -> [b]
on map(f, xs)
    tell mReturn(f)
        set lng to length of xs
        set lst to {}
        repeat with i from 1 to lng
            set end of lst to |λ|(item i of xs, i, xs)
        end repeat
        return lst
    end tell
end map

-- max :: Ord a => a -> a -> a
on max(x, y)
    if x > y then
        x
    else
        y
    end if
end max

-- min :: Ord a => a -> a -> a
on min(x, y)
    if y < x then
        y
    else
        x
    end if
end min

-- Lift 2nd class handler function into 1st class script wrapper 
-- mReturn :: Handler -> Script
on mReturn(f)
    if class of f is script then
        f
    else
        script
            property |λ| : f
        end script
    end if
end mReturn

-- scanl :: (b -> a -> b) -> b -> [a] -> [b]
on scanl(f, startValue, xs)
    tell mReturn(f)
        set v to startValue
        set lng to length of xs
        set lst to {startValue}
        repeat with i from 1 to lng
            set v to |λ|(v, item i of xs, i, xs)
            set end of lst to v
        end repeat
        return lst
    end tell
end scanl

-- scanl1 :: (a -> a -> a) -> [a] -> [a]
on scanl1(f, xs)
    if length of xs > 0 then
        scanl(f, item 1 of xs, items 2 thru -1 of xs)
    else
        {}
    end if
end scanl1

-- scanr :: (b -> a -> b) -> b -> [a] -> [b]
on scanr(f, startValue, xs)
    tell mReturn(f)
        set v to startValue
        set lng to length of xs
        set lst to {startValue}
        repeat with i from lng to 1 by -1
            set v to |λ|(v, item i of xs, i, xs)
            set end of lst to v
        end repeat
        return reverse of lst
    end tell
end scanr

-- scanr1 :: (a -> a -> a) -> [a] -> [a]
on scanr1(f, xs)
    if length of xs > 0 then
        scanr(f, item -1 of xs, items 1 thru -2 of xs)
    else
        {}
    end if
end scanr1

-- sum :: Num a => [a] -> a
on sum(xs)
    script add
        on |λ|(a, b)
            a + b
        end |λ|
    end script
    
    foldl(add, 0, xs)
end sum

-- tail :: [a] -> [a]
on tail(xs)
    if length of xs > 1 then
        items 2 thru -1 of xs
    else
        {}
    end if
end tail

-- zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
on zipWith(f, xs, ys)
    set lng to min(length of xs, length of ys)
    set lst to {}
    tell mReturn(f)
        repeat with i from 1 to lng
            set end of lst to |λ|(item i of xs, item i of ys)
        end repeat
        return lst
    end tell
end zipWith</syntaxhighlight>
{{Out}}
<syntaxhighlight lang="applescript">{2, 14, 35, 0, 0, 0, 0}</syntaxhighlight>

=={{header|Arturo}}==

<syntaxhighlight lang="arturo">cmax: function => [
    m: neg ∞
    map & 'x -> m:<=max @[m x]
]

vmin: $ => [map couple & & => min]

vsub: $ => [map couple & & 'p -> p\0 - p\1]

water: function [a][
    sum vsub vmin reverse cmax reverse a cmax a a
]

loop [
    [1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]
] 'a -> print [a "->" water a]</syntaxhighlight>

{{out}}

<pre>[1 5 3 7 2] -> 2
[5 3 7 2 6 4 5 9 1 2] -> 14
[2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1] -> 35
[5 5 5 5] -> 0
[5 6 7 8] -> 0
[8 7 7 6] -> 0
[6 7 10 7 6] -> 0</pre>

=={{header|AutoHotkey}}==
<syntaxhighlight lang="autohotkey">WCBT(oTwr){
	topL := Max(oTwr*), l := num := 0, barCh := lbarCh := "", oLvl := []
	while (++l <= topL)
		for t, h in oTwr
			oLvl[l,t] := h ? "██" : "≈≈" , oTwr[t] := oTwr[t]>0 ? oTwr[t]-1 : 0
	for l, obj in oLvl{
		while (oLvl[l, A_Index] = "≈≈")
			oLvl[l, A_Index] := "  "
		while (oLvl[l, obj.Count() +1 - A_Index] = "≈≈")
			oLvl[l, obj.Count() +1 - A_Index] := "  "
		for t, v in obj
			lbarCh .= StrReplace(v, "≈≈", "≈≈", n), num += n
		barCh := lbarCh "`n" barCh, lbarCh := ""
	}
	return [num, barCh]
}</syntaxhighlight>
Examples:<syntaxhighlight lang="autohotkey">data := [[1, 5, 3, 7, 2]
	,[5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
	,[2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
	,[5, 5, 5, 5]
	,[5, 6, 7, 8]
	,[8, 7, 7, 6]
	,[6, 7, 10, 7, 6]]
 
result := ""
for i, oTwr in data{
	inp := ""
	for i, h in oTwr
		inp .= h ", "
	inp := "[" Trim(inp, ", ") "]"
	x := WCBT(oTwr)
	result .= "Chart " inp " has " x.1 " water units`n" x.2 "------------------------`n"
}
MsgBox % result</syntaxhighlight>
{{out}}
<pre>Chart [1, 5, 3, 7, 2] has 2 water units
      ██  
      ██  
  ██≈≈██  
  ██≈≈██  
  ██████  
  ████████
██████████
------------------------
Chart [5, 3, 7, 2, 6, 4, 5, 9, 1, 2] has 14 water units
              ██    
              ██    
    ██≈≈≈≈≈≈≈≈██    
    ██≈≈██≈≈≈≈██    
██≈≈██≈≈██≈≈████    
██≈≈██≈≈████████    
██████≈≈████████    
████████████████≈≈██
████████████████████
------------------------
Chart [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1] has 35 water units
          ██                    
          ██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██    
  ██≈≈≈≈≈≈██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██    
  ██≈≈██≈≈██≈≈≈≈≈≈≈≈██≈≈████    
  ██≈≈██≈≈██≈≈██≈≈≈≈██≈≈██████  
  ██████≈≈██≈≈██≈≈≈≈██████████  
████████████≈≈████████████████  
████████████████████████████████
------------------------
Chart [5, 5, 5, 5] has 0 water units
████████
████████
████████
████████
████████
------------------------
Chart [5, 6, 7, 8] has 0 water units
      ██
    ████
  ██████
████████
████████
████████
████████
████████
------------------------
Chart [8, 7, 7, 6] has 0 water units
██      
██████  
████████
████████
████████
████████
████████
████████
------------------------
Chart [6, 7, 10, 7, 6] has 0 water units
    ██    
    ██    
    ██    
  ██████  
██████████
██████████
██████████
██████████
██████████
██████████
------------------------</pre>

=={{header|AWK}}==
<syntaxhighlight lang="awk">
# syntax: GAWK -f WATER_COLLECTED_BETWEEN_TOWERS.AWK [-v debug={0|1}]
BEGIN {
    wcbt("1,5,3,7,2")
    wcbt("5,3,7,2,6,4,5,9,1,2")
    wcbt("2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1")
    wcbt("5,5,5,5")
    wcbt("5,6,7,8")
    wcbt("8,7,7,6")
    wcbt("6,7,10,7,6")
    exit(0)
}
function wcbt(str,  ans,hl,hr,i,n,tower) {
    n = split(str,tower,",")
    for (i=n; i>=0; i--) { # scan right to left
      hr[i] = max(tower[i],(i<n)?hr[i+1]:0)
    }
    for (i=0; i<=n; i++) { # scan left to right
      hl[i] = max(tower[i],(i!=0)?hl[i-1]:0)
      ans += min(hl[i],hr[i]) - tower[i]
    }
    printf("%4d : %s\n",ans,str)
    if (debug == 1) {
      for (i=1; i<=n; i++) { printf("%-4s",tower[i]) } ; print("tower")
      for (i=1; i<=n; i++) { printf("%-4s",hl[i]) } ; print("l-r")
      for (i=1; i<=n; i++) { printf("%-4s",hr[i]) } ; print("r-l")
      for (i=1; i<=n; i++) { printf("%-4s",min(hl[i],hr[i])) } ; print("min")
      for (i=1; i<=n; i++) { printf("%-4s",min(hl[i],hr[i])-tower[i]) } ; print("sum\n")
    }
}
function max(x,y) { return((x > y) ? x : y) }
function min(x,y) { return((x < y) ? x : y) }
</syntaxhighlight>
{{out}}
<pre>
   2 : 1,5,3,7,2
  14 : 5,3,7,2,6,4,5,9,1,2
  35 : 2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1
   0 : 5,5,5,5
   0 : 5,6,7,8
   0 : 8,7,7,6
   0 : 6,7,10,7,6
</pre>

=={{header|BASIC}}==
==={{header|FreeBASIC}}===
Uses Nigel Galloway's very elegant idea, expressed verbosely so you can really see what's going on.
<syntaxhighlight lang="freebasic">type tower
    hght as uinteger
    posi as uinteger
end type

sub shellsort( a() as tower )
    'quick and dirty shellsort, not the focus of this exercise
    dim as uinteger gap = ubound(a), i, j, n=ubound(a)
    dim as tower temp
    do 
        gap = int(gap / 2.2)
        if gap=0 then gap=1
        for i=gap to n
            temp = a(i)
            j=i
            while j>=gap andalso a(j-gap).hght < temp.hght
                a(j) = a(j - gap)
                j -= gap
            wend 
            a(j) = temp
        next i
    loop until gap = 1
end sub

'heights of towers in each city prefixed by the number of towers
data 5, 1, 5, 3, 7, 2
data 10, 5, 3, 7, 2, 6, 4, 5, 9, 1, 2
data 16, 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1
data 4, 5, 5, 5, 5
data 4, 5, 6, 7, 8
data 4, 8, 7, 7, 6
data 5, 6, 7, 10, 7, 6

dim as uinteger i, n, j, first, last, water
dim as tower manhattan(0 to 1)
for i = 1 to 7
    read n
    redim manhattan( 0 to n-1 )
    for j = 0 to n-1
        read manhattan(j).hght
        manhattan(j).posi = j
    next j
    shellsort( manhattan() )
    if manhattan(0).posi < manhattan(1).posi then
        first = manhattan(0).posi
        last  = manhattan(1).posi
    else
        first = manhattan(1).posi
        last  = manhattan(0).posi
    end if
    water = manhattan(1).hght * (last-first-1)
    for j = 2 to n-1
        if first<manhattan(j).posi and manhattan(j).posi<last then water -= manhattan(j).hght
        if manhattan(j).posi < first then
             water += manhattan(j).hght * (first-manhattan(j).posi-1)
             first = manhattan(j).posi
        end if
        if manhattan(j).posi > last then
             water += manhattan(j).hght * (manhattan(j).posi-last-1)
             last = manhattan(j).posi
        end if
    next j
    print using "City configuration ## collected #### units of water."; i; water
next i</syntaxhighlight>
{{out}}
<pre>City configuration  1 collected    2 units of water.
City configuration  2 collected   14 units of water.
City configuration  3 collected   35 units of water.
City configuration  4 collected    0 units of water.
City configuration  5 collected    0 units of water.
City configuration  6 collected    0 units of water.
City configuration  7 collected    0 units of water.</pre>

==={{header|GW-BASIC}}===
{{works with|BASICA}}
<syntaxhighlight lang="gwbasic">10 DEFINT A-Z: DIM T(20): K=0
20 K=K+1: READ N: IF N=0 THEN END
30 FOR I=0 TO N-1: READ T(I): NEXT
40 W=0
50 FOR R=N-1 TO 0 STEP -1: IF T(R)=0 THEN NEXT ELSE IF R=0 THEN 110
60 B=0
70 FOR C=0 TO R
80 IF T(C)>0 THEN T(C)=T(C)-1: B=B+1 ELSE IF B>0 THEN W=W+1
90 NEXT
100 IF B>1 THEN 50
110 PRINT "Block";K;"holds";W;"water units."
120 GOTO 20
130 DATA 5,  1,5,3,7,2
140 DATA 10, 5,3,7,2,6,4,5,9,1,2
150 DATA 16, 2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1
160 DATA 4,  5,5,5,5
170 DATA 4,  5,6,7,8
180 DATA 4,  8,7,7,6
190 DATA 5,  6,7,10,7,6
200 DATA 0</syntaxhighlight>
{{out}}
<pre>Block 1 holds 2 water units.
Block 2 holds 14 water units.
Block 3 holds 35 water units.
Block 4 holds 0 water units.
Block 5 holds 0 water units.
Block 6 holds 0 water units.
Block 7 holds 0 water units.</pre>


==={{header|Nascom BASIC}}===
{{trans|FreeBasic}}
{{works with|Nascom ROM BASIC|4.7}}
<syntaxhighlight lang="basic">
10 REM Water collected between towers
20 MXN=19
30 REM Heights of towers in each city
40 REM prefixed by the number of towers
50 DATA 5,1,5,3,7,2
60 DATA 10,5,3,7,2,6,4,5,9,1,2
70 DATA 16,2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1
80 DATA 4,5,5,5,5
90 DATA 4,5,6,7,8
100 DATA 4,8,7,7,6
110 DATA 5,6,7,10,7,6
120 DIM A(MXN,1)
130 FOR I=1 TO 7
140 READ N
150 FOR J=0 TO N-1
160 READ A(J,0)
170 A(J,1)=J
180 NEXT J
190 GOSUB 390
200 IF A(0,1)>=A(1,1) THEN 220
210 FRST=A(0,1):LST=A(1,1):GOTO 230
220 FRST=A(1,1):LST=A(0,1)
230 WTR=A(1,0)*(LST-FRST-1)
240 FOR J=2 TO N-1
250 IF FRST>=A(J,1) OR A(J,1)>=LST THEN 270
260 WTR=WTR-A(J,0)
270 IF A(J,1)>=FRST THEN 300
280 WTR=WTR+A(J,0)*(FRST-A(J,1)-1)
290 FRST=A(J,1)
300 IF A(J,1)<=LST THEN 330
310 WTR=WTR+A(J,0)*(A(J,1)-LST-1)
320 LST=A(J,1)
330 NEXT J
340 PRINT "Bar chart";I;"collected";
350 PRINT WTR;"units of water."
360 NEXT I
370 END
380 REM ** ShellSort
390 GAP=N-1
400 GAP=INT(GAP/2.2)
410 IF GAP=0 THEN GAP=1
420 FOR K=GAP TO N-1
430 TH=A(K,0):TP=A(K,1)
440 L=K
450 IF L<GAP THEN 500
460 IF A(L-GAP,0)>=TH THEN 500
470 A(L,0)=A(L-GAP,0):A(L,1)=A(L-GAP,1)
480 L=L-GAP
490 GOTO 450
500 A(L,0)=TH:A(L,1)=TP
510 NEXT K
520 IF GAP<>1 THEN 400
530 RETURN
</syntaxhighlight>
{{out}}
<pre>
Bar chart 1 collected 2 units of water.
Bar chart 2 collected 14 units of water.
Bar chart 3 collected 35 units of water.
Bar chart 4 collected 0 units of water.
Bar chart 5 collected 0 units of water.
Bar chart 6 collected 0 units of water.
Bar chart 7 collected 0 units of water.
</pre>

==={{header|QuickBASIC}}===
{{trans|FreeBasic}}
<syntaxhighlight lang="qbasic">
' Water collected between towers
DECLARE SUB ShellSort (A() AS ANY)
TYPE TTowerRec
  Hght AS INTEGER
  Posi AS INTEGER
END TYPE

'heights of towers in each city prefixed by the number of towers
DATA 5, 1, 5, 3, 7, 2
DATA 10, 5, 3, 7, 2, 6, 4, 5, 9, 1, 2
DATA 16, 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1
DATA 4, 5, 5, 5, 5
DATA 4, 5, 6, 7, 8
DATA 4, 8, 7, 7, 6
DATA 5, 6, 7, 10, 7, 6

REM $DYNAMIC
DIM Manhattan(0 TO 1) AS TTowerRec
FOR I% = 1 TO 7
  READ N%
  ERASE Manhattan
  REDIM Manhattan(0 TO N% - 1) AS TTowerRec
  FOR J% = 0 TO N% - 1
    READ Manhattan(J%).Hght
    Manhattan(J%).Posi = J%
  NEXT J%
  ShellSort Manhattan()
  IF Manhattan(0).Posi < Manhattan(1).Posi THEN
    First% = Manhattan(0).Posi
    Last% = Manhattan(1).Posi
  ELSE
    First% = Manhattan(1).Posi
    Last% = Manhattan(0).Posi
  END IF
  Water% = Manhattan(1).Hght * (Last% - First% - 1)
  FOR J% = 2 TO N% - 1
    IF First% < Manhattan(J%).Posi AND Manhattan(J%).Posi < Last% THEN Water% = Water% - Manhattan(J%).Hght
    IF Manhattan(J%).Posi < First% THEN
      Water% = Water% + Manhattan(J%).Hght * (First% - Manhattan(J%).Posi - 1)
      First% = Manhattan(J%).Posi
    END IF
    IF Manhattan(J%).Posi > Last% THEN
      Water% = Water% + Manhattan(J%).Hght * (Manhattan(J%).Posi - Last% - 1)
      Last% = Manhattan(J%).Posi
    END IF
  NEXT J%
  PRINT USING "City configuration ## collected #### units of water."; I%; Water%
NEXT I%
END

REM $STATIC
SUB ShellSort (A() AS TTowerRec)
  'quick and dirty shellsort, not the focus of this exercise
  Gap% = UBOUND(A): N% = UBOUND(A)
  DIM Temp AS TTowerRec
  DO
    Gap% = INT(Gap% / 2.2)
    IF Gap% = 0 THEN Gap% = 1
    FOR I% = Gap% TO N%
      Temp = A(I%)
      J% = I%
      ' Simulated WHILE J% >= Gap% ANDALSO A(J% - Gap%).Hght < Temp.Hght
      DO
        IF J% < Gap% THEN EXIT DO
        IF A(J% - Gap%).Hght >= Temp.Hght THEN EXIT DO
        A(J%) = A(J% - Gap%)
        J% = J% - Gap%
      LOOP
      A(J%) = Temp
    NEXT I%
  LOOP UNTIL Gap% = 1
END SUB
</syntaxhighlight>
{{out}}
<pre>
City configuration  1 collected    2 units of water.
City configuration  2 collected   14 units of water.
City configuration  3 collected   35 units of water.
City configuration  4 collected    0 units of water.
City configuration  5 collected    0 units of water.
City configuration  6 collected    0 units of water.
City configuration  7 collected    0 units of water.
</pre>

==={{header|uBasic/4tH}}===
{{Trans|GW-BASIC}}
<syntaxhighlight lang="basic">Dim @t(20)

k = FUNC (_getWater (1, 5, 3, 7, 2, 1))
k = FUNC (_getWater (5, 3, 7, 2, 6, 4, 5, 9, 1, 2, k))
k = FUNC (_getWater (2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1, k))
k = FUNC (_getWater (5, 5, 5, 5, k))
k = FUNC (_getWater (5, 6, 7, 8, k))
k = FUNC (_getWater (8, 7, 7, 6, k))
k = FUNC (_getWater (6, 7, 10, 7, 6, k))
End

_getWater
  Param (1)
  Local (2)

  w = 0
  c@ = Used()

  For b@ = c@ - 1 To 0 Step -1
    @t(b@) = Pop()
  Next

  Do While FUNC(_netWater (c@)) > 1 : Loop

  Print "Block ";a@;" holds ";w;" water units."
Return (a@ + 1)

_netWater
  Param (1)
  Local (3)

  For d@ = a@-1 To 0 Step -1
    If @t(d@) Then
      If d@ = 0 Then Unloop : Return (0) : fi
    Else
      Continue
    EndIf

    b@ = 0

    For c@ = 0 To d@
      If @t(c@) > 0 Then
        @t(c@) = @t(c@) - 1
        b@ = b@ + 1
      Else
        If b@ > 0 Then w = w + 1 : fi
      EndIf
    Next

    Unloop : Return (b@)
  Next
Return (0)</syntaxhighlight>
{{Out}}
<pre>Block 1 holds 2 water units.
Block 2 holds 14 water units.
Block 3 holds 35 water units.
Block 4 holds 0 water units.
Block 5 holds 0 water units.
Block 6 holds 0 water units.
Block 7 holds 0 water units.

0 OK, 0:409</pre>

==={{header|Visual Basic .NET}}===
====Version 1====
'''Method:''' Instead of "scanning" adjoining towers for each column, this routine converts the tower data into a string representation with building blocks, empty spaces, and potential water retention sites.  The potential water retention sites are then "eroded" away where they are found to be unsupported.  This is accomplished with the '''.Replace()''' function.  The replace operations are unleashed upon the entire "block" of towers, rather than a cell at a time or a line at a time - which perhaps increases the program's execution-time, but reduces program's complexity.

The program can optionally display the interim string representation of each tower block before the final count is completed.  I've since modified it to have the same block and wavy characters are the 
[[{{FULLPAGENAME}}#version_3|REXX 9.3]] output, but used the double-wide columns, as pictured in the task definition area.
<syntaxhighlight lang="vbnet">' Convert tower block data into a string representation, then manipulate that.
Module Module1
    Sub Main(Args() As String)
        Dim shoTow As Boolean = Environment.GetCommandLineArgs().Count > 1  ' Show towers.
        Dim wta As Integer()() = {                       ' Water tower array (input data).
            New Integer() {1, 5, 3, 7, 2}, New Integer() {5, 3, 7, 2, 6, 4, 5, 9, 1, 2},
            New Integer() {2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1},
            New Integer() {5, 5, 5, 5}, New Integer() {5, 6, 7, 8},
            New Integer() {8, 7, 7, 6}, New Integer() {6, 7, 10, 7, 6}}
        Dim blk As String,                   ' String representation of a block of towers.
            lf As String = vbLf,      ' Line feed to separate floors in a block of towers.
            tb = "██", wr = "≈≈", mt = "  "    ' Tower Block, Water Retained, eMpTy space.
        For i As Integer = 0 To wta.Length - 1
            Dim bpf As Integer                    ' Count of tower blocks found per floor.
            blk = ""
            Do
                bpf = 0 : Dim floor As String = ""  ' String representation of each floor.
                For j As Integer = 0 To wta(i).Length - 1
                    If wta(i)(j) > 0 Then      ' Tower block detected, add block to floor,
                        floor &= tb : wta(i)(j) -= 1 : bpf += 1    '  reduce tower by one.
                    Else  '      Empty space detected, fill when not first or last column.
                        floor &= If(j > 0 AndAlso j < wta(i).Length - 1, wr, mt)
                    End If
                Next
                If bpf > 0 Then blk = floor & lf & blk ' Add floors until blocks are gone.
            Loop Until bpf = 0                       ' No tower blocks left, so terminate.
            ' Erode potential water retention cells from left and right.
            While blk.Contains(mt & wr) : blk = blk.Replace(mt & wr, mt & mt) : End While
            While blk.Contains(wr & mt) : blk = blk.Replace(wr & mt, mt & mt) : End While
            ' Optionaly show towers w/ water marks.
            If shoTow Then Console.Write("{0}{1}", lf, blk)
            ' Subtract the amount of non-water mark characters from the total char amount.
            Console.Write("Block {0} retains {1,2} water units.{2}", i + 1,
                                     (blk.Length - blk.Replace(wr, "").Length) \ 2, lf)
        Next
    End Sub
End Module</syntaxhighlight>
{{out}}<syntaxhighlight lang="text">Block 1 retains  2 water units.
Block 2 retains 14 water units.
Block 3 retains 35 water units.
Block 4 retains  0 water units.
Block 5 retains  0 water units.
Block 6 retains  0 water units.
Block 7 retains  0 water units.</syntaxhighlight>
Verbose output shows towers with water ("Almost equal to" characters) left in the "wells" between towers.  Just supply any command-line parameter to see it.  Use no command line parameters to see the plain output above.
<syntaxhighlight lang="text">      ██
      ██
  ██≈≈██
  ██≈≈██
  ██████
  ████████
██████████
Block 1 retains  2 water units.

              ██
              ██
    ██≈≈≈≈≈≈≈≈██
    ██≈≈██≈≈≈≈██
██≈≈██≈≈██≈≈████
██≈≈██≈≈████████
██████≈≈████████
████████████████≈≈██
████████████████████
Block 2 retains 14 water units.

          ██
          ██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██
  ██≈≈≈≈≈≈██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██
  ██≈≈██≈≈██≈≈≈≈≈≈≈≈██≈≈████
  ██≈≈██≈≈██≈≈██≈≈≈≈██≈≈██████
  ██████≈≈██≈≈██≈≈≈≈██████████
████████████≈≈████████████████
████████████████████████████████
Block 3 retains 35 water units.

████████
████████
████████
████████
████████
Block 4 retains  0 water units.

      ██
    ████
  ██████
████████
████████
████████
████████
████████
Block 5 retains  0 water units.

██
██████
████████
████████
████████
████████
████████
████████
Block 6 retains  0 water units.

    ██
    ██
    ██
  ██████
██████████
██████████
██████████
██████████
██████████
██████████
Block 7 retains  0 water units.</syntaxhighlight>

====Version 2====
'''Method:''' More conventional "scanning" method.  A Char array is used, but no Replace() statements.  Output is similar to version 1, although there is now a left margin of three spaces, the results statement is immediately to the right of the string representation of the tower blocks (instead of underneath), the verb is "hold(s)" instead of "retains", and there is a special string when the results indicate zero.

<syntaxhighlight lang="vbnet">Module Module1
    ''' <summary>
    ''' wide - Widens the aspect ratio of a linefeed separated string.
    ''' </summary>
    ''' <param name="src">A string representing a block of towers.</param>
    ''' <param name="margin">Optional padding for area to the left.</param>
    ''' <returns>A double-wide version of the string.</returns>
    Function wide(src As String, Optional margin As String = "") As String
        Dim res As String = margin : For Each ch As Char In src
            res += If(ch < " ", ch & margin, ch + ch) : Next : Return res
    End Function

    ''' <summary>
    ''' cntChar - Counts characters, also custom formats the output.
    ''' </summary>
    ''' <param name="src">The string to count characters in.</param>
    ''' <param name="ch">The character to be counted.</param>
    ''' <param name="verb">Verb to include in format.  Expecting "hold",
    '''             but can work with "retain" or "have".</param>
    ''' <returns>The count of chars found in a string, and formats a verb.</returns>
    Function cntChar(src As String, ch As Char, verb As String) As String
        Dim cnt As Integer = 0
        For Each c As Char In src : cnt += If(c = ch, 1, 0) : Next
        Return If(cnt = 0, "does not " & verb & " any",
            verb.Substring(0, If(verb = "have", 2, 4)) & "s " & cnt.ToString())
    End Function

    ''' <summary>
    ''' report - Produces a report of the number of rain units found in 
    '''          a block of towers, optionally showing the towers.
    '''          Autoincrements the blkID for each report.
    ''' </summary>
    ''' <param name="tea">An int array with tower elevations.</param>
    ''' <param name="blkID">An int of the block of towers ID.</param>
    ''' <param name="verb">The verb to use in the description.
    '''                    Defaults to "has / have".</param>
    ''' <param name="showIt">When true, the report includes a string representation
    '''                      of the block of towers.</param>
    ''' <returns>A string containing the amount of rain units, optionally preceeded by
    '''          a string representation of the towers holding any water.</returns>
    Function report(tea As Integer(),                             ' Tower elevation array.
                    ByRef blkID As Integer,                ' Block ID for the description.
                    Optional verb As String = "have",    ' Verb to use in the description.
                    Optional showIt As Boolean = False) As String    ' Show representaion.
        Dim block As String = "",                                   ' The block of towers.
            lf As String = vbLf,                           ' The separator between floors.
            rTwrPos As Integer        ' The position of the rightmost tower of this floor.
        Do
            For rTwrPos = tea.Length - 1 To 0 Step -1      ' Determine the rightmost tower
                If tea(rTwrPos) > 0 Then Exit For          '      postition on this floor.
            Next
            If rTwrPos < 0 Then Exit Do         ' When no towers remain, exit the do loop.
            ' init the floor to a space filled Char array, as wide as the block of towers.
            Dim floor As Char() = New String(" ", tea.Length).ToCharArray()
            Dim bpf As Integer = 0                  ' The count of blocks found per floor.
            For column As Integer = 0 To rTwrPos                ' Scan from left to right.
                If tea(column) > 0 Then                     ' If a tower exists here,
                    floor(column) = "█"                     ' mark the floor with a block,
                    tea(column) -= 1                    ' drop the tower elevation by one,
                    bpf += 1                           ' and advance the block count.
                ElseIf bpf > 0 Then    ' Otherwise, see if a tower is present to the left.
                    floor(column) = "≈"                           ' OK to fill with water.
                End If
            Next
            If bpf > If(showIt, 0, 1) Then       ' Continue the building only when needed.
                ' If not showing blocks, discontinue building when a single tower remains.
                ' build tower blocks string with each floor added to top.
                block = New String(floor) & If(block = "", "", lf) & block
            Else
                Exit Do                          ' Ran out of towers, so exit the do loop.
            End If
        Loop While True ' Depending on previous break statements to terminate the do loop.
        blkID += 1                                           ' increment block ID counter.
        ' format report and return it.
        Return If(showIt, String.Format(vbLf & "{0}", wide(block, "   ")), "") &
            String.Format(" Block {0} {1} water units.", blkID, cntChar(block, "≈", verb))
    End Function

    ''' <summary>
    ''' Main routine.
    ''' 
    ''' With one command line parameter, it shows tower blocks,
    '''  with no command line parameters, it shows a plain report
    '''</summary>
    Sub Main()
        Dim shoTow As Boolean = Environment.GetCommandLineArgs().Count > 1  ' Show towers.
        Dim blkCntr As Integer = 0        ' Block ID for reports.
        Dim verb As String = "hold"    ' "retain" or "have" can be used instead of "hold".
        Dim tea As Integer()() = {New Integer() {1, 5, 3, 7, 2},   ' Tower elevation data.
            New Integer() {5, 3, 7, 2, 6, 4, 5, 9, 1, 2},
            New Integer() {2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1},
            New Integer() {5, 5, 5, 5}, New Integer() {5, 6, 7, 8},
            New Integer() {8, 7, 7, 6}, New Integer() {6, 7, 10, 7, 6}}
        For Each block As Integer() In tea
            ' Produce report for each block of towers.
            Console.WriteLine(report(block, blkCntr, verb, shoTow))
        Next
    End Sub
End Module</syntaxhighlight>
Regular version 2 output:
<syntaxhighlight lang="text"> Block 1 holds 2 water units.
 Block 2 holds 14 water units.
 Block 3 holds 35 water units.
 Block 4 does not hold any water units.
 Block 5 does not hold any water units.
 Block 6 does not hold any water units.
 Block 7 does not hold any water units.</syntaxhighlight>
Sample of version 2 verbose output:
<syntaxhighlight lang="text">             ██
             ██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██
     ██≈≈≈≈≈≈██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██
     ██≈≈██≈≈██≈≈≈≈≈≈≈≈██≈≈████
     ██≈≈██≈≈██≈≈██≈≈≈≈██≈≈██████
     ██████≈≈██≈≈██≈≈≈≈██████████
   ████████████≈≈████████████████
   ████████████████████████████████ Block 3 holds 35 water units.

   ████████
   ████████
   ████████
   ████████
   ████████ Block 4 does not hold any water units.</syntaxhighlight>

==={{header|Yabasic}}===
{{trans|AWK}}
<syntaxhighlight lang="yabasic">data 7
data "1,5,3,7,2", "5,3,7,2,6,4,5,9,1,2", "2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1"
data "5,5,5,5", "5,6,7,8", "8,7,7,6", "6,7,10,7,6"

read n

for i = 1 to n
	read n$
	wcbt(n$)
next i

sub wcbt(s$)
	local tower$(1), hr(1), hl(1), n, i, ans, k
	
	n = token(s$, tower$(), ",")

	redim hr(n)
	redim hl(n)
	for i = n to 1 step -1
		if i < n then
			k = hr(i + 1)
		else
			k = 0
		end if
		hr(i) = max(val(tower$(i)), k)
	next i
	for i = 1 to n
		if i then
			k = hl(i - 1)
		else
			k = 0
		end if
		hl(i) = max(val(tower$(i)), k)
		ans = ans + min(hl(i), hr(i)) - val(tower$(i))
	next i
	print ans," ",n$
end sub</syntaxhighlight>

=={{header|C}}==
Takes the integers as input from command line, prints out usage on incorrect invocation.
<syntaxhighlight lang="c">
#include<stdlib.h>
#include<stdio.h>

int getWater(int* arr,int start,int end,int cutoff){
	int i, sum = 0;
	
	for(i=start;i<=end;i++)
		sum += ((arr[cutoff] > arr[i])?(arr[cutoff] - arr[i]):0);
	
	return sum;
}

int netWater(int* arr,int size){
	int i, j, ref1, ref2, marker, markerSet = 0,sum = 0;
	
	if(size<3)
		return 0;

	for(i=0;i<size-1;i++){
		start:if(i!=size-2 && arr[i]>arr[i+1]){
				ref1 = i;
				
				for(j=ref1+1;j<size;j++){
					if(arr[j]>=arr[ref1]){
						ref2 = j;
						
						sum += getWater(arr,ref1+1,ref2-1,ref1);

						i = ref2;
						
						goto start;
					}
					
					else if(j!=size-1 && arr[j] < arr[j+1] && (markerSet==0||(arr[j+1]>=arr[marker]))){
						marker = j+1;
						markerSet = 1;
					}
				}
				
				if(markerSet==1){
					sum += getWater(arr,ref1+1,marker-1,marker);

					i = marker;
					
					markerSet = 0;
					
					goto start;
				}
			}
		}
	
	return sum;
}

int main(int argC,char* argV[])
{
	int *arr,i;
	
	if(argC==1)
		printf("Usage : %s <followed by space separated series of integers>");
	else{
		arr = (int*)malloc((argC-1)*sizeof(int));
		
		for(i=1;i<argC;i++)
			arr[i-1] = atoi(argV[i]);

		printf("Water collected : %d",netWater(arr,argC-1));
	}
	
	return 0;
}
</syntaxhighlight>
Output :
<pre>
C:\rosettaCode>waterTowers.exe 1 5 3 7 2
Water collected : 2
C:\rosettaCode>waterTowers.exe 5 3 7 2 6 4 5 9 1 2
Water collected : 14
C:\rosettaCode>waterTowers.exe 2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1
Water collected : 35
C:\rosettaCode>waterTowers.exe 5 5 5 5
Water collected : 0
C:\rosettaCode>waterTowers.exe 8 7 7 6
Water collected : 0
C:\rosettaCode>waterTowers.exe 6 7 10 7 6
Water collected : 0
</pre>

=={{header|C sharp|C#}}==
===Version 1===
Translation from [[{{FULLPAGENAME}}#Visual_Basic_.NET|Visual Basic .NET]].  See that version 1 entry for code comment details and more sample output.
<syntaxhighlight lang="csharp">class Program
{
    static void Main(string[] args)
    {
        int[][] wta = {
            new int[] {1, 5, 3, 7, 2},   new int[] { 5, 3, 7, 2, 6, 4, 5, 9, 1, 2 },
            new int[] { 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 },
            new int[] { 5, 5, 5, 5 },    new int[] { 5, 6, 7, 8 },
            new int[] { 8, 7, 7, 6 },    new int[] { 6, 7, 10, 7, 6 }};
        string blk, lf = "\n", tb = "██", wr = "≈≈", mt = "  ";
        for (int i = 0; i < wta.Length; i++)
        {
            int bpf; blk = ""; do
            {
                string floor = ""; bpf = 0; for (int j = 0; j < wta[i].Length; j++)
                {
                    if (wta[i][j] > 0)
                    {    floor += tb; wta[i][j] -= 1; bpf += 1; }
                    else floor += (j > 0 && j < wta[i].Length - 1 ? wr : mt);
                }
                if (bpf > 0) blk = floor + lf + blk;
            } while (bpf > 0);
            while (blk.Contains(mt + wr)) blk = blk.Replace(mt + wr, mt + mt);
            while (blk.Contains(wr + mt)) blk = blk.Replace(wr + mt, mt + mt);
            if (args.Length > 0) System.Console.Write("\n{0}", blk);
            System.Console.WriteLine("Block {0} retains {1,2} water units.",
                i + 1, (blk.Length - blk.Replace(wr, "").Length) / 2);
        }
    }
}</syntaxhighlight>{{out}}<syntaxhighlight lang="text">Block 1 retains  2 water units.
Block 2 retains 14 water units.
Block 3 retains 35 water units.
Block 4 retains  0 water units.
Block 5 retains  0 water units.
Block 6 retains  0 water units.
Block 7 retains  0 water units.</syntaxhighlight>
===Version 2===
Conventional "scanning" algorithm, translated from [[{{FULLPAGENAME}}#Version_2_2|the second version of Visual Basic.NET]], but (intentionally tweaked to be) incapable of verbose output.  See that version 2 entry for code comments and details.
<syntaxhighlight lang="csharp">class Program
{
// Variable names key:
//   i Iterator (of the tower block array).
// tba Tower block array.
// tea Tower elevation array.
// rht Right hand tower column number (position).
//  wu Water units (count).
// bof Blocks on floor (count).
// col Column number in elevation array (position).
 
    static void Main(string[] args)
    {
        int i = 1; int[][] tba = {new int[] { 1, 5, 3, 7, 2 },
        new int[] { 5, 3, 7, 2, 6, 4, 5, 9, 1, 2 },
        new int[] { 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 },
        new int[] { 5, 5, 5, 5 },  new int[] { 5, 6, 7, 8 },
        new int[] { 8, 7, 7, 6 },  new int[] { 6, 7, 10, 7, 6 }};
        foreach (int[] tea in tba)
        {
            int rht, wu = 0, bof; do
            {
                for (rht = tea.Length - 1; rht >= 0; rht--)
                    if (tea[rht] > 0) break;
                if (rht < 0) break;
                bof = 0; for (int col = 0; col <= rht; col++)
                {
                    if (tea[col] > 0) { tea[col] -= 1; bof += 1; }
                    else if (bof > 0) wu++;
                }
                if (bof < 2) break;
            } while (true);
            System.Console.WriteLine(string.Format("Block {0} {1} water units.",
                i++, wu == 0 ? "does not hold any" : "holds " + wu.ToString()));
        }
    }
}</syntaxhighlight>
'''Output:'''
<pre>Block 1 holds 2 water units.
Block 2 holds 14 water units.
Block 3 holds 35 water units.
Block 4 does not hold any water units.
Block 5 does not hold any water units.
Block 6 does not hold any water units.
Block 7 does not hold any water units.</pre>

=={{header|C++}}==
<syntaxhighlight lang="cpp">
#include <iostream>
#include <vector>
#include <algorithm>

enum { EMPTY, WALL, WATER };

auto fill(const std::vector<int> b) {
  auto water = 0;
  const auto rows = *std::max_element(std::begin(b), std::end(b));
  const auto cols = std::size(b);
  std::vector<std::vector<int>> g(rows);
  for (auto& r : g) {
    for (auto i = 0; i < cols; ++i) {
      r.push_back(EMPTY);
    }
  }
  for (auto c = 0; c < cols; ++c) {
    for (auto r = rows - 1u, i = 0u; i < b[c]; ++i, --r) {
      g[r][c] = WALL;
    }
  }
  for (auto c = 0; c < cols - 1; ++c) {
    auto start_row = rows - b[c];
    while (start_row < rows) {
      if (g[start_row][c] == EMPTY) break;
      auto c2 = c + 1;
      bool hitWall = false;
      while (c2 < cols) {
        if (g[start_row][c2] == WALL) {
          hitWall = true;
          break;
        }
        ++c2;
      }
      if (hitWall) {
        for (auto i = c + 1; i < c2; ++i) {
          g[start_row][i] = WATER;
          ++water;
        }
      }
      ++start_row;
    }
  }
  return water;
}

int main() {
  std::vector<std::vector<int>> b = {
    { 1, 5, 3, 7, 2 },
    { 5, 3, 7, 2, 6, 4, 5, 9, 1, 2 },
    { 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 },
    { 5, 5, 5, 5 },
    { 5, 6, 7, 8 },
    { 8, 7, 7, 6 },
    { 6, 7, 10, 7, 6 }
  };
  for (const auto v : b) {
    auto water = fill(v);
    std::cout << water << " water drops." << std::endl;
  }
  std::cin.ignore();
  std::cin.get();
  return 0;
}</syntaxhighlight>

{{out}}
<pre>2 water drops.
14 water drops.
35 water drops.
0 water drops.
0 water drops.
0 water drops.
0 water drops.</pre>

=={{header|Clojure}}==
Similar two passes algorithm as many solutions here. First traverse left to right to find the highest tower on the left of each position, inclusive of the tower at the current position, than do the same to find the highest tower to the right of each position. Finally, compute the total water units held at any position as the difference of those two heights.

<syntaxhighlight lang="clojure">
(defn trapped-water [towers]
  (let [maxes #(reductions max %)                ; the seq of increasing max values found in the input seq
        maxl  (maxes towers)                     ; the seq of max heights to the left of each tower
        maxr  (reverse (maxes (reverse towers))) ; the seq of max heights to the right of each tower
        mins  (map min maxl maxr)]               ; minimum highest surrounding tower per position
    (reduce + (map - mins towers))))             ; sum up the trapped water per position
</syntaxhighlight>
{{out}}
<syntaxhighlight lang="clojure">
;; in the following, # is a tower block and ~ is trapped water:
;;
;;          10|
;;           9|               #
;;           8|               #
;;           7|     # ~ ~ ~ ~ #
;;           6|     # ~ # ~ ~ #
;;           5| # ~ # ~ # ~ # #
;;           4| # ~ # ~ # # # #
;;           3| # # # ~ # # # #
;;           2| # # # # # # # # ~ #
;;           1| # # # # # # # # # #
;;         ---+---------------------
;;              5 3 7 2 6 4 5 9 1 2
(trapped-water [5 3 7 2 6 4 5 9 1 2]) ;; 14
</syntaxhighlight>

=={{header|CLU}}==
<syntaxhighlight lang="clu">max = proc [T: type] (a,b: T) returns (T)
      where T has lt: proctype (T,T) returns (bool)
    if a<b then return(b)
    else return(a)
    end
end max

% based on: https://stackoverflow.com/a/42821623
water = proc (towers: sequence[int]) returns (int)
    si = sequence[int]
    
    w: int := 0
    left: int := 1
    right: int := si$size(towers)
    max_left: int := si$bottom(towers)
    max_right: int := si$top(towers)
    
    while left <= right do
        if towers[left] <= towers[right] then
            max_left := max[int](towers[left], max_left)
            w := w + max[int](max_left - towers[left], 0)
            left := left + 1
        else
            max_right := max[int](towers[right], max_right)
            w := w + max[int](max_right - towers[right], 0)
            right := right - 1
        end
    end
    return(w)
end water

start_up = proc ()
    si = sequence[int]
    ssi = sequence[si]
    
    po: stream := stream$primary_output()
    
    tests: ssi := ssi$[
        si$[1, 5, 3, 7, 2],
        si$[5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
        si$[2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
        si$[5, 5, 5, 5],
        si$[5, 6, 7, 8],
        si$[8, 7, 7, 6],
        si$[6, 7, 10, 7, 6]
    ]
    
    for test: si in ssi$elements(tests) do
        stream$puts(po, int$unparse(water(test)) || " ")
    end
end start_up</syntaxhighlight>
{{out}}
<pre>2 14 35 0 0 0 0</pre>

=={{header|Cowgol}}==
<syntaxhighlight lang="cowgol">include "cowgol.coh";
include "argv.coh";

# Count the amount of water in a given array
sub water(towers: [uint8], length: intptr): (units: uint8) is
    units := 0;
    loop
        var right := towers + length;
        loop
            right := @prev right;
            if right < towers or [right] != 0 then
                break;
            end if;
        end loop;
        if right < towers then break; end if;
        
        var blocks: uint8 := 0;
        var col := towers;
        while col <= right loop
            if [col] != 0 then
                [col] := [col] - 1;
                blocks := blocks + 1;
            elseif blocks != 0 then
                units := units + 1;
            end if;
            col := @next col;
        end loop;
        if blocks < 2 then
            break;
        end if;
    end loop;
end sub;

# Read list from the command line and print the answer
ArgvInit();
var towers: uint8[256];
var count: @indexof towers := 0;
var n32: int32;
loop
    var argmt := ArgvNext();
    if argmt == 0 as [uint8] then
        break;
    end if;
    (n32, argmt) := AToI(argmt);
    towers[count] := n32 as uint8;
    count := count + 1;
end loop;

if count == 0 then
    print("enter towers on command line\n");
    ExitWithError();
end if;

print_i8(water(&towers[0], count as intptr));
print_nl();</syntaxhighlight>

{{out}}

<pre>$ ./water.386 1 5 3 7 2
2
$ ./water.386 5 3 7 2 6 4 5 9 1 2
14
$ ./water.386 2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1
35
$ ./water.386 5 5 5 5
0
$ ./water.386 5 6 7 8
0
$ ./water.386 8 7 7 6
0
$ ./water.386 6 7 10 7 6
0</pre>

=={{header|D}}==
{{Trans|C#}}
<syntaxhighlight lang="d">import std.stdio;

void main() {
    int i = 1;
    int[][] tba = [
        [ 1, 5, 3, 7, 2 ],
        [ 5, 3, 7, 2, 6, 4, 5, 9, 1, 2 ],
        [ 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 ],
        [ 5, 5, 5, 5 ],
        [ 5, 6, 7, 8 ],
        [ 8, 7, 7, 6 ],
        [ 6, 7, 10, 7, 6 ]
    ];

    foreach (tea; tba) {
        int rht, wu, bof;
        do {
            for (rht = tea.length - 1; rht >= 0; rht--) {
                if (tea[rht] > 0) {
                    break;
                }
            }

            if (rht < 0) {
                break;
            }

            bof = 0;
            for (int col = 0; col <= rht; col++) {
                if (tea[col] > 0) {
                    tea[col] -= 1; bof += 1;
                } else if (bof > 0) {
                    wu++;
                }
            }
            if (bof < 2) {
                break;
            }
        } while (true);

        write("Block ", i++);
        if (wu == 0) {
            write(" does not hold any");
        } else {
            write(" holds ", wu);
        }
        writeln(" water units.");
    }
}</syntaxhighlight>

{{out}}
<pre>Block 1 holds 2 water units.
Block 2 holds 14 water units.
Block 3 holds 35 water units.
Block 4 does not hold any water units.
Block 5 does not hold any water units.
Block 6 does not hold any water units.
Block 7 does not hold any water units.</pre>

=={{header|Delphi}}==
{{works with|Delphi|6.0}}
{{libheader|SysUtils,StdCtrls}}
The program builds a matrix of the towers and scans each line looking for pairs of towers that trap water.

<syntaxhighlight lang="Delphi">

var Towers1: array [0..4] of integer = (1, 5, 3, 7, 2);
var Towers2: array [0..9] of integer = (5, 3, 7, 2, 6, 4, 5, 9, 1, 2);
var Towers3: array [0..15] of integer = (2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1);
var Towers4: array [0..3] of integer = (5, 5, 5, 5);
var Towers5: array [0..3] of integer = (5, 6, 7, 8);
var Towers6: array [0..3] of integer = (8, 7, 7, 6);
var Towers7: array [0..4] of integer = (6, 7, 10, 7, 6);


type TMatrix = array of array of boolean;

function ArrayToMatrix(Towers: array of integer): TMatrix;
{Convert Tower Array to Matrix for analysis}
var Max,I,X,Y: integer;
begin
Max:=0;
for I:=0 to High(Towers) do if Towers[I]>=Max then Max:=Towers[I];
SetLength(Result,Length(Towers),Max);
for Y:=0 to High(Result[0]) do
 for X:=0 to High(Result) do Result[X,Y]:=Towers[X]>(Max-Y);
end;


procedure DisplayMatrix(Memo: TMemo; Matrix: TMatrix);
{Display a matrix}
var X,Y: integer;
var S: string;
begin
for Y:=0 to High(Matrix[0]) do
	begin
	S:='[';
	for X:=0 to High(Matrix) do
		begin
		if Matrix[X,Y] then S:=S+'#'
		else S:=S+' ';
		end;
	S:=S+']';
	Memo.Lines.Add(S);
	end;
end;


function GetWaterStorage(Matrix: TMatrix): integer;
{Analyze matrix to get water storage amount}
var X,Y,Cnt: integer;
var Inside: boolean;
begin
Result:=0;
{Scan each row of matrix to see if it is storing water}
for Y:=0 to High(Matrix[0]) do
	begin
	Inside:=False;
	Cnt:=0;
 	for X:=0 to High(Matrix) do
		begin
		{Test if this is a tower}
		if Matrix[X,Y] then
			begin
			{if so, we may be inside trough}
			Inside:=True;
			{If Cnt>0 there was a previous tower}
			{And we've impounded water }
			Result:=Result+Cnt;
			{Start new count with new tower}
			Cnt:=0;
			end
		else if Inside then Inc(Cnt);	{Count potential impounded water}
		end;
	end;
end;


procedure ShowWaterLevels(Memo: TMemo; Towers: array of integer);
{Analyze the water storage of towers and display result}
var Water: integer;
var Matrix: TMatrix;
begin
Matrix:=ArrayToMatrix(Towers);
DisplayMatrix(Memo,Matrix);
Water:=GetWaterStorage(Matrix);
Memo.Lines.Add('Storage: '+IntToStr(Water)+CRLF);
end;


procedure WaterLevel(Memo: TMemo);
begin
ShowWaterLevels(Memo,Towers1);
ShowWaterLevels(Memo,Towers2);
ShowWaterLevels(Memo,Towers3);
ShowWaterLevels(Memo,Towers4);
ShowWaterLevels(Memo,Towers5);
ShowWaterLevels(Memo,Towers6);
ShowWaterLevels(Memo,Towers7);
end;




</syntaxhighlight>
{{out}}
<pre>
[     ]
[   # ]
[   # ]
[ # # ]
[ # # ]
[ ### ]
[ ####]
Storage: 2

[          ]
[       #  ]
[       #  ]
[  #    #  ]
[  # #  #  ]
[# # # ##  ]
[# # ####  ]
[### ####  ]
[######## #]
Storage: 14

[                ]
[     #          ]
[     #       #  ]
[ #   #       #  ]
[ # # #    # ##  ]
[ # # # #  # ### ]
[ ### # #  ##### ]
[###### ######## ]
Storage: 35

[    ]
[####]
[####]
[####]
[####]
Storage: 0

[    ]
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Elapsed Time: 171.444 ms.

</pre>

=={{header|EasyLang}}==

<syntaxhighlight lang="easylang">
proc water h[] . .
   n = len h[]
   len left[] n
   len right[] n
   for i = 1 to n
      max = higher max h[i]
      left[i] = max
   .
   max = 0
   for i = n downto 1
      max = higher max h[i]
      right[i] = max
   .
   for i = 1 to n
      sum += (lower left[i] right[i]) - h[i]
   .
   print sum
.
repeat
   s$ = input
   until s$ = ""
   water number strsplit s$ " "
.
# 
input_data
1 5 3 7 2
5 3 7 2 6 4 5 9 1 2
2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1
5 5 5 5
5 6 7 8
8 7 7 6
6 7 10 7 6

</syntaxhighlight>

=={{header|Erlang}}==
Implements a version that uses recursion to solve the problem functionally, using two passes without requiring list reversal or modifications. On the list iteration from head to tail, gather the largest element seen so far (being the highest one on the left). Once the list is scanned, each position returns the highest tower to its right as reported by its follower, along with the amount of water seen so far, which can then be used to calculate the value at the current position. Back at the first list element, the final result is gathered.

<syntaxhighlight lang="erlang">
-module(watertowers).
-export([towers/1, demo/0]).

towers(List) -> element(2, tower(List, 0)).

tower([], _) -> {0,0};
tower([H|T], MaxLPrev) ->
    MaxL = max(MaxLPrev, H),
    {MaxR, WaterAcc} = tower(T, MaxL),
    {max(MaxR,H), WaterAcc+max(0, min(MaxR,MaxL)-H)}.

demo() ->
    Cases = [[1, 5, 3, 7, 2],
             [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
             [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
             [5, 5, 5, 5],
             [5, 6, 7, 8],
             [8, 7, 7, 6],
             [6, 7, 10, 7, 6]],
    [io:format("~p -> ~p~n", [Case, towers(Case)]) || Case <- Cases],
    ok.
</syntaxhighlight>

{{out}}
<pre>
1> watertowers:demo().
[1,5,3,7,2] -> 2
[5,3,7,2,6,4,5,9,1,2] -> 14
[2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1] -> 35
[5,5,5,5] -> 0
[5,6,7,8] -> 0
[8,7,7,6] -> 0
[6,7,10,7,6] -> 0
ok
</pre>

=={{header|F_Sharp|F#}}==
see http://stackoverflow.com/questions/24414700/water-collected-between-towers/43779936#43779936 for an explanation of this code. It is proportional to the number of towers. Although the examples on stackoverflow claim this, the n they use is actually the distance between the two end towers and not the number of towers. Consider the case of a tower of height 5 at 1, a tower of height 10 at 39, and a tower of height 3 at 101. 
<syntaxhighlight lang="fsharp">
(*
A solution I'd show to Euclid !!!!.
Nigel Galloway May 4th., 2017
*)
let solve n =
  let (n,_)::(i,e)::g = n|>List.sortBy(fun n->(-(snd n)))
  let rec fn i g e l =
    match e with
    | (n,e)::t when n < i -> fn n g t (l+(i-n-1)*e)
    | (n,e)::t when n > g -> fn i n t (l+(n-g-1)*e)
    | (n,t)::e            -> fn i g e (l-t)
    | _                   -> l
  fn (min n i) (max n i) g (e*(abs(n-i)-1))
</syntaxhighlight>
{{out}}
<pre>
solve [(1,1);(2,5);(3,3);(4,7);(5,2)] -> 2
solve [(1,5);(2,3);(3,7);(4,2);(5,6);(6,4);(7,5);(8,9);(9,1);(10,2)] -> 14
solve [(1,2);(2,6);(3,3);(4,5);(5,2);(6,8);(7,1);(8,4);(9,2);(10,2);(11,5);(12,3);(13,5);(14,7);(15,4);(16,1)] -> 35
solve [(1,5);(2,5);(3,5);(4,5)] -> 0
solve [(1,5);(2,6);(3,7);(4,8)] -> 0
solve [(1,8);(2,7);(3,7);(4,6)] -> 0
solve [(1,6);(2,7);(3,10);(4,7);(5,6)] -> 0
solve [(1,5);(39,10);(101,3)] -> 368
</pre>

=={{header|Factor}}==
<syntaxhighlight lang="factor">USING: formatting kernel math.statistics math.vectors sequences ;

: area ( seq -- n )
    [ cum-max ] [ <reversed> cum-max reverse vmin ] [ v- sum ] tri ;

{
    { 1 5 3 7 2 }
    { 5 3 7 2 6 4 5 9 1 2 }
    { 2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1 }
    { 5 5 5 5 }
    { 5 6 7 8 }
    { 8 7 7 6 }
    { 6 7 10 7 6 }
} [ dup area "%[%d, %] -> %d\n" printf ] each</syntaxhighlight>
{{out}}
<pre>
{ 1, 5, 3, 7, 2 } -> 2
{ 5, 3, 7, 2, 6, 4, 5, 9, 1, 2 } -> 14
{ 2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 } -> 35
{ 5, 5, 5, 5 } -> 0
{ 5, 6, 7, 8 } -> 0
{ 8, 7, 7, 6 } -> 0
{ 6, 7, 10, 7, 6 } -> 0
</pre>

=={{header|Go}}==
<syntaxhighlight lang="go">
package main

import "fmt"

func maxl(hm []int ) []int{
	res := make([]int,len(hm))
	max := 1
	for i := 0; i < len(hm);i++{
		if(hm[i] > max){
			max = hm[i]
		}
		res[i] = max;
	}
	return res
}
func maxr(hm []int ) []int{
	res := make([]int,len(hm))
	max := 1
	for i := len(hm) - 1 ; i >= 0;i--{
		if(hm[i] > max){
			max = hm[i]
		}
		res[i] = max;
	}
	return res
}
func min(a,b []int)  []int {
	res := make([]int,len(a))
	for i := 0; i < len(a);i++{
		if a[i] >= b[i]{
			res[i] = b[i]
		}else {
			res[i] = a[i]
		}
	}
	return res
}
func diff(hm, min []int) []int {
	res := make([]int,len(hm))
	for i := 0; i < len(hm);i++{
		if min[i] > hm[i]{
			res[i] = min[i] - hm[i]
		}
	}
	return res
}
func sum(a []int) int {
	res := 0
	for i := 0; i < len(a);i++{
		res += a[i]
	}
	return res
}

func waterCollected(hm []int) int {
	maxr := maxr(hm)
	maxl := maxl(hm)
	min := min(maxr,maxl)
	diff := diff(hm,min)
	sum := sum(diff)
	return sum
}


func main() {
	fmt.Println(waterCollected([]int{1, 5, 3, 7, 2}))
	fmt.Println(waterCollected([]int{5, 3, 7, 2, 6, 4, 5, 9, 1, 2}))
	fmt.Println(waterCollected([]int{2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1}))
	fmt.Println(waterCollected([]int{5, 5, 5, 5}))
	fmt.Println(waterCollected([]int{5, 6, 7, 8}))
	fmt.Println(waterCollected([]int{8, 7, 7, 6}))
	fmt.Println(waterCollected([]int{6, 7, 10, 7, 6}))
}</syntaxhighlight>

{{out}}
<pre>
2
14
35
0
0
0
0
</pre>

=={{header|Groovy}}==

<syntaxhighlight lang="groovy">
Integer waterBetweenTowers(List<Integer> towers) {
    // iterate over the vertical axis. There the amount of water each row can hold is 
    // the number of empty spots, minus the empty spots at the beginning and end
    return (1..towers.max()).collect { height -> 
        // create a string representing the row, '#' for tower material and ' ' for air
        // use .trim() to remove spaces at beginning and end and then count remaining spaces
        towers.collect({ it >= height ? "#" : " " }).join("").trim().count(" ")
    }.sum()
}

tasks = [
    [1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]
]

tasks.each {
    println "$it => total water: ${waterBetweenTowers it}"
}
</syntaxhighlight>

{{out}}
<pre>
[1, 5, 3, 7, 2] => total water: 2
[5, 3, 7, 2, 6, 4, 5, 9, 1, 2] => total water: 14
[2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1] => total water: 35
[5, 5, 5, 5] => total water: 0
[5, 6, 7, 8] => total water: 0
[8, 7, 7, 6] => total water: 0
[6, 7, 10, 7, 6] => total water: 0
</pre>

=={{header|Haskell}}==

Following the approach of slightly modified [http://stackoverflow.com/users/1416525/cdk cdk]'s Haskell solution at [http://stackoverflow.com/questions/24414700/amazon-water-collected-between-towers/ Stack Overflow]. As recommended in [http://h2.jaguarpaw.co.uk/posts/data-structures-matter/ Programming as if the Correct Data Structure (and Performance) Mattered] it uses [http://hackage.haskell.org/package/vector-0.12.0.1/docs/Data-Vector-Unboxed.html Vector] instead of Array:

<syntaxhighlight lang="haskell">import Data.Vector.Unboxed (Vector)
import qualified Data.Vector.Unboxed as V

waterCollected :: Vector Int -> Int
waterCollected =
  V.sum .            -- Sum of the water depths over each of
  V.filter (> 0) .   -- the columns that are covered by some water.
  (V.zipWith (-) =<< -- Where coverages are differences between:
   (V.zipWith min .  -- the lower water level in each case of:
    V.scanl1 max <*> -- highest wall to left, and
    V.scanr1 max))   -- highest wall to right.

main :: IO ()
main =
  mapM_
    (print . waterCollected . V.fromList)
    [ [1, 5, 3, 7, 2]
    , [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
    , [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
    , [5, 5, 5, 5]
    , [5, 6, 7, 8]
    , [8, 7, 7, 6]
    , [6, 7, 10, 7, 6]
    ]</syntaxhighlight>
{{Out}}
<pre>2
14
35
0
0
0
0</pre>


Or, using Data.List for simplicity - no need to prioritize performance here - and adding diagrams:

<syntaxhighlight lang="haskell">import Data.List (replicate, transpose)

-------------- WATER COLLECTED BETWEEN TOWERS ------------

towerPools :: [Int] -> [(Int, Int)]
towerPools =
  zipWith min . scanl1 max <*> scanr1 max
    >>= zipWith ((<*>) (,) . (-))


--------------------------- TEST -------------------------
main :: IO ()
main =
  mapM_
    (putStrLn . display . towerPools)
    [ [1, 5, 3, 7, 2],
      [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
      [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
      [5, 5, 5, 5],
      [5, 6, 7, 8],
      [8, 7, 7, 6],
      [6, 7, 10, 7, 6]
    ]

------------------------- DIAGRAMS -----------------------

display :: [(Int, Int)] -> String
display = (<>) . showTowers <*> (('\n' :) . showLegend)

showTowers :: [(Int, Int)] -> String
showTowers xs =
  let upper = maximum (fst <$> xs)
   in '\n' :
      ( unlines
          . transpose
          . fmap
            ( \(x, d) ->
                concat $
                  replicate (upper - (x + d)) " "
                    <> replicate d "x"
                    <> replicate x "█"
            )
      )
        xs

showLegend :: [(Int, Int)] -> String
showLegend =
  ((<>) . show . fmap fst)
    <*> ((" -> " <>) . show . foldr ((+) . snd) 0)</syntaxhighlight>
{{Out}}
<pre>
   █ 
   █ 
 █x█ 
 █x█ 
 ███ 
 ████
█████

[1,5,3,7,2] -> 2

       █  
       █  
  █xxxx█  
  █x█xx█  
█x█x█x██  
█x█x████  
███x████  
████████x█
██████████

[5,3,7,2,6,4,5,9,1,2] -> 14

     █          
     █xxxxxxx█  
 █xxx█xxxxxxx█  
 █x█x█xxxx█x██  
 █x█x█x█xx█x███ 
 ███x█x█xx█████ 
██████x████████ 
████████████████

[2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1] -> 35

████
████
████
████
████

[5,5,5,5] -> 0

   █
  ██
 ███
████
████
████
████
████

[5,6,7,8] -> 0

█   
███ 
████
████
████
████
████
████

[8,7,7,6] -> 0

  █  
  █  
  █  
 ███ 
█████
█████
█████
█████
█████
█████

[6,7,10,7,6] -> 0</pre>

=={{header|J}}==
Inspired by [[#Julia]].

'''Solution:'''
<syntaxhighlight lang="j">collectLevels =: >./\ <. >./\.                          NB. collect levels after filling
waterLevels=: collectLevels - ]                         NB. water levels for each tower
collectedWater=: +/@waterLevels                         NB. sum the units of water collected
printTowers =: ' ' , [: |.@|: '#~' #~ ] ,. waterLevels  NB. print a nice graph of towers and water</syntaxhighlight>

'''Examples:'''
<syntaxhighlight lang="j">   collectedWater 5 3 7 2 6 4 5 9 1 2
14
   printTowers 5 3 7 2 6 4 5 9 1 2

       #
       #
  #~~~~#
  #~#~~#
#~#~#~##
#~#~####
###~####
########~#
##########

NB. Test cases
   TestTowers =: <@".;._2 noun define
1 5 3 7 2
5 3 7 2 6 4 5 9 1 2
2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1
5 5 5 5
5 6 7 8
8 7 7 6
6 7 10 7 6
)
   TestResults =: 2 14 35 0 0 0 0
   TestResults -: collectedWater &> TestTowers  NB. check tests
1</syntaxhighlight>

=={{header|Java}}==
{{trans|D}}
<syntaxhighlight lang="java">public class WaterBetweenTowers {
    public static void main(String[] args) {
        int i = 1;
        int[][] tba = new int[][]{
            new int[]{1, 5, 3, 7, 2},
            new int[]{5, 3, 7, 2, 6, 4, 5, 9, 1, 2},
            new int[]{2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1},
            new int[]{5, 5, 5, 5},
            new int[]{5, 6, 7, 8},
            new int[]{8, 7, 7, 6},
            new int[]{6, 7, 10, 7, 6}
        };

        for (int[] tea : tba) {
            int rht, wu = 0, bof;
            do {
                for (rht = tea.length - 1; rht >= 0; rht--) {
                    if (tea[rht] > 0) {
                        break;
                    }
                }

                if (rht < 0) {
                    break;
                }

                bof = 0;
                for (int col = 0; col <= rht; col++) {
                    if (tea[col] > 0) {
                        tea[col]--;
                        bof += 1;
                    } else if (bof > 0) {
                        wu++;
                    }
                }
                if (bof < 2) {
                    break;
                }
            } while (true);

            System.out.printf("Block %d", i++);
            if (wu == 0) {
                System.out.print(" does not hold any");
            } else {
                System.out.printf(" holds %d", wu);
            }
            System.out.println(" water units.");
        }
    }
}</syntaxhighlight>
{{out}}
<pre>Block 1 holds 2 water units.
Block 2 holds 14 water units.
Block 3 holds 35 water units.
Block 4 does not hold any water units.
Block 5 does not hold any water units.
Block 6 does not hold any water units.
Block 7 does not hold any water units.
</pre>

=={{header|JavaScript}}==
===ES5===
{{Trans|Haskell}}
<syntaxhighlight lang="javascript">(function () {
    'use strict';

    // waterCollected :: [Int] -> Int
    var waterCollected = function (xs) {
        return sum(                   // water above each bar
            zipWith(function (a, b) {
                    return a - b;     // difference between water level and bar
                },
                zipWith(min,          // lower of two flanking walls
                    scanl1(max, xs),  // highest walls to left
                    scanr1(max, xs)   // highest walls to right
                ), 
                xs                    // tops of bars
            )
            .filter(function (x) {
                return x > 0;         // only bars with water above them
            })
        );
    };

    // GENERIC FUNCTIONS ----------------------------------------

    // zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
    var zipWith = function (f, xs, ys) {
        var ny = ys.length;
        return (xs.length <= ny ? xs : xs.slice(0, ny))
            .map(function (x, i) {
                return f(x, ys[i]);
            });
    };

    // scanl1 is a variant of scanl that has no starting value argument
    // scanl1 :: (a -> a -> a) -> [a] -> [a]
    var scanl1 = function (f, xs) {
        return xs.length > 0 ? scanl(f, xs[0], xs.slice(1)) : [];
    };

    // scanr1 is a variant of scanr that has no starting value argument
    // scanr1 :: (a -> a -> a) -> [a] -> [a]
    var scanr1 = function (f, xs) {
        return xs.length > 0 ? scanr(f, xs.slice(-1)[0], xs.slice(0, -1)) : [];
    };

    // scanl :: (b -> a -> b) -> b -> [a] -> [b]
    var scanl = function (f, startValue, xs) {
        var lst = [startValue];
        return xs.reduce(function (a, x) {
            var v = f(a, x);
            return lst.push(v), v;
        }, startValue), lst;
    };

    // scanr :: (b -> a -> b) -> b -> [a] -> [b]
    var scanr = function (f, startValue, xs) {
        var lst = [startValue];
        return xs.reduceRight(function (a, x) {
            var v = f(a, x);
            return lst.push(v), v;
        }, startValue), lst.reverse();
    };

    // sum :: (Num a) => [a] -> a
    var sum = function (xs) {
        return xs.reduce(function (a, x) {
            return a + x;
        }, 0);
    };

    // max :: Ord a => a -> a -> a
    var max = function (a, b) {
        return a > b ? a : b;
    };

    // min :: Ord a => a -> a -> a
    var min = function (a, b) {
        return b < a ? b : a;
    };

    // TEST ---------------------------------------------------
    return [
        [1, 5, 3, 7, 2],
        [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
        [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
        [5, 5, 5, 5],
        [5, 6, 7, 8],
        [8, 7, 7, 6],
        [6, 7, 10, 7, 6]
    ].map(waterCollected);

    //--> [2, 14, 35, 0, 0, 0, 0]
})();</syntaxhighlight>

{{Out}}
<syntaxhighlight lang="javascript">[2, 14, 35, 0, 0, 0, 0]</syntaxhighlight>

===ES6===
{{Trans|Haskell}}
<syntaxhighlight lang="javascript">(() => {
    "use strict";

    // --------- WATER COLLECTED BETWEEN TOWERS ----------

    // waterCollected :: [Int] -> Int
    const waterCollected = xs =>
        sum(filter(lt(0))(
            zipWith(subtract)(xs)(
                zipWith(min)(
                    scanl1(max)(xs)
                )(
                    scanr1(max)(xs)
                )
            )
        ));


    // ---------------------- TEST -----------------------
    const main = () => [
        [1, 5, 3, 7, 2],
        [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
        [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
        [5, 5, 5, 5],
        [5, 6, 7, 8],
        [8, 7, 7, 6],
        [6, 7, 10, 7, 6]
    ].map(waterCollected);


    // --------------------- GENERIC ---------------------

    // Tuple (,) :: a -> b -> (a, b)
    const Tuple = a =>
        b => ({
            type: "Tuple",
            "0": a,
            "1": b,
            length: 2
        });


    // filter :: (a -> Bool) -> [a] -> [a]
    const filter = p =>
        // The elements of xs which match
        // the predicate p.
        xs => [...xs].filter(p);


    // lt (<) :: Ord a => a -> a -> Bool
    const lt = a =>
        b => a < b;


    // max :: Ord a => a -> a -> a
    const max = a =>
        // b if its greater than a,
        // otherwise a.
        b => a > b ? a : b;


    // min :: Ord a => a -> a -> a
    const min = a =>
        b => b < a ? b : a;


    // scanl :: (b -> a -> b) -> b -> [a] -> [b]
    const scanl = f => startValue => xs =>
        xs.reduce((a, x) => {
            const v = f(a[0])(x);

            return Tuple(v)(a[1].concat(v));
        }, Tuple(startValue)([startValue]))[1];


    // scanl1 :: (a -> a -> a) -> [a] -> [a]
    const scanl1 = f =>
        // scanl1 is a variant of scanl that
        // has no starting value argument.
        xs => xs.length > 0 ? (
            scanl(f)(
                xs[0]
            )(xs.slice(1))
        ) : [];


    // scanr :: (a -> b -> b) -> b -> [a] -> [b]
    const scanr = f =>
        startValue => xs => xs.reduceRight(
            (a, x) => {
                const v = f(x)(a[0]);

                return Tuple(v)([v].concat(a[1]));
            }, Tuple(startValue)([startValue])
        )[1];


    // scanr1 :: (a -> a -> a) -> [a] -> [a]
    const scanr1 = f =>
        // scanr1 is a variant of scanr that has no
        // seed-value argument, and assumes that
        // xs is not empty.
        xs => xs.length > 0 ? (
            scanr(f)(
                xs.slice(-1)[0]
            )(xs.slice(0, -1))
        ) : [];


    // subtract :: Num -> Num -> Num
    const subtract = x =>
        y => y - x;


    // sum :: [Num] -> Num
    const sum = xs =>
        // The numeric sum of all values in xs.
        xs.reduce((a, x) => a + x, 0);


    // zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
    const zipWith = f =>
        // A list constructed by zipping with a
        // custom function, rather than with the
        // default tuple constructor.
        xs => ys => xs.map(
            (x, i) => f(x)(ys[i])
        ).slice(
            0, Math.min(xs.length, ys.length)
        );

    // MAIN ---
    return main();
})();</syntaxhighlight>
{{Out}}
<syntaxhighlight lang="javascript">[2, 14, 35, 0, 0, 0, 0]</syntaxhighlight>

=={{header|jq}}==
{{trans|Kotlin}}
{{works with|jq}}
'''Works with gojq, the Go implementation of jq'''

<syntaxhighlight lang="jq">def waterCollected:
  . as $tower
  | ($tower|length) as $n
  | ([0] + [range(1;$n) | ($tower[0:.]  | max) ]) as $highLeft
  | (      [range(1;$n) | ($tower[.:$n] | max) ] + [0]) as $highRight
  | [ range(0;$n) | [ ([$highLeft[.], $highRight[.] ]| min) - $tower[.], 0 ] | max]
  | add ;
 
def towers: [
    [1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]
];

towers[]
| "\(waterCollected) from \(.)"</syntaxhighlight>
{{out}}
As for [[#Kotlin]] and others.
=={{header|Julia}}==
Inspired to [[#Python]].

<syntaxhighlight lang="julia">using Printf

function watercollected(towers::Vector{Int})
    high_lft = vcat(0, accumulate(max, towers[1:end-1]))
    high_rgt = vcat(reverse(accumulate(max, towers[end:-1:2])), 0)
    waterlvl = max.(min.(high_lft, high_rgt) .- towers, 0)
    return waterlvl
end

function towerprint(towers, levels)
    ctowers = copy(towers)
    clevels = copy(levels)
    hmax = maximum(towers)
    ntow = length(towers)
    for h in hmax:-1:1
        @printf("%2i |", h)
        for j in 1:ntow
            if ctowers[j] + clevels[j] ≥ h
                if clevels[j] > 0
                    cell = "≈≈"
                    clevels[j] -= 1
                else
                    cell = "NN"
                    ctowers[j] -= 1
                end
            else
                cell = "  "
            end
            print(cell)
        end
        println("|")
    end


    println("   " * join(lpad(t, 2) for t in levels) * ": Water lvl")
    println("   " * join(lpad(t, 2) for t in towers) * ": Tower lvl")
end

for towers in [[1, 5, 3, 7, 2], [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5], [5, 6, 7, 8], [8, 7, 7, 6], [6, 7, 10, 7, 6]]
    towerprint(towers, watercollected(towers))
    println()
end</syntaxhighlight>

{{out}}
<pre> 7 |      NN  |
 6 |      NN  |
 5 |  NN≈≈NN  |
 4 |  NN≈≈NN  |
 3 |  NNNNNN  |
 2 |  NNNNNNNN|
 1 |NNNNNNNNNN|
    0 0 2 0 0: Water lvl
    1 5 3 7 2: Tower lvl

 9 |              NN    |
 8 |              NN    |
 7 |    NN≈≈≈≈≈≈≈≈NN    |
 6 |    NN≈≈NN≈≈≈≈NN    |
 5 |NN≈≈NN≈≈NN≈≈NNNN    |
 4 |NN≈≈NN≈≈NNNNNNNN    |
 3 |NNNNNN≈≈NNNNNNNN    |
 2 |NNNNNNNNNNNNNNNN≈≈NN|
 1 |NNNNNNNNNNNNNNNNNNNN|
    0 2 0 5 1 3 2 0 1 0: Water lvl
    5 3 7 2 6 4 5 9 1 2: Tower lvl

 8 |          NN                    |
 7 |          NN≈≈≈≈≈≈≈≈≈≈≈≈≈≈NN    |
 6 |  NN≈≈≈≈≈≈NN≈≈≈≈≈≈≈≈≈≈≈≈≈≈NN    |
 5 |  NN≈≈NN≈≈NN≈≈≈≈≈≈≈≈NN≈≈NNNN    |
 4 |  NN≈≈NN≈≈NN≈≈NN≈≈≈≈NN≈≈NNNNNN  |
 3 |  NNNNNN≈≈NN≈≈NN≈≈≈≈NNNNNNNNNN  |
 2 |NNNNNNNNNNNN≈≈NNNNNNNNNNNNNNNN  |
 1 |NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN|
    0 0 3 1 4 0 6 3 5 5 2 4 2 0 0 0: Water lvl
    2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1: Tower lvl

 5 |NNNNNNNN|
 4 |NNNNNNNN|
 3 |NNNNNNNN|
 2 |NNNNNNNN|
 1 |NNNNNNNN|
    0 0 0 0: Water lvl
    5 5 5 5: Tower lvl

 8 |      NN|
 7 |    NNNN|
 6 |  NNNNNN|
 5 |NNNNNNNN|
 4 |NNNNNNNN|
 3 |NNNNNNNN|
 2 |NNNNNNNN|
 1 |NNNNNNNN|
    0 0 0 0: Water lvl
    5 6 7 8: Tower lvl

 8 |NN      |
 7 |NNNNNN  |
 6 |NNNNNNNN|
 5 |NNNNNNNN|
 4 |NNNNNNNN|
 3 |NNNNNNNN|
 2 |NNNNNNNN|
 1 |NNNNNNNN|
    0 0 0 0: Water lvl
    8 7 7 6: Tower lvl

10 |    NN    |
 9 |    NN    |
 8 |    NN    |
 7 |  NNNNNN  |
 6 |NNNNNNNNNN|
 5 |NNNNNNNNNN|
 4 |NNNNNNNNNN|
 3 |NNNNNNNNNN|
 2 |NNNNNNNNNN|
 1 |NNNNNNNNNN|
    0 0 0 0 0: Water lvl
    6 710 7 6: Tower lvl
</pre>

=={{header|Kotlin}}==
{{trans|Python}}
<syntaxhighlight lang="scala">// version 1.1.2

fun waterCollected(tower: IntArray): Int {
    val n = tower.size
    val highLeft = listOf(0) + (1 until n).map { tower.slice(0 until it).max()!! }
    val highRight = (1 until n).map { tower.slice(it until n).max()!! } + 0
    return (0 until n).map { maxOf(minOf(highLeft[it], highRight[it]) - tower[it], 0) }.sum()
}

fun main(args: Array<String>) {
    val towers = listOf(
        intArrayOf(1, 5, 3, 7, 2),
        intArrayOf(5, 3, 7, 2, 6, 4, 5, 9, 1, 2),
        intArrayOf(2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1),
        intArrayOf(5, 5, 5, 5),
        intArrayOf(5, 6, 7, 8),
        intArrayOf(8, 7, 7, 6),
        intArrayOf(6, 7, 10, 7, 6)
    )
    for (tower in towers) { 
        println("${"%2d".format(waterCollected(tower))} from ${tower.contentToString()}")
    }
}</syntaxhighlight>

{{out}}
<pre>
 2 from [1, 5, 3, 7, 2]
14 from [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
35 from [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
 0 from [5, 5, 5, 5]
 0 from [5, 6, 7, 8]
 0 from [8, 7, 7, 6]
 0 from [6, 7, 10, 7, 6]
</pre>

=={{header|Lua}}==
{{trans|C#}}
<syntaxhighlight lang="lua">function waterCollected(i,tower)
    local length = 0
    for _ in pairs(tower) do
        length = length + 1
    end

    local wu = 0
    repeat
        local rht = length - 1
        while rht >= 0 do
            if tower[rht + 1] > 0 then
                break
            end
            rht = rht - 1
        end
        if rht < 0 then
            break
        end

        local bof = 0
        local col = 0
        while col <= rht do
            if tower[col + 1] > 0 then
                tower[col + 1] = tower[col + 1] - 1
                bof = bof + 1
            elseif bof > 0 then
                wu = wu + 1
            end
            col = col + 1
        end
        if bof < 2 then
            break
        end
    until false
    if wu == 0 then
        print(string.format("Block %d does not hold any water.", i))
    else
        print(string.format("Block %d holds %d water units.", i, wu))
    end
end

function main()
    local towers = {
        {1, 5, 3, 7, 2},
        {5, 3, 7, 2, 6, 4, 5, 9, 1, 2},
        {2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1},
        {5, 5, 5, 5},
        {5, 6, 7, 8},
        {8, 7, 7, 6},
        {6, 7, 10, 7, 6}
    }

    for i,tbl in pairs(towers) do
        waterCollected(i,tbl)
    end
end

main()</syntaxhighlight>
{{out}}
<pre>Block 1 holds 2 water units.
Block 2 holds 14 water units.
Block 3 holds 35 water units.
Block 4 does not hold any water.
Block 5 does not hold any water.
Block 6 does not hold any water.
Block 7 does not hold any water.</pre>

=={{header|M2000 Interpreter}}==
===Scan min-max for each bar===
<syntaxhighlight lang="m2000 interpreter">
Module Water {
      Flush ' empty stack
      Data (1, 5, 3, 7, 2)
      Data (5, 3, 7, 2, 6, 4, 5, 9, 1, 2)
      Data (2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1)
      Data (5, 5, 5, 5), (5, 6, 7, 8),(8, 7, 7, 6)
      Data (6, 7, 10, 7, 6)
      bars=stack.size  ' mark stack frame
      Dim bar()
      for bar=1 to bars 
            bar()=Array  ' pop an array from stack
            acc=0
            For i=1 to len(bar())-2
                  level1=bar(i)
                  level2=level1
                  m=each(bar(), i+1, 1)
                  while m
                        if array(m)>level1 then level1=array(m)
                  End While
                  n=each(bar(), i+1, -1)
                  while n
                        if array(n)>level2 then level2=array(n)
                  End While
                  acc+=max.data(min(level1, level2)-bar(i), 0)
            Next i
            Data acc  ' push to end value
      Next bar
      finalwater=[]   ' is a stack object
      Print finalwater
}
Water
</syntaxhighlight>
===Drain method===
Module Water2 {
      Flush ' empty stack
      Data (1, 5, 3, 7, 2)
      Data (5, 3, 7, 2, 6, 4, 5, 9, 1, 2)
      Data (2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1)
      Data (5, 5, 5, 5), (5, 6, 7, 8),(8, 7, 7, 6)
      Data (6, 7, 10, 7, 6)
      bars=stack.size  ' mark stack frame
      Dim bar()
      For bar=1 to bars 
            bar()=Array  ' pop an array from stack
            acc=0
            range=bar()#max()-bar()#min()
            if range>0 then 
                  dim water(len(bar()))=bar()#max()
                  water(0)=bar(0)
                  water(len(bar())-1)=bar(len(bar())-1)
                  For j=1 to range-1
                        For i=1 to len(bar())-2
                              if water(i)>bar(i) then if water(i-1)<water(i) Then water(i)-- 
                        Next i
                        For i=len(bar())-2 to 1
                              if water(i)>bar(i) then if water(i+1)<water(i) Then water(i)--
                        Next i
                  Next j
                  Data water()#sum()-bar()#sum()
            Else
                  Data 0
            End if
      Next bar
      finalwater=[]
      Print finalwater
}
Water2
<syntaxhighlight lang="m2000 interpreter">
</syntaxhighlight>
===Faster Method===
{{trans|AWK}}
<syntaxhighlight lang="m2000 interpreter">
Module Water3 {
      Flush ' empty stack
      Data (1, 5, 3, 7, 2)
      Data (5, 3, 7, 2, 6, 4, 5, 9, 1, 2)
      Data (2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1)
      Data (5, 5, 5, 5), (5, 6, 7, 8),(8, 7, 7, 6)
      Data (6, 7, 10, 7, 6)
      bars=stack.size  ' mark stack frame
      Dim bar()
      for bar=1 to bars 
            bar()=Array  ' pop an array from stack
            acc=0
            n=len(bar())-1
            dim hl(n+1), hr(n+1)
            For i=n to 0
                  hr(i)=max.data(bar(i), if(i<n->hr(i+1), 0))
            Next i
            For i=0 to n
                  hl(i)=max.data(bar(i), if(i>0->hl(i-1), 0))
                  acc+=min.data(hl(i), hr(i))-bar(i)
            Next i
            Data acc  ' push to end value
      Next bar
      finalwater=[]   ' is a stack object
      Print finalwater
}
Water3
</syntaxhighlight>

{{out}}
<pre>
 2 14 35 0 0 0 0
</pre >

=={{header|Mathematica}} / {{header|Wolfram Language}}==
<syntaxhighlight lang="mathematica">ClearAll[waterbetween]
waterbetween[h_List] := Module[{mi, ma, ch},
  {mi, ma} = MinMax[h];
  Sum[
   ch = h - i;
   Count[
    Flatten@
     Position[
      ch, _?Negative], _?(Between[
       MinMax[Position[ch, _?NonNegative]]])]
   ,
   {i, mi + 1, ma}
   ]
  ]
h = {{1, 5, 3, 7, 2}, {5, 3, 7, 2, 6, 4, 5, 9, 1, 2}, {2, 6, 3, 5, 2, 
    8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1}, {5, 5, 5, 5}, {5, 6, 7, 8}, {8, 
    7, 7, 6}, {6, 7, 10, 7, 6}};
waterbetween /@ h</syntaxhighlight>
{{out}}
<pre>{2, 14, 35, 0, 0, 0, 0}</pre>

=={{header|Nim}}==
<syntaxhighlight lang="nim">import math, sequtils, sugar

proc water(barChart: seq[int], isLeftPeak = false, isRightPeak = false): int =
  if len(barChart) <= 2:
    return
  if isLeftPeak and isRightPeak:
    return sum(barChart[1..^2].map(x=>min(barChart[0], barChart[^1])-x))
  var i: int
  if isLeftPeak:
    i = maxIndex(barChart[1..^1])+1
  else:
    i = maxIndex(barChart[0..^2])
  return water(barChart[0..i], isLeftPeak, true)+water(barChart[i..^1], true, isRightPeak)

const barCharts = [
    @[1, 5, 3, 7, 2],
    @[5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    @[2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    @[5, 5, 5, 5],
    @[5, 6, 7, 8],
    @[8, 7, 7, 6],
    @[6, 7, 10, 7, 6]]
const waterUnits = barCharts.map(chart=>water(chart, false, false))
echo(waterUnits)
</syntaxhighlight>
{{out}}
<pre>
 @[2, 14, 35, 0, 0, 0, 0]
</pre >

=={{header|Pascal}}==
{{works with|Delphi|7}}
{{works with|Free Pascal}}
<syntaxhighlight lang="pascal">
program RainInFlatland;

{$IFDEF FPC} // Free Pascal
  {$MODE Delphi}
{$ELSE}      // Delphi
  {$APPTYPE CONSOLE}
{$ENDIF}

uses SysUtils;
type THeight = integer;
// Heights could be f.p., but some changes to the code would be needed:
// (1) the inc function isn't available for f.p. values,
// (2) the print-out would need extra formatting.

{------------------------------------------------------------------------------
Find highest tower; if there are 2 or more equal highest, choose any.
Then fill troughs so that on going towards the highest tower, from the
  left-hand or right-hand end, there are no steps down.
Amount of filling required equals amount of water collected.
}
function FillTroughs( const h : array of THeight) : THeight;
var
  m, i, i_max : integer;
  h_max : THeight;
begin
  result := 0;
  m := High( h); // highest index, 0-based; there are m + 1 towers
  if (m <= 1) then exit; // result = 0 if <= 2 towers

  // Find highest tower and its index in the array.
  h_max := h[0];
  i_max := 0;
  for i := 1 to m do begin
    if h[i] > h_max then begin
      h_max := h[i];
      i_max := i;
    end;
  end;
  // Fill troughs from left-hand end to highest tower
  h_max := h[0];
  for i := 1 to i_max - 1 do begin
    if h[i] < h_max then inc( result, h_max - h[i])
                    else h_max := h[i];
  end;
  // Fill troughs from right-hand end to highest tower
  h_max := h[m];
  for i := m - 1 downto i_max + 1 do begin
    if h[i] < h_max then inc( result, h_max - h[i])
                    else h_max := h[i];
  end;
end;

{-------------------------------------------------------------------------
Wrapper for the above: finds amount of water, and prints input and result.
}
procedure CalcAndPrint( h : array of THeight);
var
  water : THeight;
  j : integer;
begin
  water := FillTroughs( h);
  Write( water:5, ' <-- [');
  for j := 0 to High( h) do begin
    Write( h[j]);
    if j < High(h) then Write(', ') else WriteLn(']');
  end;
end;

{---------------------------------------------------------------------------
Main routine.
}
begin
  CalcAndPrint([1,5,3,7,2]);
  CalcAndPrint([5,3,7,2,6,4,5,9,1,2]);
  CalcAndPrint([2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1]);
  CalcAndPrint([5,5,5,5]);
  CalcAndPrint([5,6,7,8]);
  CalcAndPrint([8,7,7,6]);
  CalcAndPrint([6,7,10,7,6]);
end.
</syntaxhighlight>
{{out}}
<pre>
    2 <-- [1, 5, 3, 7, 2]
   14 <-- [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
   35 <-- [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
    0 <-- [5, 5, 5, 5]
    0 <-- [5, 6, 7, 8]
    0 <-- [8, 7, 7, 6]
    0 <-- [6, 7, 10, 7, 6]
</pre>

=={{header|Perl}}==
<syntaxhighlight lang="perl">use Modern::Perl;
use List::Util qw{ min max sum };

sub water_collected {
    my @t = map { { TOWER => $_, LEFT => 0, RIGHT => 0, LEVEL => 0 } } @_;

    my ( $l, $r ) = ( 0, 0 );
    $_->{LEFT}  = ( $l = max( $l, $_->{TOWER} ) ) for @t;
    $_->{RIGHT} = ( $r = max( $r, $_->{TOWER} ) ) for reverse @t;
    $_->{LEVEL} = min( $_->{LEFT}, $_->{RIGHT} )  for @t;

    return sum map { $_->{LEVEL} > 0 ? $_->{LEVEL} - $_->{TOWER} : 0 } @t;
}

say join ' ', map { water_collected( @{$_} ) } (
    [ 1, 5,  3, 7, 2 ],
    [ 5, 3,  7, 2, 6, 4, 5, 9, 1, 2 ],
    [ 2, 6,  3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 ],
    [ 5, 5,  5, 5 ],
    [ 5, 6,  7, 8 ],
    [ 8, 7,  7, 6 ],
    [ 6, 7, 10, 7, 6 ],
);</syntaxhighlight>
{{Out}}
<pre>2 14 35 0 0 0 0</pre>

=={{header|Phix}}==
=== inefficient one-pass method ===
<!--<syntaxhighlight lang="phix">(phixonline)-->
 <span style="color: #008080;">with</span> <span style="color: #008080;">javascript_semantics</span>
 <span style="color: #008080;">function</span> <span style="color: #000000;">collect_water</span><span style="color: #0000FF;">(</span><span style="color: #004080;">sequence</span> <span style="color: #000000;">heights</span><span style="color: #0000FF;">)</span>
     <span style="color: #004080;">integer</span> <span style="color: #000000;">res</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">0</span>
     <span style="color: #008080;">for</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">=</span><span style="color: #000000;">2</span> <span style="color: #008080;">to</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">)-</span><span style="color: #000000;">1</span> <span style="color: #008080;">do</span>
         <span style="color: #004080;">integer</span> <span style="color: #000000;">lm</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">1</span><span style="color: #0000FF;">..</span><span style="color: #000000;">i</span><span style="color: #0000FF;">-</span><span style="color: #000000;">1</span><span style="color: #0000FF;">]),</span>
                 <span style="color: #000000;">rm</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">+</span><span style="color: #000000;">1</span><span style="color: #0000FF;">..$]),</span>
                 <span style="color: #000000;">d</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">min</span><span style="color: #0000FF;">(</span><span style="color: #000000;">lm</span><span style="color: #0000FF;">,</span><span style="color: #000000;">rm</span><span style="color: #0000FF;">)-</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span>
         <span style="color: #000000;">res</span> <span style="color: #0000FF;">+=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">0</span><span style="color: #0000FF;">,</span><span style="color: #000000;">d</span><span style="color: #0000FF;">)</span>
     <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
     <span style="color: #008080;">return</span> <span style="color: #000000;">res</span>
 <span style="color: #008080;">end</span> <span style="color: #008080;">function</span>
  
 <span style="color: #008080;">constant</span> <span style="color: #000000;">tests</span> <span style="color: #0000FF;">=</span> <span style="color: #0000FF;">{{</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">3</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">},</span>
                   <span style="color: #0000FF;">{</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">3</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">6</span><span style="color: #0000FF;">,</span><span style="color: #000000;">4</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">9</span><span style="color: #0000FF;">,</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">},</span>
                   <span style="color: #0000FF;">{</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">6</span><span style="color: #0000FF;">,</span><span style="color: #000000;">3</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">8</span><span style="color: #0000FF;">,</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #000000;">4</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">3</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">4</span><span style="color: #0000FF;">,</span><span style="color: #000000;">1</span><span style="color: #0000FF;">},</span>
                   <span style="color: #0000FF;">{</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">},</span>
                   <span style="color: #0000FF;">{</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">6</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">8</span><span style="color: #0000FF;">},</span>
                   <span style="color: #0000FF;">{</span><span style="color: #000000;">8</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">6</span><span style="color: #0000FF;">},</span>
                   <span style="color: #0000FF;">{</span><span style="color: #000000;">6</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">10</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">6</span><span style="color: #0000FF;">}}</span>
  
 <span style="color: #008080;">for</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">=</span><span style="color: #000000;">1</span> <span style="color: #008080;">to</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">tests</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">do</span>
     <span style="color: #004080;">sequence</span> <span style="color: #000000;">ti</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">tests</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span>
     <span style="color: #7060A8;">printf</span><span style="color: #0000FF;">(</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"%35s : %d\n"</span><span style="color: #0000FF;">,{</span><span style="color: #7060A8;">sprint</span><span style="color: #0000FF;">(</span><span style="color: #000000;">ti</span><span style="color: #0000FF;">),</span><span style="color: #000000;">collect_water</span><span style="color: #0000FF;">(</span><span style="color: #000000;">ti</span><span style="color: #0000FF;">)})</span>
 <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
<!--</syntaxhighlight>-->
{{out}}
<pre>
                        {1,5,3,7,2} : 2
              {5,3,7,2,6,4,5,9,1,2} : 14
  {2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1} : 35
                          {5,5,5,5} : 0
                          {5,6,7,8} : 0
                          {8,7,7,6} : 0
                       {6,7,10,7,6} : 0
</pre>
=== more efficient two-pass version ===
<!--<syntaxhighlight lang="phix">(phixonline)-->
 <span style="color: #008080;">with</span> <span style="color: #008080;">javascript_semantics</span>
 <span style="color: #008080;">function</span> <span style="color: #000000;">collect_water</span><span style="color: #0000FF;">(</span><span style="color: #004080;">sequence</span> <span style="color: #000000;">heights</span><span style="color: #0000FF;">)</span>
 
     <span style="color: #004080;">integer</span> <span style="color: #000000;">left_max</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">1</span><span style="color: #0000FF;">],</span>
             <span style="color: #000000;">right_max</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">heights</span><span style="color: #0000FF;">[$]</span>
     <span style="color: #004080;">sequence</span> <span style="color: #000000;">left_height</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">deep_copy</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">),</span>
              <span style="color: #000000;">right_height</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">deep_copy</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">)</span>
 
     <span style="color: #008080;">for</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">=</span><span style="color: #000000;">2</span> <span style="color: #008080;">to</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">)-</span><span style="color: #000000;">1</span> <span style="color: #008080;">do</span>
         <span style="color: #000000;">left_max</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">],</span><span style="color: #000000;">left_max</span><span style="color: #0000FF;">)</span>
         <span style="color: #000000;">left_height</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">left_max</span>
         <span style="color: #000000;">right_max</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[-</span><span style="color: #000000;">i</span><span style="color: #0000FF;">],</span><span style="color: #000000;">right_max</span><span style="color: #0000FF;">)</span>
         <span style="color: #000000;">right_height</span><span style="color: #0000FF;">[-</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">right_max</span>
     <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
 
     <span style="color: #004080;">sequence</span> <span style="color: #000000;">mins</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">sq_min</span><span style="color: #0000FF;">(</span><span style="color: #000000;">left_height</span><span style="color: #0000FF;">,</span><span style="color: #000000;">right_height</span><span style="color: #0000FF;">),</span>
              <span style="color: #000000;">diffs</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">sq_sub</span><span style="color: #0000FF;">(</span><span style="color: #000000;">mins</span><span style="color: #0000FF;">,</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">)</span>            
 
     <span style="color: #008080;">return</span> <span style="color: #7060A8;">sum</span><span style="color: #0000FF;">(</span><span style="color: #000000;">diffs</span><span style="color: #0000FF;">)</span>
 <span style="color: #008080;">end</span> <span style="color: #008080;">function</span>
<!--</syntaxhighlight>-->
(same output)

=== pretty print routine ===
<!--<syntaxhighlight lang="phix">(phixonline)-->
 <span style="color: #008080;">with</span> <span style="color: #008080;">javascript_semantics</span>
 <span style="color: #7060A8;">requires</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"1.0.2"</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- (bugfix in p2js.js/$sidii(), 20/4/22)</span>
 <span style="color: #008080;">procedure</span> <span style="color: #000000;">print_water</span><span style="color: #0000FF;">(</span><span style="color: #004080;">sequence</span> <span style="color: #000000;">heights</span><span style="color: #0000FF;">)</span>
     <span style="color: #004080;">integer</span> <span style="color: #000000;">res</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">0</span><span style="color: #0000FF;">,</span> <span style="color: #000000;">l</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">)</span>
     <span style="color: #004080;">sequence</span> <span style="color: #000000;">towers</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">repeat</span><span style="color: #0000FF;">(</span><span style="color: #7060A8;">repeat</span><span style="color: #0000FF;">(</span><span style="color: #008000;">' '</span><span style="color: #0000FF;">,</span><span style="color: #000000;">l</span><span style="color: #0000FF;">),</span><span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">))</span>
     <span style="color: #008080;">for</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">=</span><span style="color: #000000;">1</span> <span style="color: #008080;">to</span> <span style="color: #000000;">l</span> <span style="color: #008080;">do</span>
         <span style="color: #008080;">for</span> <span style="color: #000000;">j</span><span style="color: #0000FF;">=</span><span style="color: #000000;">1</span> <span style="color: #008080;">to</span> <span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span> <span style="color: #008080;">do</span>
             <span style="color: #000000;">towers</span><span style="color: #0000FF;">[-</span><span style="color: #000000;">j</span><span style="color: #0000FF;">][</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #008000;">'#'</span>
         <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
         <span style="color: #008080;">if</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">></span><span style="color: #000000;">1</span> <span style="color: #008080;">and</span> <span style="color: #000000;">i</span><span style="color: #0000FF;"><</span><span style="color: #000000;">l</span> <span style="color: #008080;">then</span>
             <span style="color: #004080;">integer</span> <span style="color: #000000;">lm</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">1</span><span style="color: #0000FF;">..</span><span style="color: #000000;">i</span><span style="color: #0000FF;">-</span><span style="color: #000000;">1</span><span style="color: #0000FF;">]),</span>
                     <span style="color: #000000;">rm</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">max</span><span style="color: #0000FF;">(</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">+</span><span style="color: #000000;">1</span><span style="color: #0000FF;">..$]),</span>
                     <span style="color: #000000;">m</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">min</span><span style="color: #0000FF;">(</span><span style="color: #000000;">lm</span><span style="color: #0000FF;">,</span><span style="color: #000000;">rm</span><span style="color: #0000FF;">)</span>
             <span style="color: #008080;">for</span> <span style="color: #000000;">j</span><span style="color: #0000FF;">=</span><span style="color: #000000;">heights</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]+</span><span style="color: #000000;">1</span> <span style="color: #008080;">to</span> <span style="color: #000000;">m</span> <span style="color: #008080;">do</span>
                 <span style="color: #000000;">towers</span><span style="color: #0000FF;">[-</span><span style="color: #000000;">j</span><span style="color: #0000FF;">][</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #008000;">'~'</span>
                 <span style="color: #000000;">res</span> <span style="color: #0000FF;">+=</span> <span style="color: #000000;">1</span>
             <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
         <span style="color: #008080;">end</span> <span style="color: #008080;">if</span>
     <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
     <span style="color: #7060A8;">printf</span><span style="color: #0000FF;">(</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"%s\ncollected:%d\n"</span><span style="color: #0000FF;">,{</span><span style="color: #7060A8;">join</span><span style="color: #0000FF;">(</span><span style="color: #000000;">towers</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"\n"</span><span style="color: #0000FF;">),</span><span style="color: #000000;">res</span><span style="color: #0000FF;">})</span>
 <span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
 
 <span style="color: #000000;">print_water</span><span style="color: #0000FF;">({</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">3</span><span style="color: #0000FF;">,</span><span style="color: #000000;">7</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">6</span><span style="color: #0000FF;">,</span><span style="color: #000000;">4</span><span style="color: #0000FF;">,</span><span style="color: #000000;">5</span><span style="color: #0000FF;">,</span><span style="color: #000000;">9</span><span style="color: #0000FF;">,</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">})</span>
<!--</syntaxhighlight>-->
{{out}}
<pre>
       #
       #
  #~~~~#
  #~#~~#
#~#~#~##
#~#~####
###~####
########~#
##########
collected:14
</pre>

=={{header|Phixmonti}}==
{{trans|Phix}}
<syntaxhighlight lang="phixmonti">include ..\Utilitys.pmt

def collect_water
    0 var res
    len 1 - 2 swap 2 tolist
    for
        var i
        1 i 1 - slice max >ps
        len i - 1 + i swap slice max >ps
        i get ps> ps> min swap - 
        0 max res + var res
    endfor
    drop
    res
enddef

( ( 1 5 3 7 2 ) 
  ( 5 3 7 2 6 4 5 9 1 2 ) 
  ( 2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1 ) 
  ( 5 5 5 5 ) 
  ( 5 6 7 8 ) 
  ( 8 7 7 6 ) 
  ( 6 7 10 7 6 ) )

len for
    get dup print " : " print collect_water ?
endfor</syntaxhighlight>
{{out}}
<pre>[1, 5, 3, 7, 2] : 2
[5, 3, 7, 2, 6, 4, 5, 9, 1, 2] : 14
[2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1] : 35
[5, 5, 5, 5] : 0
[5, 6, 7, 8] : 0
[8, 7, 7, 6] : 0
[6, 7, 10, 7, 6] : 0

=== Press any key to exit ===</pre>

=={{header|PicoLisp}}==
<syntaxhighlight lang="picolisp">(de water (Lst)
   (sum
      '((A)
         (cnt
            nT
            (clip (mapcar '((B) (>= B A)) Lst)) ) )
      (range 1 (apply max Lst)) ) )
(println
   (mapcar
      water
      (quote
         (1 5 3 7 2)
         (5 3 7 2 6 4 5 9 1 2)
         (2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1)
         (5 5 5 5)
         (5 6 7 8)
         (8 7 7 6)
         (6 7 10 7 6) ) ) )</syntaxhighlight>
{{out}}
<pre>(2 14 35 0 0 0 0)</pre>

=={{header|Python}}==
Based on the algorithm explained at [http://stackoverflow.com/questions/24414700/amazon-water-collected-between-towers/32135773#32135773 Stack Overflow]:

<syntaxhighlight lang="python">def water_collected(tower):
    N = len(tower)
    highest_left = [0] + [max(tower[:n]) for n in range(1,N)]
    highest_right = [max(tower[n:N]) for n in range(1,N)] + [0]
    water_level = [max(min(highest_left[n], highest_right[n]) - tower[n], 0)
        for n in range(N)]
    print("highest_left:  ", highest_left)
    print("highest_right: ", highest_right)
    print("water_level:   ", water_level)
    print("tower_level:   ", tower)
    print("total_water:   ", sum(water_level))
    print("")
    return sum(water_level)

towers = [[1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]]

[water_collected(tower) for tower in towers]</syntaxhighlight>
{{Out}}
<pre>
highest_left:   [0, 1, 5, 5, 7]
highest_right:  [7, 7, 7, 2, 0]
water_level:    [0, 0, 2, 0, 0]
tower_level:    [1, 5, 3, 7, 2]
total_water:    2

highest_left:   [0, 5, 5, 7, 7, 7, 7, 7, 9, 9]
highest_right:  [9, 9, 9, 9, 9, 9, 9, 2, 2, 0]
water_level:    [0, 2, 0, 5, 1, 3, 2, 0, 1, 0]
tower_level:    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
total_water:    14

highest_left:   [0, 2, 6, 6, 6, 6, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8]
highest_right:  [8, 8, 8, 8, 8, 7, 7, 7, 7, 7, 7, 7, 7, 4, 1, 0]
water_level:    [0, 0, 3, 1, 4, 0, 6, 3, 5, 5, 2, 4, 2, 0, 0, 0]
tower_level:    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
total_water:    35

highest_left:   [0, 5, 5, 5]
highest_right:  [5, 5, 5, 0]
water_level:    [0, 0, 0, 0]
tower_level:    [5, 5, 5, 5]
total_water:    0

highest_left:   [0, 5, 6, 7]
highest_right:  [8, 8, 8, 0]
water_level:    [0, 0, 0, 0]
tower_level:    [5, 6, 7, 8]
total_water:    0

highest_left:   [0, 8, 8, 8]
highest_right:  [7, 7, 6, 0]
water_level:    [0, 0, 0, 0]
tower_level:    [8, 7, 7, 6]
total_water:    0

highest_left:   [0, 6, 7, 10, 10]
highest_right:  [10, 10, 7, 6, 0]
water_level:    [0, 0, 0, 0, 0]
tower_level:    [6, 7, 10, 7, 6]
total_water:    0

[2, 14, 35, 0, 0, 0, 0]</pre>


Or, expressed in terms of '''itertools.accumulate''', and showing diagrams:

<syntaxhighlight lang="python">'''Water collected between towers'''

from itertools import accumulate
from functools import reduce
from operator import add


# ---------------------- TOWER POOLS -----------------------

# towerPools :: [Int] -> [(Int, Int)]
def towerPools(towers):
    '''Tower heights with water depths.
    '''
    def towerAndWater(level, tower):
        return tower, level - tower

    waterlevels = map(
        min,
        accumulate(towers, max),
        reversed(list(
            accumulate(reversed(towers), max)
        )),
    )
    return list(map(towerAndWater, waterlevels, towers))


# ------------------------ DIAGRAMS ------------------------

# showTowers :: [(Int, Int)] -> String
def showTowers(xs):
    '''Diagrammatic representation.
    '''
    upper = max(xs, key=fst)[0]

    def row(xd):
        return ' ' * (upper - add(*xd)) + (
            snd(xd) * 'x' + '██' * fst(xd)
        )
    return unlines([
        ''.join(x) for x in zip(*map(row, xs))
    ])


# showLegend :: (Int, Int)] -> String
def showLegend(xs):
    '''String display of tower heights and
       total sum of trapped water units.
    '''
    towers, depths = zip(*xs)
    return showList(towers) + (
        ' -> ' + str(sum(depths))
    )


# -------------------------- TEST --------------------------
# main :: IO ()
def main():
    '''Water collected in various flooded bar charts.'''
    def diagram(xs):
        return showTowers(xs) + '\n\n' + (
            showLegend(xs) + '\n\n'
        )

    print(unlines(
        map(compose(diagram, towerPools), [
            [1, 5, 3, 7, 2],
            [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
            [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
            [5, 5, 5, 5],
            [5, 6, 7, 8],
            [8, 7, 7, 6],
            [6, 7, 10, 7, 6]
        ])
    ))


# ------------------------ GENERIC -------------------------

# compose :: ((a -> a), ...) -> (a -> a)
def compose(*fs):
    '''Composition, from right to left,
       of a series of functions.
    '''
    def go(f, g):
        return lambda x: f(g(x))
    return reduce(go, fs, lambda x: x)


# fst :: (a, b) -> a
def fst(tpl):
    '''First member of a pair.'''
    return tpl[0]


# showList :: [a] -> String
def showList(xs):
    '''Stringification of a list.'''
    return '[' + ','.join(str(x) for x in xs) + ']'


# snd :: (a, b) -> b
def snd(tpl):
    '''Second member of a pair.'''
    return tpl[1]


# unlines :: [String] -> String
def unlines(xs):
    '''A single string formed by the intercalation
       of a list of strings with the newline character.
    '''
    return '\n'.join(xs)


# MAIN ---
if __name__ == '__main__':
    main()
</syntaxhighlight>
{{Out}}
<pre>   █ 
   █ 
 █x█ 
 █x█ 
 ███ 
 ████
█████
█████

[1,5,3,7,2] -> 2


       █  
       █  
  █xxxx█  
  █x█xx█  
█x█x█x██  
█x█x████  
███x████  
████████x█
██████████
██████████

[5,3,7,2,6,4,5,9,1,2] -> 14


     █          
     █xxxxxxx█  
 █xxx█xxxxxxx█  
 █x█x█xxxx█x██  
 █x█x█x█xx█x███ 
 ███x█x█xx█████ 
██████x████████ 
████████████████
████████████████

[2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1] -> 35


████
████
████
████
████
████
████
████
████
████

[5,5,5,5] -> 0


   █
  ██
 ███
████
████
████
████
████
████
████
████
████
████

[5,6,7,8] -> 0


█   
███ 
████
████
████
████
████
████
████
████
████
████
████
████

[8,7,7,6] -> 0


  █  
  █  
  █  
 ███ 
█████
█████
█████
█████
█████
█████
█████
█████
█████
█████
█████
█████

[6,7,10,7,6] -> 0</pre>

=={{header|Quackery}}==

<syntaxhighlight lang="Quackery">  [ $ "turtleduck.qky" loadfile ] now!

  [ dup 0 = iff drop done
    dup 2 times
      [ 20 * 1 walk
        1 4 turn
        20 1 walk
        1 4 turn ] ]        is bar      (   [ -->   )

  [ tuck size unrot
    -1 4 turn
    witheach
      [ dup
        ' [ 158 151 147 ]
        dup colour
        fill bar
        dup 20 * 1 fly
        dip
          [ behead
           ' [ 162 197 208 ]
           dup colour
           fill bar ]
        -20 * 1 fly
        1 4 turn
        20 1 fly
        -1 4 turn ]
      drop
      1 4 turn
      -20 * 1 fly ]         is chart    ( [ [ -->   )

  [ [] 0 rot witheach
      [ max dup dip join ]
    drop ]                  is rightmax (   [ --> [ )

  [ reverse
    rightmax
    reverse ]               is leftmax  (   [ --> [ )

  [ [] unrot
    witheach
      [ over i^ peek
        min swap dip join ]
    drop  ]                 is mins     (   [ --> [ )

  [ [] unrot
    witheach
      [ over i^ peek
        - swap dip join ]
    drop  ]                 is diffs    (   [ --> [ )

  [ 0 swap witheach + ]     is sum      (   [ --> n )

  [ dup 2dup rightmax
    swap leftmax
    mins diffs chart ]      is task1    (   [ -->   )

  [ dup dup rightmax
    swap leftmax
    mins diffs sum ]        is task2    (   [ -->   )

  turtle
  10 frames
  -540 1 fly

    ' [ [ 1 5 3 7 2 ]
      [ 5 3 7 2 6 4 5 9 1 2 ]
      [ 2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1 ]
      [ 5 5 5 5 ]
      [ 5 6 7 8 ]
      [ 8 7 7 6 ]
      [ 6 7 10 7 6 ] ]
  dup
  witheach
      [ dup size swap
        task1
        1+ 20 * 1 fly ] 
  witheach 
    [ task2 echo sp ]
  1 frames</syntaxhighlight>

{{out}}

I see from the discussion page that drawing the towers wasn't part of the task. Here they are anyway.

"What is the use of a book," thought Alice, "without pictures or conversations?"

[[File:Quackery - Water collected between towers.png|thumb|center]]

<pre>2 14 35 0 0 0 0</pre>

=={{header|Racket}}==
<syntaxhighlight lang="racket">#lang racket/base
(require racket/match)

(define (water-collected-between-towers towers)
  (define (build-tallest-left/rev-list t mx/l rv)
    (match t
      [(list) rv]
      [(cons a d)
       (define new-mx/l (max a mx/l))
       (build-tallest-left/rev-list d new-mx/l (cons mx/l rv))]))

  (define (collect-from-right t tallest/l mx/r rv)
    (match t
      [(list) rv]
      [(cons a d)
       (define new-mx/r (max a mx/r))
       (define new-rv (+ rv (max (- (min new-mx/r (car tallest/l)) a) 0)))
       (collect-from-right d (cdr tallest/l) new-mx/r new-rv)]))

  (define reversed-left-list (build-tallest-left/rev-list towers 0 null))  
  (collect-from-right (reverse towers) reversed-left-list 0 0))

(module+ test
  (require rackunit)
  (check-equal?
   (let ((towerss
          '[[1 5 3 7 2]
            [5 3 7 2 6 4 5 9 1 2]
            [2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1]
            [5 5 5 5]
            [5 6 7 8]
            [8 7 7 6]
            [6 7 10 7 6]]))
     (map water-collected-between-towers towerss))
   (list 2 14 35 0 0 0 0)))</syntaxhighlight>

When run produces no output -- meaning that the tests have run successfully.

=={{header|Raku}}==
(formerly Perl 6)
{{Trans|Haskell}}

<syntaxhighlight lang="raku" line>sub max_l ( @a ) {  [\max] @a }
sub max_r ( @a ) { ([\max] @a.reverse).reverse }

sub water_collected ( @towers ) {
    return 0 if @towers <= 2;

    my @levels = max_l(@towers) »min« max_r(@towers);

    return ( @levels »-« @towers ).grep( * > 0 ).sum;
}

say map &water_collected,
    [ 1, 5,  3, 7, 2 ],
    [ 5, 3,  7, 2, 6, 4, 5, 9, 1, 2 ],
    [ 2, 6,  3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 ],
    [ 5, 5,  5, 5 ],
    [ 5, 6,  7, 8 ],
    [ 8, 7,  7, 6 ],
    [ 6, 7, 10, 7, 6 ],
;</syntaxhighlight>
{{Out}}
<pre>(2 14 35 0 0 0 0)</pre>

=={{header|REXX}}==
===version 1===
<syntaxhighlight lang="rexx">/* REXX */
Call bars '1 5 3 7 2'
Call bars '5 3 7 2 6 4 5 9 1 2'
Call bars '2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1'
Call bars '5 5 5 5'
Call bars '5 6 7 8'
Call bars '8 7 7 6'
Call bars '6 7 10 7 6'
Exit
bars:
Parse Arg bars
bar.0=words(bars)
high=0
box.=' '
Do i=1 To words(bars)
  bar.i=word(bars,i)
  high=max(high,bar.i)
  Do j=1 To bar.i
    box.i.j='x'
    End
  End
m=1
w=0
Do Forever
  Do i=m+1 To bar.0
    If bar.i>bar.m Then
      Leave
    End
  If i>bar.0 Then Leave
  n=i
  Do i=m+1 To n-1
    w=w+bar.m-bar.i
    Do j=bar.i+1 To bar.m
      box.i.j='*'
      End
    End
  m=n
  End
m=bar.0
Do Forever
  Do i=bar.0 To 1 By -1
    If bar.i>bar.m Then
      Leave
    End
  If i<1 Then Leave
  n=i
  Do i=m-1 To n+1 By -1
    w=w+bar.m-bar.i
    Do j=bar.i+1 To bar.m
      box.i.j='*'
      End
    End
  m=n
  End
Say bars '->' w
Call show
Return
show:
Do j=high To 1 By -1
  ol=''
  Do i=1 To bar.0
    ol=ol box.i.j
    End
  Say ol
  End
Return</syntaxhighlight>
{{out}}
<pre>1 5 3 7 2 -> 2
       x
       x
   x * x
   x * x
   x x x
   x x x x
 x x x x x
5 3 7 2 6 4 5 9 1 2 -> 14
               x
               x
     x * * * * x
     x * x * * x
 x * x * x * x x
 x * x * x x x x
 x x x * x x x x
 x x x x x x x x * x
 x x x x x x x x x x
2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1 -> 35
           x
           x * * * * * * * x
   x * * * x * * * * * * * x
   x * x * x * * * * x * x x
   x * x * x * x * * x * x x x
   x x x * x * x * * x x x x x
 x x x x x x * x x x x x x x x
 x x x x x x x x x x x x x x x x
5 5 5 5 -> 0
 x x x x
 x x x x
 x x x x
 x x x x
 x x x x
5 6 7 8 -> 0
       x
     x x
   x x x
 x x x x
 x x x x
 x x x x
 x x x x
 x x x x
8 7 7 6 -> 0
 x
 x x x
 x x x x
 x x x x
 x x x x
 x x x x
 x x x x
 x x x x
6 7 10 7 6 -> 0
     x
     x
     x
   x x x
 x x x x x
 x x x x x
 x x x x x
 x x x x x
 x x x x x
 x x x x x</pre>

===version 2, simple numeric list output===
<syntaxhighlight lang="rexx">/*REXX program calculates and displays the amount of rainwater collected between towers.*/
       call tower  1  5  3  7  2
       call tower  5  3  7  2  6  4  5  9  1  2
       call tower  2  6  3  5  2  8  1  4  2  2  5  3  5  7  4  1
       call tower  5  5  5  5
       call tower  5  6  7  8
       call tower  8  7  7  6
       call tower  6  7 10  7  6
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
tower: procedure; arg y;  #=words(y); t.=0; L.=0 /*the T. array holds the tower heights.*/
            do j=1  for #;    t.j= word(y, j)    /*construct the towers,                */
            _= j-1;           L.j= max(t._, L._) /*    "      "  left─most tallest tower*/
            end   /*j*/
       R.=0
            do b=#  by -1  for #;  _= b+1; R.b= max(t._, R._) /*right─most tallest tower*/
            end   /*b*/
       w.=0                                                       /*rainwater collected.*/
            do f=1  for #;  if t.f>=L.f | t.f>=R.f  then iterate  /*rain between towers?*/
            w.f= min(L.f, R.f) - t.f;     w.00= w.00 + w.f        /*rainwater collected.*/
            end   /*f*/
       say right(w.00, 9) 'units of rainwater collected for: '  y /*display water units.*/
       return</syntaxhighlight>
{{out|output|text=&nbsp;}}
<pre>
        2 units of rainwater collected for:  1 5 3 7 2
       14 units of rainwater collected for:  5 3 7 2 6 4 5 9 1 2
       35 units of rainwater collected for:  2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1
        0 units of rainwater collected for:  5 5 5 5
        0 units of rainwater collected for:  5 6 7 8
        0 units of rainwater collected for:  8 7 7 6
        0 units of rainwater collected for:  6 7 10 7 6
</pre>

===version 3, with ASCII art===
This REXX version shows a scale &nbsp; (showing the number of floors in the building) &nbsp; and a representation of the towers and water collected.

It tries to protect the aspect ratio by showing the buildings as in this task's preamble.
<syntaxhighlight lang="rexx">/*REXX program calculates and displays the amount of rainwater collected between towers.*/
       call tower  1  5  3  7  2
       call tower  5  3  7  2  6  4  5  9  1  2
       call tower  2  6  3  5  2  8  1  4  2  2  5  3  5  7  4  1
       call tower  5  5  5  5
       call tower  5  6  7  8
       call tower  8  7  7  6
       call tower  6  7 10  7  6
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
tower: procedure; arg y; #= words(y); t.=0; L.=0 /*the T. array holds the tower heights.*/
            do j=1  for #;  t.j=word(y,j); _=j-1 /*construct the towers;  max height.   */
            L.j=max(t._, L._); t.0=max(t.0, t.j) /*left-most tallest tower; build scale.*/
            end   /*j*/
       R.=0
            do b=#  by -1  for #; _= b+1;  R.b= max(t._, R._) /*right-most tallest tower*/
            end   /*b*/
       w.=0                                                       /*rainwater collected.*/
            do f=1  for #;  if t.f>=L.f | t.f>=R.f  then iterate  /*rain between towers?*/
            w.f= min(L.f, R.f) - t.f;     w.00= w.00 + w.f        /*rainwater collected.*/
            end   /*f*/
       if w.00==0  then w.00= 'no'               /*pretty up wording for "no rainwater".*/
       ratio= 2                                  /*used to maintain a good aspect ratio.*/
       p.=                                       /*P.  stores plot versions of towers.  */
            do c=0  to #;  cc= c * ratio         /*construct the plot+scale for display.*/
              do h=1  for t.c+w.c;    glyph= '█' /*maybe show a floor of some tower(s). */
                       if h>t.c  then glyph= '≈' /*  "     "  rainwater between towers. */
              if c==0  then p.h= overlay(right(h, 9)         , p.h,  1   ) /*tower scale*/
                       else p.h= overlay(copies(glyph,ratio) , p.h, 10+cc) /*build tower*/
              end   /*h*/
            end     /*c*/
       p.1= overlay(w.00  'units of rainwater collected', p.1, 15*ratio+#) /*append text*/
            do z=t.0  by -1  to 0;     say p.z   /*display various tower floors & water.*/
            end     /*z*/
       return</syntaxhighlight>
{{out|output|text=&nbsp;}}
<pre>
        7        ██
        6        ██
        5    ██≈≈██
        4    ██≈≈██
        3    ██████
        2    ████████
        1  ██████████             2 units of rainwater collected

        9                ██
        8                ██
        7      ██≈≈≈≈≈≈≈≈██
        6      ██≈≈██≈≈≈≈██
        5  ██≈≈██≈≈██≈≈████
        4  ██≈≈██≈≈████████
        3  ██████≈≈████████
        2  ████████████████≈≈██
        1  ████████████████████        14 units of rainwater collected

        8            ██
        7            ██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██
        6    ██≈≈≈≈≈≈██≈≈≈≈≈≈≈≈≈≈≈≈≈≈██
        5    ██≈≈██≈≈██≈≈≈≈≈≈≈≈██≈≈████
        4    ██≈≈██≈≈██≈≈██≈≈≈≈██≈≈██████
        3    ██████≈≈██≈≈██≈≈≈≈██████████
        2  ████████████≈≈████████████████
        1  ████████████████████████████████  35 units of rainwater collected

        5  ████████
        4  ████████
        3  ████████
        2  ████████
        1  ████████              no units of rainwater collected

        8        ██
        7      ████
        6    ██████
        5  ████████
        4  ████████
        3  ████████
        2  ████████
        1  ████████              no units of rainwater collected

        8  ██
        7  ██████
        6  ████████
        5  ████████
        4  ████████
        3  ████████
        2  ████████
        1  ████████              no units of rainwater collected

       10      ██
        9      ██
        8      ██
        7    ██████
        6  ██████████
        5  ██████████
        4  ██████████
        3  ██████████
        2  ██████████
        1  ██████████             no units of rainwater collected
</pre>

=={{header|RPL}}==
{{trans|Python}}
{{works with|HP|49/50}}
 « DUPDUP SIZE 1 - NDUPN →LIST 
   DUP 1 « 1 NSUB SUB 0 + « MAX » STREAM » DOSUBS 0 SWAP +           <span style="color:grey">@ the seq of max heights to the left of each tower</span>
   SWAP 1 « NSUB 1 + OVER SIZE SUB 0 + « MAX » STREAM » DOSUBS 0 +   <span style="color:grey">@ the seq of max heights to the right of each tower</span>
   MIN SWAP -
   1 « 0 MAX » DOLIST ∑LIST
 » '<span style="color:blue">WATER</span>' STO
 
 « { {1 5 3 7 2}
     {5 3 7 2 6 4 5 9 1 2}
     {2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1}
     {5 5 5 5}
     {5 6 7 8}
     {8 7 7 6}
     {6 7 10 7 6} }
   1 « <span style="color:blue">WATER</span> » DOLIST 
 » '<span style="color:blue">TASK</span>' STO
{{out}}
<pre>
1: { 2 14 35 0 0 0 0 }
</pre>

=={{header|Ruby}}==
<syntaxhighlight lang="ruby">
def a(array)
n=array.length
left={}
right={}
left[0]=array[0]
i=1
loop do
   break if i >=n
left[i]=[left[i-1],array[i]].max
   i += 1  
end
right[n-1]=array[n-1]
i=n-2
loop do 
break if i<0
 right[i]=[right[i+1],array[i]].max
i-=1
end
i=0
water=0
loop do
break if i>=n
water+=[left[i],right[i]].min-array[i]
i+=1
end
puts water
end

a([ 5, 3,  7, 2, 6, 4, 5, 9, 1, 2 ])
a([ 2, 6,  3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 ])
a([ 5, 5,  5, 5 ])
a([ 5, 6,  7, 8 ])
a([ 8, 7,  7, 6 ])
a([ 6, 7, 10, 7, 6 ])
return</syntaxhighlight>
'''output'''
<pre>
14
35
0
0
0
0
</pre>

=={{header|Rust}}==
<syntaxhighlight lang="rust">
use std::cmp::min;

fn getfill(pattern: &[usize]) -> usize {
    let mut total = 0;
    for (idx, val) in pattern.iter().enumerate() {
        let l_peak = pattern[..idx].iter().max();
        let r_peak = pattern[idx + 1..].iter().max();
        if l_peak.is_some() && r_peak.is_some() {
            let peak = min(l_peak.unwrap(), r_peak.unwrap());
            if peak > val {
                total += peak - val;
            }
        }
    }
    total
}

fn main() {
    let patterns = vec![
        vec![1, 5, 3, 7, 2],
        vec![5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
        vec![2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
        vec![5, 5, 5, 5],
        vec![5, 6, 7, 8],
        vec![8, 7, 7, 6],
        vec![6, 7, 10, 7, 6],
    ];

    for pattern in patterns {
        println!("pattern: {:?}, fill: {}", &pattern, getfill(&pattern));
    }
}
</syntaxhighlight>
'''output'''
<pre>
pattern: [1, 5, 3, 7, 2], fill: 2
pattern: [5, 3, 7, 2, 6, 4, 5, 9, 1, 2], fill: 14
pattern: [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1], fill: 35
pattern: [5, 5, 5, 5], fill: 0
pattern: [5, 6, 7, 8], fill: 0
pattern: [8, 7, 7, 6], fill: 0
pattern: [6, 7, 10, 7, 6], fill: 0
</pre>

=={{header|Scala}}==
===No sweat.===
{{Out}}See it yourself by running in your browser either by [https://scalafiddle.io/sf/jx29Ace/0 ScalaFiddle (ES aka JavaScript, non JVM)] or [https://scastie.scala-lang.org/5nXQHfR0T0iauHNWSTRntg Scastie (remote JVM)].
{{libheader|Scala Concise}}
{{libheader|Scala Parallel Programming}}
{{libheader|Scala Time complexity O(n)}}
{{libheader|ScalaFiddle qualified}}
{{libheader|Scastie qualified}}
{{works with|Scala|2.13}}
<syntaxhighlight lang="scala">import scala.collection.parallel.CollectionConverters.VectorIsParallelizable

// Program to find maximum amount of water
// that can be trapped within given set of bars.
object TrappedWater extends App {
  private val barLines = List(
    Vector(1, 5, 3, 7, 2),
    Vector(5, 3, 7, 2, 6, 4, 5, 9, 1, 2),
    Vector(2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1),
    Vector(5, 5, 5, 5),
    Vector(5, 6, 7, 8),
    Vector(8, 7, 7, 6),
    Vector(6, 7, 10, 7, 6)).zipWithIndex

  // Method for maximum amount of water
  private def sqBoxWater(barHeights: Vector[Int]): Int = {
    def maxOfLeft = barHeights.par.scanLeft(0)(math.max).tail
    def maxOfRight = barHeights.par.scanRight(0)(math.max).init

    def waterlevels = maxOfLeft.zip(maxOfRight)
      .map { case (maxL, maxR) => math.min(maxL, maxR) }

    waterlevels.zip(barHeights).map { case (level, towerHeight) => level - towerHeight }.sum
  }

  barLines.foreach(barSet =>
    println(s"Block ${barSet._2 + 1} could hold max. ${sqBoxWater(barSet._1)} units."))

}</syntaxhighlight>

=={{header|Scheme}}==
<syntaxhighlight lang="scheme">(import (scheme base)
        (scheme write))

(define (total-collected chart)
  (define (highest-left vals curr)
    (if (null? vals)
      (list curr)
      (cons curr
            (highest-left (cdr vals) (max (car vals) curr)))))
  (define (highest-right vals curr)
    (reverse (highest-left (reverse vals) curr)))
  ;
  (if (< (length chart) 3) ; catch the end cases
    0
    (apply +
           (map (lambda (l c r) 
                  (if (or (<= l c)
                          (<= r c))
                    0
                    (- (min l r) c)))
                (highest-left chart 0)
                chart 
                (highest-right chart 0)))))

(for-each 
  (lambda (chart)
    (display chart) (display " -> ") (display (total-collected chart)) (newline))
  '((1 5 3 7 2)
    (5 3 7 2 6 4 5 9 1 2)
    (2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1)
    (5 5 5 5)
    (5 6 7 8)
    (8 7 7 6)
    (6 7 10 7 6)))</syntaxhighlight>
{{out}}
<pre>(1 5 3 7 2) -> 2
(5 3 7 2 6 4 5 9 1 2) -> 14
(2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1) -> 35
(5 5 5 5) -> 0
(5 6 7 8) -> 0
(8 7 7 6) -> 0
(6 7 10 7 6) -> 0
(3 1 2) -> 1
(1) -> 0
() -> 0
(1 2) -> 0</pre>

=={{header|Sidef}}==
<syntaxhighlight lang="ruby">func max_l(Array a, m = a[0]) {
    gather { a.each {|e| take(m = max(m, e)) } }
}

func max_r(Array a) {
    max_l(a.flip).flip
}

func water_collected(Array towers) {
    var levels = (max_l(towers) »min« max_r(towers))
    (levels »-« towers).grep{ _ > 0 }.sum
}

[
    [ 1, 5,  3, 7, 2 ],
    [ 5, 3,  7, 2, 6, 4, 5, 9, 1, 2 ],
    [ 2, 6,  3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1 ],
    [ 5, 5,  5, 5 ],
    [ 5, 6,  7, 8 ],
    [ 8, 7,  7, 6 ],
    [ 6, 7, 10, 7, 6 ],
].map { water_collected(_) }.say</syntaxhighlight>
{{out}}
<pre>
[2, 14, 35, 0, 0, 0, 0]
</pre>

=={{header|Swift}}==
<syntaxhighlight lang="swift">// Based on this answer from Stack Overflow:
// https://stackoverflow.com/a/42821623

func waterCollected(_ heights: [Int]) -> Int {
    guard heights.count > 0 else {
        return 0
    }
    var water = 0
    var left = 0, right = heights.count - 1
    var maxLeft = heights[left], maxRight = heights[right]
    
    while left < right {
        if heights[left] <= heights[right] {
            maxLeft = max(heights[left], maxLeft)
            water += maxLeft - heights[left]
            left += 1
        } else {
            maxRight = max(heights[right], maxRight)
            water += maxRight - heights[right]
            right -= 1
        }
    }
    return water
}

for heights in [[1, 5, 3, 7, 2],
                [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
                [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
                [5, 5, 5, 5],
                [5, 6, 7, 8],
                [8, 7, 7, 6],
                [6, 7, 10, 7, 6]] {
    print("water collected = \(waterCollected(heights))")
}</syntaxhighlight>

{{out}}
<pre>
water collected = 2
water collected = 14
water collected = 35
water collected = 0
water collected = 0
water collected = 0
water collected = 0
</pre>

=={{header|Tailspin}}==
<syntaxhighlight lang="tailspin">
templates histogramWater
  $ -> \( @: 0"1";
    [$... -> ($)"1"-> { leftMax: $ -> #, value: ($)"1" } ] !
    when <$@..> do @: $; $ !
    otherwise $@ !
  \) -> \( @: { rightMax: 0"1", sum: 0"1" };
    $(last..1:-1)... -> #
    $@.sum !
    when <{ value: <$@.rightMax..> }> do @.rightMax: $.value;
    when <{ value: <$.leftMax..> }> do !VOID
    when <{ leftMax: <..$@.rightMax>}> do @.sum: $@.sum + $.leftMax - $.value;
    otherwise  @.sum: $@.sum + $@.rightMax - $.value;
  \) !
end histogramWater

[[1, 5, 3, 7, 2],
  [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
  [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
  [5, 5, 5, 5],
  [5, 6, 7, 8],
  [8, 7, 7, 6],
  [6, 7, 10, 7, 6]]... -> '$ -> histogramWater; water in $;$#10;' -> !OUT::write
</syntaxhighlight>

{{out}}
<pre>
2"1" water in [1, 5, 3, 7, 2]
14"1" water in [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
35"1" water in [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
0"1" water in [5, 5, 5, 5]
0"1" water in [5, 6, 7, 8]
0"1" water in [8, 7, 7, 6]
0"1" water in [6, 7, 10, 7, 6]
</pre>

=={{header|Tcl}}==
Tcl makes for a surprisingly short and readable implementation, next to some of the more functional-oriented languages.
<syntaxhighlight lang="tcl">namespace path {::tcl::mathfunc ::tcl::mathop}

proc flood {ground} {
    set lefts [
        set d 0
        lmap g $ground {
            set d [max $d $g]
        }
    ]
    set ground [lreverse $ground]
    set rights [
        set d 0
        lmap g $ground {
            set d [max $d $g]
        }
    ]
    set rights [lreverse $rights]
    set ground [lreverse $ground]
    set water [lmap l $lefts r $rights {min $l $r}]
    set depths [lmap g $ground w $water {- $w $g}]
    + {*}$depths
}

foreach p {
    {5 3 7 2 6 4 5 9 1 2}
    {1 5 3 7 2}
    {5 3 7 2 6 4 5 9 1 2}
    {2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1}
    {5 5 5 5}
    {5 6 7 8}
    {8 7 7 6}
    {6 7 10 7 6}
} {
    puts [flood $p]:\t$p
}</syntaxhighlight>

{{out}}
<pre>14:        5 3 7 2 6 4 5 9 1 2
2:      1 5 3 7 2
14:     5 3 7 2 6 4 5 9 1 2
35:     2 6 3 5 2 8 1 4 2 2 5 3 5 7 4 1
0:      5 5 5 5
0:      5 6 7 8
0:      8 7 7 6
0:      6 7 10 7 6</pre>

=={{header|Wren}}==
{{trans|Kotlin}}
{{libheader|Wren-math}}
{{libheader|Wren-fmt}}
<syntaxhighlight lang="wren">import "./math" for Math, Nums
import "./fmt" for Fmt

var waterCollected = Fn.new { |tower|
    var n = tower.count
    var highLeft = [0] + (1...n).map { |i| Nums.max(tower[0...i]) }.toList
    var highRight = (1...n).map { |i| Nums.max(tower[i...n]) }.toList + [0]
    var t = (0...n).map { |i| Math.max(Math.min(highLeft[i], highRight[i]) - tower[i], 0) }
    return Nums.sum(t)
}

var towers = [
    [1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]
]
for (tower in towers) Fmt.print("$2d from $n", waterCollected.call(tower), tower)</syntaxhighlight>

{{out}}
<pre>
 2 from [1, 5, 3, 7, 2]
14 from [5, 3, 7, 2, 6, 4, 5, 9, 1, 2]
35 from [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1]
 0 from [5, 5, 5, 5]
 0 from [5, 6, 7, 8]
 0 from [8, 7, 7, 6]
 0 from [6, 7, 10, 7, 6]
</pre>

=={{header|XPL0}}==
<syntaxhighlight lang="xpl0">func WaterCollected(Array, Width);      \Return amount of water collected
int  Array, Width, Height, I, Row, Col, Left, Right, Water;
[Water:= 0;  Height:= 0;
for I:= 0 to Width-1 do                         \find max height
    if Array(I) > Height then Height:= Array(I);
for Row:= 2 to Height do
    for Col:= 1 to Width-2 do                   \(zero-based)
        if Row > Array(Col) then                \empty location
            [Left:= false;  Right:= false;      \check for barriers
            for I:= 0 to Width-1 do
                if Array(I) >= Row then         \have barrier
                    [if I < Col then Left:= true;
                     if I > Col then Right:= true;
                    ];
            if Left & Right then Water:= Water+1;
            ];
return Water;
];

int Towers, I;
[Towers:=[[1, 5, 3, 7, 2],
          [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
          [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
          [5, 5, 5, 5],
          [5, 6, 7, 8],
          [8, 7, 7, 6],
          [6, 7, 10, 7, 6],
          [0]];         \for determining sub-array lengths
for I:= 0 to 7-1 do
    [IntOut( 0, WaterCollected(Towers(I), (Towers(I+1)-Towers(I))/4) );
    ChOut(0, ^ );
    ];
]</syntaxhighlight>

{{out}}
<pre>
2 14 35 0 0 0 0 
</pre>

=={{header|zkl}}==
{{trans|Haskell}}
<syntaxhighlight lang="zkl">fcn waterCollected(walls){
     // compile max wall heights from left to right and right to left
     // then each pair is left/right wall of that cell.
     // Then the min of each wall pair == water height for that cell
   scanl(walls,(0).max)     // scan to right, f is max(0,a,b)
  .zipWith((0).MAX.min,     // f is MAX.min(a,b) == min(a,b)
           scanl(walls.reverse(),(0).max).reverse()) // right to left
     // now subtract the wall height from the water level and add 'em up
   .zipWith('-,walls).filter('>(0)).sum(0);
} 
fcn scanl(xs,f,i=0){ // aka reduce but save list of results
   xs.reduce('wrap(s,x,a){ s=f(s,x); a.append(s); s },i,ss:=List()); 
   ss
} // scanl((1,5,3,7,2),max,0) --> (1,5,5,7,7)</syntaxhighlight>
<syntaxhighlight lang="zkl">T( T(1, 5, 3, 7, 2), T(5, 3, 7, 2, 6, 4, 5, 9, 1, 2),
   T(2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1),
   T(5, 5, 5, 5), T(5, 6, 7, 8),T(8, 7, 7, 6),
   T(6, 7, 10, 7, 6) )
.pump(List, waterCollected).println();</syntaxhighlight>
{{out}}
<pre>
L(2,14,35,0,0,0,0)
</pre>