Look-and-say sequence

Look-and-say sequence
You are encouraged to solve this task according to the task description, using any language you may know.

The   Look and say sequence   is a recursively defined sequence of numbers studied most notably by   John Conway.

The   look-and-say sequence   is also known as the   Morris Number Sequence,   after cryptographer Robert Morris,   and the puzzle   What is the next number in the sequence 1,   11,   21,   1211,   111221?   is sometimes referred to as the Cuckoo's Egg,   from a description of Morris in Clifford Stoll's book   The Cuckoo's Egg.

Sequence Definition

• Take a decimal number
• Look at the number, visually grouping consecutive runs of the same digit.
• Say the number, from left to right, group by group; as how many of that digit there are - followed by the digit grouped.

This becomes the next number of the sequence.

An example:

• Starting with the number 1,   you have one 1 which produces 11
• Starting with 11,   you have two 1's.   I.E.:   21
• Starting with 21,   you have one 2, then one 1.   I.E.:   (12)(11) which becomes 1211
• Starting with 1211,   you have one 1, one 2, then two 1's.   I.E.:   (11)(12)(21) which becomes 111221

Task

Write a program to generate successive members of the look-and-say sequence.

Related tasks

•   Fours is the number of letters in the ...
•   Number names
•   Self-describing numbers
•   Self-referential sequence
•   Spelling of ordinal numbers

See also

•   Look-and-Say Numbers (feat John Conway), A Numberphile Video.
•   This task is related to, and an application of, the Run-length encoding task.
•   Sequence A005150 on The On-Line Encyclopedia of Integer Sequences.

11l

Translation of: Python

F lookandsay(=number)
   V result = ‘’
   V repeat = number[0]
   number = number[1..]‘ ’
   V times = 1

   L(actual) number
      I actual != repeat
         result ‘’= String(times)‘’repeat
         times = 1
         repeat = actual
      E
         times++
   R result

V num = ‘1’

L 10
   print(num)
   num = lookandsay(num)

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

8080 Assembly

bdos:		equ	5	; CP/M calls
puts:		equ	9

nmemb:		equ	15	; Change this to print more or fewer members

		org	100h

		
		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
		;; Generate and output members of the sequence
		mvi	b,nmemb	; Counter 
		
outloop:	push	b	; Preserve counter across calls
		
		mvi	c,puts	; Output current member
		lxi	d,memb
		call	bdos	; And newline	
		mvi	c,puts
		lxi	d,newline
		call	bdos
		
		lxi	h,memb	; Generate next member
		call	looksay
		
		pop	b	; Restore counter
		dcr	b	; Done yet?
		jnz	outloop
		rst	0
		
		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
		;; Given a $-terminated string under HL, representing
		;; a member of the look and say sequence, generate
		;; the next one in place (ish). The memory after the
		;; string is assumed to be free. 
looksay:	push	h	; Save start of string on stack
		mov	d,h	; And in DE
		mov	e,l
		mvi	a,'$'	; Find end of string
findend:	cmp	m
		inx	h
		jnz	findend
		xchg		; HL=string, DE=destination
		push	d	; Save start of new string on stack
		
lookchar:	mvi	b,0	; Zero counter
lookloop:	mov	a,m	; Get current character
		inr	b	; Compare next character
		inx	h
		cmp	m	; While it is the same, keep going
		jz	lookloop
		
		mov	c,a	; Keep character
		mvi	a,'0'	; There are B amount of these characters
		add	b	
		stax	d	; Store the amount
		inx	d	; And in the next location
		mov	a,c 	; Store the character
		stax	d
		inx	d
		
		mvi	a,'$'	; Are we done? 
		cmp	m
		jnz	lookchar	; If not, do next character
		stax	d	; If yes, terminate new string
		
		;; Free up memory by copying the new string to where the old
		;; string began. 
		pop	d	; Start of new string
		pop	h	; Start of old string
copyloop:	ldax	d	; Get char from new string
		mov	m,a	; Store char where old string was
		cpi	'$'	; are we done yet?
		inx	d
		inx	h
		jnz	copyloop
		ret 
		
		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
newline:	db	13,10,'$'
		;; This is where the string will be stored.
memb:		db	'1$'	; First item
		; Due to how CP/M loads programs, the memory after here
		; is free until we hit the stack.

8086 Assembly

		bits	16
		cpu	8086
puts:		equ	9h	; MS/DOS system call to print a string
nmemb:		equ	15	; Change this to print more or fewer members
section		.text
		org	100h
		mov	cx,nmemb	; CX = how many members to print
outloop:	mov	dx,memb		; Print current member
		mov	ah,puts
		int	21h
		mov	dx,newline	; Print newline
		int	21h
		mov	di,memb		; Generate next member
		call	looksay
		loop	outloop		; Decrease CX, and loop until zero.
		ret			; Go back to DOS.
		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
		;;; Given a look and say string in ES:DI, generate the next
		;;; one in place. Assumption: DS = ES.
looksay:	push	cx		; Keep the counter register
		mov	si,di		; Store pointer to string begin in SI
		mov	bx,di		; And another in BX
		mov	al,'$'		; Find the end of the string
		xor	cx,cx		; Max. 65535 tries
		dec	cx
		repne	scasb		; The 8086 has dedicated string search
		mov	dx,di		; Store copy of start of new str in DX
		;;; Process one character
.procchar:	mov	al,'0'		; Set counter to ASCII 0
		mov	ah,[bx]		; Get current character of string
		cmp	ah,'$'		; Done?
		je	.done
.samechar:	inc	bx 		; Increment pointer
		inc	al		; Increment counter
		cmp	ah,[bx]		; Still the same character?
		je	.samechar	; If yes, test next character
		mov	[di],ax		; Store counter and character
		inc	di		; Move ahead two characters
		inc	di
		jmp	.procchar	; Do next character
		;;; Copy new string into old location
.done:		mov	byte [di],'$'	; Terminate the string
		mov	cx,di		; Calculate how many bytes to copy
		sub	cx,dx		; end + 1 - start, so one too few here
		shr 	cx,1		; Divide by 2 = words
		inc	cx		; Compensate for the missing +1
		mov	di,dx		; Pointer to begin of new string
		xchg	si,di		; Set SI = new string and DI = old
		rep	movsw		; Copy 16 bits at a time
		pop	cx		; Restore counter register
		ret
section		.data
newline:	db	13,10,'$'	; Newline to print in between members
memb:		db	'1$'	; This is where the current member is stored

ABC

HOW TO RETURN look.and.say seq:
    PUT "" IN result
    PUT 0 IN n
    PUT "" IN c
    FOR ch IN seq:
        SELECT:
            c=ch:
                PUT n+1 IN n
            ELSE:
                IF n>0: PUT result^"`n`"^c IN result
                PUT 1 IN n
                PUT ch IN c
    RETURN result^"`n`"^c

PUT "1" IN item

FOR i IN {1..14}:
    WRITE item/
    PUT look.and.say item IN item

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211

Action!

BYTE FUNC GetLength(CHAR ARRAY s BYTE pos)
  CHAR c
  BYTE len

  c=s(pos)
  len=1
  DO
    pos==+1
    IF pos<=s(0) AND s(pos)=c THEN
      len==+1
    ELSE
      EXIT
    FI
  OD
RETURN (len)

PROC Append(CHAR ARRAY text,suffix)
  BYTE POINTER srcPtr,dstPtr
  BYTE len

  len=suffix(0)
  IF text(0)+len>255 THEN
    len=255-text(0)
  FI
  IF len THEN
    srcPtr=suffix+1
    dstPtr=text+text(0)+1
    MoveBlock(dstPtr,srcPtr,len)
    text(0)==+suffix(0)
  FI
RETURN

PROC LookAndSay(CHAR ARRAY in,out)
  BYTE pos,len
  CHAR ARRAY tmp(5)

  pos=1 len=0 out(0)=0
  WHILE pos<=in(0)
  DO
    len=GetLength(in,pos)
    StrB(len,tmp)
    Append(out,tmp)
    out(0)==+1
    out(out(0))=in(pos)
    pos==+len
  OD
RETURN

PROC Main()
  CHAR ARRAY s1(256),s2(256)
  BYTE i

  SCopy(s1,"1")
  PrintE(s1)
  FOR i=1 TO 11
  DO
    IF (i&1)=0 THEN
      LookAndSay(s2,s1)
      PrintE(s1)
    ELSE
      LookAndSay(s1,s2)
      PrintE(s2)
    FI
  OD
RETURN

Output:

Screenshot from Atari 8-bit computer

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

Ada

with Ada.Text_IO, Ada.Strings.Fixed;
use  Ada.Text_IO, Ada.Strings, Ada.Strings.Fixed;

function "+" (S : String) return String is
   Item : constant Character := S (S'First);
begin
   for Index in S'First + 1..S'Last loop
      if Item /= S (Index) then
         return Trim (Integer'Image (Index - S'First), Both) & Item & (+(S (Index..S'Last)));
      end if;
   end loop;
   return Trim (Integer'Image (S'Length), Both) & Item;
end "+";

This function can be used as follows:

Put_Line (+"1");
Put_Line (+(+"1"));
Put_Line (+(+(+"1")));
Put_Line (+(+(+(+"1"))));
Put_Line (+(+(+(+(+"1")))));
Put_Line (+(+(+(+(+(+"1"))))));
Put_Line (+(+(+(+(+(+(+"1")))))));
Put_Line (+(+(+(+(+(+(+(+"1"))))))));
Put_Line (+(+(+(+(+(+(+(+(+"1")))))))));
Put_Line (+(+(+(+(+(+(+(+(+(+"1"))))))))));

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

ALGOL 68

Translation of: Ada

Works with: ALGOL 68 version Standard - no extensions to language used

Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386

Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386

OP + = (STRING s)STRING:
BEGIN
   CHAR item = s[LWB s];
   STRING out;
   FOR index FROM LWB s + 1 TO UPB s DO
      IF item /= s [index] THEN
         out := whole(index - LWB s, 0) + item + (+(s [index:UPB s]));
         GO TO return out
      FI
   OD;
   out := whole (UPB s, 0) + item;
   return out: out
END  # + #;

OP + = (CHAR s)STRING: 
  + STRING(s);

print ((+"1", new line));
print ((+(+"1"), new line));
print ((+(+(+"1")), new line));
print ((+(+(+(+"1"))), new line));
print ((+(+(+(+(+"1")))), new line));
print ((+(+(+(+(+(+"1"))))), new line));
print ((+(+(+(+(+(+(+"1")))))), new line));
print ((+(+(+(+(+(+(+(+"1"))))))), new line));
print ((+(+(+(+(+(+(+(+(+"1")))))))), new line));
print ((+(+(+(+(+(+(+(+(+(+"1"))))))))), new line))

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

ALGOL-M

begin
    string(1) function digit(n);
    integer n;
    case n of begin
        digit := "0";   digit := "1";   digit := "2";
        digit := "3";   digit := "4";   digit := "5";
        digit := "6";   digit := "7";   digit := "8";
        digit := "9";
    end;
    
    string(1) array cur[1:128];
    string(1) array next[1:128];
    integer curlen, i, cnt, j, n;
    
    cur[1] := "1";
    curlen := 1;
    
    for n := 1 step 1 until 15 do begin
        write("");
        for i := 1 step 1 until curlen do 
            writeon(cur[i]);
        
        i := j := 1;
        while i <= curlen do begin
            cnt := 1;
            while cur[i + cnt] = cur[i] do 
                cnt := cnt + 1;
            next[j] := digit(cnt);
            next[j + 1] := cur[i];
            j := j + 2;
            i := i + cnt;
        end;
        
        for i := 1 step 1 until j-1 do
            cur[i] := next[i];
        
        curlen := j - 1;
    end;
end

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

APL

  ⎕IO←0
  d←{(1↓⍵)-¯1↓⍵}
  f←{m←(0≠d ⍵),1 ⋄ ,(d ¯1,m/⍳⍴⍵),[.5](m/⍵)}
  {(f⍣⍵) ,1}¨⍳10

This is an ugly little APL2 function that accepts a numeric vector (or scalar) and returns the result. Apologies for the labeled loop...

R←LNS V;T
R←0⍴0                    ⍝ initiate empty reply
LOOP:T←↑⍴↑(=\V)⊂V←,V     ⍝ t is the length of the 1st digit's run
R←R,T,↑V                 ⍝ append t and the 1st digit
→(0≠↑⍴V←T↓V)/LOOP        ⍝ drop t digits and iterate

AppleScript

on lookAndSay(startNumber, howMany)
    if (howMany < 1) then return {}
    
    -- The numbers are handled as lists of digit-value integers for efficiency and output as a list of strings.
    script o
        property previousNumber : {}
        property newNumber : {}
        property output : {}
    end script
    
    -- "Digitise" the start number.
    repeat
        set beginning of o's newNumber to startNumber mod 10 as integer
        set startNumber to startNumber div 10
        if (startNumber is 0) then exit repeat
    end repeat
    -- Add it to the output as text and successively derive the remaining numbers.
    set astid to AppleScript's text item delimiters
    set AppleScript's text item delimiters to ""
    set end of o's output to o's newNumber as text
    repeat (howMany - 1) times
        set o's previousNumber to o's newNumber
        set o's newNumber to {}
        set i to 1
        set previousLength to (o's previousNumber's length)
        set currentDigit to beginning of o's previousNumber
        repeat with j from 2 to previousLength
            set thisDigit to item j of o's previousNumber
            if (thisDigit is not currentDigit) then
                set end of o's newNumber to j - i
                set end of o's newNumber to currentDigit
                set i to j
                set currentDigit to thisDigit
            end if
        end repeat
        set end of o's newNumber to previousLength - i + 1
        set end of o's newNumber to currentDigit
        
        set end of o's output to o's newNumber as text
    end repeat
    set AppleScript's text item delimiters to astid
    
    return o's output
end lookAndSay

-- Test code:
return lookAndSay(1, 10)

Output:

{"1", "11", "21", "1211", "111221", "312211", "13112221", "1113213211", "31131211131221", "13211311123113112211"}

Arturo

lookAndSay: function [n][
    if n=0 -> return "1"
    previous: lookAndSay n-1

    result: new ""
    currentCounter: 0
    currentCh: first previous
    loop previous 'ch [
        if? currentCh <> ch [
            if not? zero? currentCounter ->
                'result ++ (to :string currentCounter) ++ currentCh
            currentCounter: 1
            currentCh: ch
        ]
        else ->
            currentCounter: currentCounter + 1
    ]
    'result ++ (to :string currentCounter) ++ currentCh
    return result
]

loop 0..10 'x [
    print [x "->" lookAndSay x]
]

Output:

0 -> 1 
1 -> 11 
2 -> 21 
3 -> 1211 
4 -> 111221 
5 -> 312211 
6 -> 13112221 
7 -> 1113213211 
8 -> 31131211131221 
9 -> 13211311123113112211 
10 -> 11131221133112132113212221

AutoHotkey

AutoExecute:
    Gui, -MinimizeBox
    Gui, Add, Edit, w500 r20 vInput, 1
    Gui, Add, Button, x155 w100 Default, &Calculate
    Gui, Add, Button, xp+110 yp wp, E&xit
    Gui, Show,, Look-and-Say sequence
Return


ButtonCalculate:
    Gui, Submit, NoHide
    GuiControl,, Input, % LookAndSay(Input)
Return


GuiClose:
ButtonExit:
    ExitApp
Return


;---------------------------------------------------------------------------
LookAndSay(Input) {
;--------------------------------------------------------------------------- 
    ; credit for this function goes to AutoHotkey forum member Laslo 
    ; http://www.autohotkey.com/forum/topic44657-161.html
    ;-----------------------------------------------------------------------
    Loop, Parse, Input          ; look at every digit
        If (A_LoopField = d)    ; I've got another one! (of the same value)
            c += 1                  ; Let's count them ...
        Else {                  ; No, this one is different!
            r .= c d                ; remember what we've got so far
            c := 1                  ; It is the first one in a row
            d := A_LoopField        ; Which one is it?
        }
    Return, r c d
}

AWK

function lookandsay(a)
{
  s = ""
  c = 1
  p = substr(a, 1, 1)
  for(i=2; i <= length(a); i++) {
    if ( p == substr(a, i, 1) ) {
      c++
    } else {
      s = s sprintf("%d%s", c, p)
      p = substr(a, i, 1)
      c = 1
    }
  }
  s = s sprintf("%d%s", c, p)
  return s
}

BEGIN {
  b = "1"
  print b
  for(k=1; k <= 10; k++) {
    b = lookandsay(b)
    print b
  }
}

BASIC

Works with: QBasic

Works with: QuickBasic

Works with: BASICA

Works with: Chipmunk Basic

Works with: GW-BASIC

Works with: MSX BASIC version any

Works with: Run BASIC

Works with: Just BASIC

10 DEFINT A-Z: I$="1"
20 FOR Z=1 TO 15
30 PRINT I$
40 O$=""
50 FOR I=1 TO LEN(I$)
60 C=1
70 IF MID$(I$,I,1)=MID$(I$,I+C,1) THEN C=C+1: GOTO 70
80 O$=O$+CHR$(C+48)+MID$(I$,I,1)
90 I=I+C-1
100 NEXT I
110 I$=O$
120 NEXT Z

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

Applesoft BASIC

Translation of: BASIC

10 I$="1"
20 FOR Z=1 TO 15
30  PRINT I$
40  O$=""
50  FOR I=1 TO LEN(I$)
60   C=1
70   IF MID$(I$,I,1)=MID$(I$,I+C,1) THEN C=C+1: GOTO 70
80   O$=O$+CHR$(C+48)+MID$(I$,I,1)
90   I=I+C-1
100  NEXT I
110  I$=O$
120 NEXT Z

Chipmunk Basic

Works with: Chipmunk Basic version 3.6.4

100 cls
110 dim x$(2)
120 i = 0 ' índice de cadena de entrada
130 x$(i) = "1"
140 input "Indica cuantas repeticiones: ",r
150 print "Secuencia:"
160 print x$(i)
170 for n = 1 to r-1
180   j = 1-i ' índice de cadena de salida
190   x$(j) = ""
200   k = 1
210   while k <= len(x$(i))
220     k0 = k+1
230     while ((k0 <= len(x$(i))) and (mid$(x$(i),k,1) = mid$(x$(i),k0,1)))
240       k0 = k0+1
250     wend
260     x$(j) = x$(j)+str$(k0-k)+mid$(x$(i),k,1)
270     k = k0
280   wend
290   i = j
300   print x$(j)
310 next n
320 end

Output:

Similar to FreeBASIC entry.

GW-BASIC

The BASIC solution works without any changes.

MSX Basic

Works with: MSX BASIC version any

The BASIC solution works without any changes.

True BASIC

LET i$ = "1"
FOR z = 1 TO 15
    PRINT i$
    LET o$ = ""
    FOR i = 1 TO LEN(i$)
        LET c = 1
        DO WHILE (i$)[i:i+1-1] = (i$)[i+c:i+c+1-1]
           LET c = c+1
        LOOP
        LET o$ = o$ & CHR$(c+48) & (i$)[i:i+1-1]
        LET i = i+c-1
    NEXT i
    LET i$ = o$
NEXT z
END

Output:

Similar to FreeBASIC entry.

BASIC256

# look and say

dim a$(2)

i = 0  # input string index

a$[i] = "1"

print a$[i]

for n=1 to 10
  j = 1 - i  # output string index
  a$[j] = ""
  k = 1
  while (k <= length(a$[i]))
    k0 = k + 1
    while ((k0 <= length(a$[i])) and (mid(a$[i], k, 1) = mid(a$[i], k0, 1)))
      k0 = k0 + 1
    end while
    a$[j] += string(k0 - k) + mid(a$[i], k, 1)
    k = k0
  end while
  i = j
  print a$[j]
next n

BBC BASIC

      number$ = "1"
      FOR i% = 1 TO 10
        number$ = FNlooksay(number$)
        PRINT number$
      NEXT
      END
      
      DEF FNlooksay(n$)
      LOCAL i%, j%, c$, o$
      i% = 1
      REPEAT
        c$ = MID$(n$,i%,1)
        j% = i% + 1
        WHILE MID$(n$,j%,1) = c$
          j% += 1
        ENDWHILE
        o$ += STR$(j%-i%) + c$
        i% = j%
      UNTIL i% > LEN(n$)
      = o$

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

BCPL

get "libhdr"

manifest $(
    amount = 15
    bufsize = 128
$)

let move(dest,src) be
$(  until !src = 0 do
    $(  !dest := !src
        dest := dest + 1
        src := src + 1
    $)
    !dest := 0
$)

let count(v) = valof
$(  let i=1
    while v!i = !v do i := i + 1
    resultis i
$)

let looksay(in,out) be
$(  until !in = 0 do
    $(  let n = count(in)
        out!0 := n
        out!1 := !in
        out := out + 2
        in := in + n
    $)
    !out := 0
$)

let show(v) be
$(  until !v = 0 do
    $(  writen(!v)
        v := v + 1
    $)
    wrch('*N')
$)

let start() be
$(  let buf1 = vec bufsize and buf2 = vec bufsize
    buf1!0 := 1
    buf1!1 := 0
    for n = 1 to amount do
    $(  show(buf1)
        looksay(buf1,buf2)
        move(buf1,buf2)
    $)
$)

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

BQN

LookSay ← ∾´((⊑∾˜ ≠+'0'˙)¨1↓((+`»≠⊢)⊸⊔))

>((⌈´≠¨)↑¨⊢) LookSay⍟(↕15)"1"

Output:

┌─                                                                                
╵"1                                                                               
  11                                                                              
  21                                                                              
  1211                                                                            
  111221                                                                          
  312211                                                                          
  13112221                                                                        
  1113213211                                                                      
  31131211131221                                                                  
  13211311123113112211                                                            
  11131221133112132113212221                                                      
  3113112221232112111312211312113211                                              
  1321132132111213122112311311222113111221131221                                  
  11131221131211131231121113112221121321132132211331222113112211                  
  311311222113111231131112132112311321322112111312211312111322212311322113212221" 
                                                                                 ┘

Bracmat

In this example we use a non-linear pattern and a negation of a pattern: the end of e sequence of equal digits is (1) the end of the string or (2) the start of a sequence starting with a different digit.

( 1:?number
& 0:?lines
&   whl
  ' ( 1+!lines:~>10:?lines
    & :?say                           { This will accumulate all that has to be said after one iteration. }
    & 0:?begin
    & ( @( !number                    { Pattern matching. The '@' indicates we're looking in a string rather than a tree structure. }
         :   ?
             (   [!begin
                 %@?digit
                 ?
                 [?end
                 ( (|(%@:~!digit) ?)  { The %@ guarantees we're testing one character - not less (%) and not more (@). The ? takes the rest. }
                 & !say !end+-1*!begin !digit:?say
                 & !end:?begin        { When backtracking, 'begin' advances to the begin of the next sequence, or to the end of the string. }
                 )
             & ~                      { fail! This forces backtracking. Backtracking stops when all begin positions have been tried. }
             )
         )
      | out$(str$!say:?number)        { After backtracking, output string and set number to string for next iteration. }
      )
    )
);

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

C

This program will not stop until killed or running out of memory.

#include <stdio.h>
#include <stdlib.h>

int main()
{
	char *a = malloc(2), *b = 0, *x, c;
	int cnt, len = 1;

	for (sprintf(a, "1"); (b = realloc(b, len * 2 + 1)); a = b, b = x) {
		puts(x = a);
		for (len = 0, cnt = 1; (c = *a); ) {
			if (c == *++a)
				cnt++;
			else if (c) {
				len += sprintf(b + len, "%d%c", cnt, c);
				cnt = 1;
			}
		}
	}

	return 0;
}

C#

using System;
using System.Text;
using System.Linq;

class Program
{
    static string lookandsay(string number)
    {
        StringBuilder result = new StringBuilder();

        char repeat = number[0];
        number = number.Substring(1, number.Length-1)+" ";
        int times = 1;
      
        foreach (char actual in number)
        {
            if (actual != repeat)
            {
                result.Append(Convert.ToString(times)+repeat);
                times = 1;
                repeat = actual;
            }
            else
            {
                times += 1;
            }
        }
        return result.ToString();
    }

    static void Main(string[] args)
    {
        string num = "1"; 

        foreach (int i in Enumerable.Range(1, 10)) {
             Console.WriteLine(num);
             num = lookandsay(num);             
        }
    }
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Alternate version using Regex (C#2 syntax only):

using System;
using System.Text.RegularExpressions;

namespace RosettaCode_Cs_LookAndSay
{
    public class Program
    {
        public static int Main(string[] args)
        {
            Array.Resize<string>(ref args, 2);
            string ls = args[0] ?? "1";
            int n;
            if (!int.TryParse(args[1], out n)) n = 10;
            do {
                Console.WriteLine(ls);
                if (--n <= 0) break;
                ls = say(look(ls));
            } while(true);

            return 0;
        }

        public static string[] look(string input)
        {
            int i = -1;
            return Array.FindAll(Regex.Split(input, @"((\d)\2*)"),
                delegate(string p) { ++i; i %= 3; return i == 1; }
            );
        }

        public static string say(string[] groups)
        {
            return string.Concat(
                Array.ConvertAll<string, string>(groups,
                    delegate(string p) { return string.Concat(p.Length, p[0]); }
                )
            );
        }
    }
}

Output:

(with args

1 15):

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

C++

#include <iostream>
#include <sstream>
#include <string>

std::string lookandsay(const std::string& s)
{
    std::ostringstream r;

    for (std::size_t i = 0; i != s.length();) {
        auto new_i = s.find_first_not_of(s[i], i + 1);

        if (new_i == std::string::npos)
            new_i = s.length();

        r << new_i - i << s[i];
        i = new_i;
    }
    return r.str();
}

int main()
{
    std::string laf = "1";

    std::cout << laf << '\n';
    for (int i = 0; i < 10; ++i) {
        laf = lookandsay(laf);
        std::cout << laf << '\n';
    }
}

Ceylon

shared void run() {
	
    function lookAndSay(Integer|String input) {
        
        variable value digits = if (is Integer input) then input.string else input;
        value builder = StringBuilder();
        
        while (exists currentChar = digits.first) {
            if (exists index = digits.firstIndexWhere((char) => char != currentChar)) {
                digits = digits[index...];
                builder.append("``index````currentChar``");
            }
            else {
                builder.append("``digits.size````currentChar``");
                break;
            }
        }
        
        return builder.string;
    }
    
    variable String|Integer result = 1;
    print(result);
    for (i in 1..14) {
        result = lookAndSay(result);
        print(result);
    }
}

Clojure

No ugly int-to-string-and-back conversions.

(defn digits-seq
  "Returns a seq of the digits of a number (L->R)."
  [n]
  (loop [digits (), number n]
    (if (zero? number) (seq digits)
        (recur (cons (mod number 10) digits)
               (quot number 10)))))

(defn join-digits
  "Converts a digits-seq back in to a number."
  [ds]
  (reduce (fn [n d] (+ (* 10 n) d)) ds))

(defn look-and-say [n]
  (->> n digits-seq (partition-by identity)
       (mapcat (juxt count first)) join-digits))

Output:

user> (take 8 (iterate look-and-say 1))
(1 11 21 1211 111221 312211 13112221 1113213211)

---

The math above is lovely, Clojure using Java's regular expressions is also powerful:

(defn look-and-say
  [n]
  (->> (re-seq #"(.)\1*" n)
       (mapcat (comp (juxt count first) first))
       (apply str)))

(take 12 (iterate look-and-say "1"))

Output:

("1"
 "11"
 "21"
 "1211"
 "111221"
 "312211"
 "13112221"
 "1113213211"
 "31131211131221"
 "13211311123113112211"
 "11131221133112132113212221"
 "3113112221232112111312211312113211")

CLU

look_and_say = proc (s: string) returns (string)
    out: array[char] := array[char]$[]
    count: int := 0
    last: char := '\000'
    
    for c: char in string$chars(s) do
        if c ~= last then   
            if count ~= 0 then
                array[char]$addh(out, char$i2c(count + 48))
                array[char]$addh(out, last)
            end
            last := c
            count := 1
        else
            count := count + 1
        end
    end
    
    array[char]$addh(out, char$i2c(count + 48))
    array[char]$addh(out, last)
    return (string$ac2s(out))
end look_and_say

start_up = proc ()
    lines = 15
    
    po: stream := stream$primary_output()
    cur: string := "1"
    
    for i: int in int$from_to(1, lines) do
        stream$putl(po, cur)
        cur := look_and_say(cur)
    end
end start_up

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

COBOL

        IDENTIFICATION DIVISION.
        PROGRAM-ID. LOOK-AND-SAY-SEQ.
        
        DATA DIVISION.
        WORKING-STORAGE SECTION.
        01 SEQUENCES.
           02 CUR-SEQ       PIC X(80) VALUE "1".
           02 CUR-CHARS     REDEFINES CUR-SEQ 
                            PIC X OCCURS 80 TIMES INDEXED BY CI.
           02 CUR-LENGTH    PIC 99 COMP VALUE 1.
           02 NEXT-SEQ      PIC X(80).
           02 NEXT-CHARS    REDEFINES NEXT-SEQ 
                            PIC X OCCURS 80 TIMES INDEXED BY NI.
        01 ALG-STATE.
           02 STEP-AMOUNT   PIC 99 VALUE 14.
           02 ITEM-COUNT    PIC 9.
           
        PROCEDURE DIVISION.
        LOOK-AND-SAY.
            DISPLAY CUR-SEQ.
            SET CI TO 1.
            SET NI TO 1.
        MAKE-NEXT-ENTRY.
            MOVE 0 TO ITEM-COUNT.
            IF CI IS GREATER THAN CUR-LENGTH GO TO STEP-DONE.
        TALLY-ITEM.
            ADD 1 TO ITEM-COUNT.
            SET CI UP BY 1.
            IF CI IS NOT GREATER THAN CUR-LENGTH
               AND CUR-CHARS(CI) IS EQUAL TO CUR-CHARS(CI - 1) 
                GO TO TALLY-ITEM.
        INSERT-ENTRY.
            MOVE ITEM-COUNT TO NEXT-CHARS(NI).
            MOVE CUR-CHARS(CI - 1) TO NEXT-CHARS(NI + 1).
            SET NI UP BY 2.
            GO TO MAKE-NEXT-ENTRY.
        STEP-DONE.
            MOVE NEXT-SEQ TO CUR-SEQ.
            SET NI DOWN BY 1.
            SET CUR-LENGTH TO NI.
            SUBTRACT 1 FROM STEP-AMOUNT.
            IF STEP-AMOUNT IS NOT EQUAL TO ZERO GO TO LOOK-AND-SAY.
            STOP RUN.

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211

Common Lisp

(defun compress (array &key (test 'eql) &aux (l (length array)))
  "Compresses array by returning a list of conses each of whose car is
a number of occurrences and whose cdr is the element occurring.  For
instance, (compress \"abb\") produces ((1 . #\a) (2 . #\b))."
  (if (zerop l) nil
    (do* ((i 1 (1+ i))
          (segments (acons 1 (aref array 0) '())))
         ((eql i l) (nreverse segments))
      (if (funcall test (aref array i) (cdar segments))
        (incf (caar segments))
        (setf segments (acons 1 (aref array i) segments))))))

(defun next-look-and-say (number)
  (reduce #'(lambda (n pair)
              (+ (* 100 n)
                 (* 10 (car pair))
                 (parse-integer (string (cdr pair)))))
          (compress (princ-to-string number))
          :initial-value 0))

Example use:

(next-look-and-say 9887776666) ;=> 19283746

Straight character counting:

(defun look-and-say (s)
   (let ((out (list (char s 0) 0)))
     (loop for x across s do
	   (if (char= x (first out))
	     (incf (second out))
	     (setf out (list* x 1 out))))
     (format nil "~{~a~^~}" (nreverse out))))

(loop for s = "1" then (look-and-say s)
      repeat 10
      do (write-line s))

Cowgol

include "cowgol.coh";
include "strings.coh";

# Given a string with a member of the look-and-say sequence,
# generate the next member of the sequence.
sub LookSay(cur: [uint8], next: [uint8]) is
    while [cur] != 0 loop
        var count: uint8 := 1;
        var curch: uint8 := [cur];
        
        # count how many of this character we have
        loop
            cur := @next cur;
            if [cur] != curch then break; end if;
            count := count + 1;
        end loop;
        
        # add it and its count to the next sequence
        [next] := '0' + count;
        next := @next next;
        [next] := curch;
        next := @next next;
    end loop;
    [next] := 0;
end sub;

# amount of members to print
# (don't forget to enlarge the buffers if you make this bigger)
var members: uint8 := 15;

# define two buffers
var curbuf: uint8[255];
var nextbuf: uint8[255];

# start with "1"
CopyString("1", &curbuf as [uint8]);

# generate and print successive members
while members > 0 loop
    print(&curbuf as [uint8]);
    print_nl();
    LookSay(&curbuf as [uint8], &nextbuf as [uint8]);
    CopyString(&nextbuf as [uint8], &curbuf as [uint8]);
    members := members - 1; 
end loop;

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

Crystal

Translation of: Ruby

The simplest one:

class String
  def lookandsay
    gsub(/(.)\1*/){ |s| s.size.to_s + s[0] }
  end
end
 
ss = '1'
12.times { puts ss; ss = ss.to_s.lookandsay }

Translation of: Ruby from Perl

def lookandsay(str)
  str.gsub(/(.)\1*/) { |s| s.size.to_s + $1 }
end
   
num = "1"
12.times { puts num;  num = lookandsay(num) }

Translation of: Ruby

Using Enumerable#chunks

def lookandsay(str)  
  str.chars.chunks(&.itself).map{ |(c, x)| x.size.to_s + c }.join
end
   
num = "1"
12.times { puts num; num = lookandsay(num) }

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

D

Short Functional Version

import std.stdio, std.algorithm, std.range;

enum say = (in string s) pure => s.group.map!q{ text(a[1],a[0]) }.join;

void main() {
    "1".recurrence!((t, n) => t[n - 1].say).take(8).writeln;
}

Output:

["1", "11", "21", "1211", "111221", "312211", "13112221", "1113213211"]

Beginner Imperative Version

import std.stdio, std.conv, std.array;

pure string lookAndSay(string s){
  auto result = appender!string;
  auto i=0, j=i+1;
  while(i<s.length){
    while(j<s.length && s[i]==s[j]) j++;
    result.put( text(j-i) ~ s[i] );
    i = j++;
  }
  return result.data;
}
void main(){ 
  auto s="1";
  for(auto i=0; i<10; i++) 
    (s = s.lookAndSay).writeln;	
}

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Fast Imperative Version

import core.stdc.stdio, std.math, std.conv, std.algorithm, std.array;

void showLookAndSay(bool showArrays)(in uint n) nothrow {
    if (n == 0) // No sequences to generate and show.
        return;

    enum Digit : char { nil = '\0', one = '1', two = '2', thr = '3' }

    // Allocate an approximate upper bound size for the array.
    static Digit* allocBuffer(in uint m) nothrow {
        immutable len = cast(size_t)(100 + 1.05 *
                                     exp(0.269 * m + 0.2686)) + 1;
        auto a = len.uninitializedArray!(Digit[]);
        printf("Allocated %d bytes.\n", a.length * Digit.sizeof);
        return a.ptr;
    }

    // Can't be expressed in the D type system:
    // a1 and a2 are immutable pointers to mutable data.
    auto a1 = allocBuffer(n % 2 ? n : n - 1);
    auto a2 = allocBuffer(n % 2 ? n - 1 : n);
    printf("\n");

    a1[0] = Digit.one;
    size_t len1 = 1;
    a1[len1] = Digit.nil;

    foreach (immutable i; 0 .. n - 1) {
        static if (showArrays)
            printf("%2u: %s\n", i + 1, a1);
        else
            printf("%2u: n. digits: %u\n", i + 1, len1);
        auto p1 = a1,
             p2 = a2;

        S0: final switch (*p1++) with (Digit) { // Initial state.
                case nil: goto END;
                case one: goto S1;
                case two: goto S2;
                case thr: goto S3;
            }
        S1: final switch (*p1++) with (Digit) {
                case nil: *p2++ = one; *p2++ = one; goto END;
                case one: goto S11;
                case two: *p2++ = one; *p2++ = one; goto S2;
                case thr: *p2++ = one; *p2++ = one; goto S3;
            }
        S2: final switch (*p1++) with (Digit) {
                case nil: *p2++ = one; *p2++ = two; goto END;
                case one: *p2++ = one; *p2++ = two; goto S1;
                case two: goto S22;
                case thr: *p2++ = one; *p2++ = two; goto S3;
            }
        S3: final switch (*p1++) with (Digit) {
                case nil: *p2++ = one; *p2++ = thr; goto END;
                case one: *p2++ = one; *p2++ = thr; goto S1;
                case two: *p2++ = one; *p2++ = thr; goto S2;
                case thr: goto S33;
            }
        S11: final switch (*p1++) with (Digit) {
                case nil: *p2++ = two; *p2++ = one; goto END;
                case one: *p2++ = thr; *p2++ = one; goto S0;
                case two: *p2++ = two; *p2++ = one; goto S2;
                case thr: *p2++ = two; *p2++ = one; goto S3;
            }
        S22: final switch (*p1++) with (Digit) {
                case nil: *p2++ = two; *p2++ = two; goto END;
                case one: *p2++ = two; *p2++ = two; goto S1;
                case two: *p2++ = thr; *p2++ = two; goto S0;
                case thr: *p2++ = two; *p2++ = two; goto S3;
            }
        S33: final switch (*p1++) with (Digit) {
                case nil: *p2++ = two; *p2++ = thr; goto END;
                case one: *p2++ = two; *p2++ = thr; goto S1;
                case two: *p2++ = two; *p2++ = thr; goto S2;
                case thr: *p2++ = thr; *p2++ = thr; goto S0;
            }
        END:
            immutable len2 = p2 - a2;
            a2[len2] = Digit.nil;
            a1.swap(a2);
            len1 = len2;
    }

    static if (showArrays)
        printf("%2u: %s\n", n, a1);
    else
        printf("%2u: n. digits: %u\n", n, len1);
}

void main(in string[] args) {
    immutable n = (args.length == 2) ? args[1].to!uint : 10;
    n.showLookAndSay!true;
}

Output:

Allocated 116 bytes.
Allocated 121 bytes.

 1: 1
 2: 11
 3: 21
 4: 1211
 5: 111221
 6: 312211
 7: 13112221
 8: 1113213211
 9: 31131211131221
10: 13211311123113112211

With:

70.showLookAndSay!false;

Output:

Allocated 158045069 bytes.
Allocated 206826462 bytes.

 1: n. digits: 1
 2: n. digits: 2
 3: n. digits: 2
 4: n. digits: 4
 5: n. digits: 6
...
60: n. digits: 12680852
61: n. digits: 16530884
62: n. digits: 21549544
63: n. digits: 28091184
64: n. digits: 36619162
65: n. digits: 47736936
66: n. digits: 62226614
67: n. digits: 81117366
68: n. digits: 105745224
69: n. digits: 137842560
70: n. digits: 179691598

Using the LDC2 compiler with n=70 the run-time is about 3.74 seconds.

Intermediate Version

This mostly imperative version is intermediate in both speed and code size:

void main(in string[] args) {
    import std.stdio, std.conv, std.algorithm, std.array, std.string;

    immutable n = (args.length == 2) ? args[1].to!uint : 10;
    if (n == 0)
        return;

    auto seq = ['1'];
    writefln("%2d: n. digits: %d", 1, seq.length);
    foreach (immutable i; 2 .. n + 1) {
        Appender!(typeof(seq)) result;
        foreach (const digit, const count; seq.representation.group) {
            result ~= "123"[count - 1];
            result ~= digit;
        }
        seq = result.data;
        writefln("%2d: n. digits: %d", i, seq.length);
    }
}

The output is the same as the second version.

If you modify the first program to print only the lengths of the strings (with a .map!(s => s.length)), compiling with LDC2 the run-times of the three versions with n=55 are about 31.1, 0.10 and 0.23 seconds.

More Direct Version

Translated and modified from C code by Reddit user "skeeto": http://www.reddit.com/r/dailyprogrammer/comments/2ggy30/9152014_challenge180_easy_looknsay/

Using ideas from: http://www.njohnston.ca/2010/10/a-derivation-of-conways-degree-71-look-and-say-polynomial/

This recursive version is able to generate very large sequences in a short time without memory for the intermediate sequence (and with stack space proportional to the sequence order).

import core.stdc.stdio, std.conv;

// On Windows this uses the printf from the Microsoft C runtime,
// that doesn't handle real type and some of the C99 format
// specifiers, but it's faster for bulk printing.
version (LDC) version (Windows)
extern(C) nothrow @nogc int printf(const char*, ...);

// http://www.njohnston.ca/2010/10/a-derivation-of-conways-degree-71-look-and-say-polynomial/
struct Sequence {
    string seq;
    uint[6] next;
}

immutable Sequence[93] sequences = [
    {"", []},
    {"1112", [63]},
    {"1112133", [64, 62]},
    {"111213322112", [65]},
    {"111213322113", [66]},
    {"1113", [68]},
    {"11131", [69]},
    {"111311222112", [84, 55]},
    {"111312", [70]},
    {"11131221", [71]},
    {"1113122112", [76]},
    {"1113122113", [77]},
    {"11131221131112", [82]},
    {"111312211312", [78]},
    {"11131221131211", [79]},
    {"111312211312113211", [80]},
    {"111312211312113221133211322112211213322112", [81, 29, 91]},
    {"111312211312113221133211322112211213322113", [81, 29, 90]},
    {"11131221131211322113322112", [81, 30]},
    {"11131221133112", [75, 29, 92]},
    {"1113122113322113111221131221", [75, 32]},
    {"11131221222112", [72]},
    {"111312212221121123222112", [73]},
    {"111312212221121123222113", [74]},
    {"11132", [83]},
    {"1113222", [86]},
    {"1113222112", [87]},
    {"1113222113", [88]},
    {"11133112", [89, 92]},
    {"12", [1]},
    {"123222112", [3]},
    {"123222113", [4]},
    {"12322211331222113112211", [2, 61, 29, 85]},
    {"13", [5]},
    {"131112", [28]},
    {"13112221133211322112211213322112", [24, 33, 61, 29, 91]},
    {"13112221133211322112211213322113", [24, 33, 61, 29, 90]},
    {"13122112", [7]},
    {"132", [8]},
    {"13211", [9]},
    {"132112", [10]},
    {"1321122112", [21]},
    {"132112211213322112", [22]},
    {"132112211213322113", [23]},
    {"132113", [11]},
    {"1321131112", [19]},
    {"13211312", [12]},
    {"1321132", [13]},
    {"13211321", [14]},
    {"132113212221", [15]},
    {"13211321222113222112", [18]},
    {"1321132122211322212221121123222112", [16]},
    {"1321132122211322212221121123222113", [17]},
    {"13211322211312113211", [20]},
    {"1321133112", [6, 61, 29, 92]},
    {"1322112", [26]},
    {"1322113", [27]},
    {"13221133112", [25, 29, 92]},
    {"1322113312211", [25, 29, 67]},
    {"132211331222113112211", [25, 29, 85]},
    {"13221133122211332", [25, 29, 68, 61, 29, 89]},
    {"22", [61]},
    {"3", [33]},
    {"3112", [40]},
    {"3112112", [41]},
    {"31121123222112", [42]},
    {"31121123222113", [43]},
    {"3112221", [38, 39]},
    {"3113", [44]},
    {"311311", [48]},
    {"31131112", [54]},
    {"3113112211", [49]},
    {"3113112211322112", [50]},
    {"3113112211322112211213322112", [51]},
    {"3113112211322112211213322113", [52]},
    {"311311222", [47, 38]},
    {"311311222112", [47, 55]},
    {"311311222113", [47, 56]},
    {"3113112221131112", [47, 57]},
    {"311311222113111221", [47, 58]},
    {"311311222113111221131221", [47, 59]},
    {"31131122211311122113222", [47, 60]},
    {"3113112221133112", [47, 33, 61, 29, 92]},
    {"311312", [45]},
    {"31132", [46]},
    {"311322113212221", [53]},
    {"311332", [38, 29, 89]},
    {"3113322112", [38, 30]},
    {"3113322113", [38, 31]},
    {"312", [34]},
    {"312211322212221121123222113", [36]},
    {"312211322212221121123222112", [35]},
    {"32112", [37]}
];

void evolve(in uint seq, in uint n) nothrow @nogc {
    if (n <= 0) {
        printf(sequences[seq].seq.ptr);
    } else {
        foreach (immutable next; sequences[seq].next) {
            if (next == 0)
                break;
            evolve(next, n - 1);
        }
    }
}

void main(in string[] args) {
    immutable uint n = (args.length != 2) ? 10 : args[1].to!uint;

    immutable base = 8;
    immutable string[base] results = ["", "1", "11", "21", "1211",
                                      "111221", "312211", "13112221"];
    if (n < base) {
        printf("%s\n", results[n].ptr);
        return;
    }

    evolve(24, n - base);
    evolve(39, n - base);
    '\n'.putchar;
}

Delphi

See Pascal.

Draco

\util.g

proc nonrec look_and_say(*char inp, outp) void:
    char cur;
    byte count;
    channel output text outch;
    open(outch, outp);
    while cur := inp*; cur ~= '\e' do
        count := 1;
        while 
            inp := inp + 1;
            inp* = cur
        do  
            count := count + 1
        od;
        write(outch; count, cur)
    od;
    close(outch)
corp

proc nonrec main() void:
    [256] char buf1, buf2;
    byte i;
    byte LINES = 14;
    
    CharsCopy(&buf1[0], "1");
    for i from 1 upto LINES do
        writeln(&buf1[0]);
        look_and_say(&buf1[0], &buf2[0]);
        CharsCopy(&buf1[0], &buf2[0])
    od
corp

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211

E

def lookAndSayNext(number :int) {
  var seen := null
  var count := 0
  var result := ""
  def put() {
    if (seen != null) {
      result += count.toString(10) + E.toString(seen)
    }
  }
  for ch in number.toString(10) {
    if (ch != seen) {
      put()
      seen := ch
      count := 0
    }
    count += 1
  }
  put()
  return __makeInt(result, 10)
}

var number := 1
for _ in 1..20 {
  println(number)
  number := lookAndSayNext(number)
}

EasyLang

Translation of: AWK

proc lookandsay . a$ .
   c = 1
   p$ = substr a$ 1 1
   for i = 2 to len a$
      if p$ = substr a$ i 1
         c += 1
      else
         s$ &= c & p$
         p$ = substr a$ i 1
         c = 1
      .
   .
   s$ &= c & p$
   swap a$ s$
.
b$ = 1
print b$
for k = 1 to 10
   lookandsay b$
   print b$
.

EchoLisp

(lib 'math) ;; for (number->list) = explode function
(lib 'list) ;; (group)

(define (next L)
	(for/fold (acc null) ((g (group L)))
		(append acc (list (length g) (first g)))))
		

(define (task n starter)
(for/fold (L (number->list starter)) ((i n))
	(writeln (list->string L))
	(next L)))

Output:

(task 10 1)
1    
11    
21    
1211    
111221    
312211    
13112221    
1113213211    
31131211131221    
13211311123113112211

Elixir

defmodule LookAndSay do
  def next(n) do
    Enum.chunk_by(to_char_list(n), &(&1))
    |> Enum.map(fn cl=[h|_] -> Enum.concat(to_char_list(length cl), [h]) end)
    |> Enum.concat
    |> List.to_integer
  end
  
  def sequence_from(n) do
    Stream.iterate n, &(next/1)
  end
  
  def main([start_str|_]) do
    {start_val,_} = Integer.parse(start_str)
    IO.inspect sequence_from(start_val) |> Enum.take 9
  end
  
  def main([]) do
    main(["1"])
  end
end

LookAndSay.main(System.argv)

Output:

[1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, 31131211131221]

Regex version:

defmodule RC do
  def look_and_say(n) do
    Regex.replace(~r/(.)\1*/, to_string(n), fn x,y -> [to_string(String.length(x)),y] end)
    |> String.to_integer
  end
end

IO.inspect Enum.reduce(1..9, [1], fn _,acc -> [RC.look_and_say(hd(acc)) | acc] end) |> Enum.reverse

Output:

[1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, 31131211131221,
 13211311123113112211]

Erlang

-module(str).
-export([look_and_say/1, look_and_say/2]).

%% converts a single number
look_and_say([H|T]) -> lists:reverse(look_and_say(T,H,1,"")).

%% converts and accumulates as a loop
look_and_say(_, 0) -> [];
look_and_say(Start, Times) when Times > 0 ->
    [Start | look_and_say(look_and_say(Start), Times-1)].

%% does the actual conversion for a number
look_and_say([], Current, N, Acc) ->
    [Current, $0+N | Acc];
look_and_say([H|T], H, N, Acc) ->
    look_and_say(T, H, N+1, Acc);
look_and_say([H|T], Current, N, Acc) ->
    look_and_say(T, H, 1, [Current, $0+N | Acc]).

Output:

1> c(str).
{ok,str}
2> str:look_and_say("1").
"11"
3> str:look_and_say("111221").
"312211"
4> str:look_and_say("1",10).
["1","11","21","1211","111221","312211","13112221",
 "1113213211","31131211131221","13211311123113112211"]

ERRE

PROGRAM LOOK

PROCEDURE LOOK_AND_SAY(N$->N$)
      LOCAL I%,J%,C$,O$
      I%=1
      REPEAT
        C$=MID$(N$,I%,1)
        J%=I%+1
        WHILE MID$(N$,J%,1)=C$ DO
          J%+=1
        END WHILE
        O$+=MID$(STR$(J%-I%),2)+C$
        I%=J%
      UNTIL I%>LEN(N$)
      N$=O$
END PROCEDURE

BEGIN
      NUMBER$="1"
      FOR I%=1 TO 10 DO
        LOOK_AND_SAY(NUMBER$->NUMBER$)
        PRINT(NUMBER$)
      END FOR
END PROGRAM

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

F#

Library functions somehow missing in F# out of the box (but present in haskell)

let rec brk p lst =
  match lst with
  | [] -> (lst, lst)
  | x::xs ->
    if p x
    then ([], lst)
    else
      let (ys, zs) = brk p xs
      (x::ys, zs)

let span p lst = brk (not << p) lst

let rec groupBy eq lst =
  match lst with
  | [] ->  []
  | x::xs ->
    let (ys,zs) = span (eq x) xs
	(x::ys)::groupBy eq zs

let group lst : list<list<'a>> when 'a : equality = groupBy (=) lst

Implementation

let lookAndSay =
  let describe (xs: char list) =
    List.append (List.ofSeq <| (List.length xs).ToString()) [List.head xs]
  let next xs = List.collect describe (group xs)
  let toStr xs = String (Array.ofList xs)
  Seq.map toStr <| Seq.unfold (fun xs -> Some (xs, next xs)) ['1']

let getNthLookAndSay n = Seq.nth n lookAndSay

Seq.take 10 lookAndSay

Factor

: (look-and-say) ( str -- )
    unclip-slice swap [ 1 ] 2dip [
        2dup = [ drop [ 1 + ] dip ] [
            [ [ number>string % ] dip , 1 ] dip
        ] if
    ] each [ number>string % ] [ , ] bi* ;

: look-and-say ( str -- str' ) [ (look-and-say) ] "" make ;

"1" 10 [ dup print look-and-say ] times print

Fennel

(fn look-and-say [t]
  (let [lst t
        ret []]
    (while (> (length lst) 0)
      (var (num cnt) (values (table.remove lst 1) 1))
      (while (= num (. lst 1))
        (set cnt (+ cnt 1))
        (when (> (length lst) 0)
          (set num (table.remove lst 1))))
      (tset ret (+ (length ret) 1) cnt)
      (tset ret (+ (length ret) 1) num))
    ret))

(var lst [1])
(for [i 1 10]
  (print (table.concat lst))
  (set lst (look-and-say lst)))

Alternative solution

(fn look-and-say [s]
  (var ret [])
  (var (num cnt) (values (s:sub 1 1) 1))
  (for [i 2 (length s)]
    (var cur-num (s:sub i i))
    (if (= num cur-num)
        (set cnt (+ cnt 1))
        (do
          (table.insert ret (.. cnt num))
          (set cnt 1)
          (set num cur-num))))
  (table.insert ret (.. cnt num))
  (table.concat ret))

(var str "1")
(for [i 1 10]
  (print str)
  (set str (look-and-say str)))

FOCAL

01.10 A "HOW MANY",M
01.20 S B(0)=1;S B(1)=0
01.30 F Z=1,M;D 4;D 2
01.40 Q

02.10 S X=0;S Y=0
02.15 I (B(X)),2.5
02.17 S CN=0
02.20 S CN=CN+1
02.25 S X=X+1
02.30 I (FABS(B(X)-B(X-1))),2.2
02.35 S C(Y)=CN;S C(Y+1)=B(X-1)
02.40 S Y=Y+2
02.45 G 2.15
02.50 S C(Y)=0
02.55 F X=0,Y;S B(X)=C(X)

03.10 I (A-9)3.2;T "9";R
03.20 I (A-8)3.3;T "8";R
03.30 I (A-7)3.4;T "7";R
03.40 I (A-6)3.5;T "6";R
03.50 I (A-5)3.6;T "5";R
03.60 I (A-4)3.7;T "4";R
03.70 I (A-3)3.8;T "3";R
03.80 I (A-2)3.9;T "2";R
03.90 T "1"

04.10 S X=0
04.20 S A=B(X)
04.30 I (-A)4.4;T !;R
04.40 D 3
04.50 S X=X+1
04.60 G 4.2

Output:

HOW MANY:12
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

Forth

create buf1 256 allot
create buf2 256 allot
buf1 value src
buf2 value dest

s" 1" src place

: append-run ( digit run -- )
  dest count +
  tuck c!  1+ c!
  dest c@ 2 + dest c! ;

: next-look-and-say
  0 dest c!
  src 1+ c@  [char] 0  ( digit run )
  src count bounds do
    over i c@ =
    if   1+
    else append-run  i c@ [char] 1
    then
  loop
  append-run
  src dest to src to dest ;

: look-and-say ( n -- )
  0 do next-look-and-say  cr src count type loop ;

10 look-and-say

Fortran

module LookAndSay
  implicit none

contains

  subroutine look_and_say(in, out)
    character(len=*), intent(in) :: in
    character(len=*), intent(out) :: out

    integer :: i, c
    character(len=1) :: x
    character(len=2) :: d

    out = ""
    c = 1
    x = in(1:1)
    do i = 2, len(trim(in))
       if ( x == in(i:i) ) then
          c = c + 1
       else
          write(d, "(I2)") c
          out = trim(out) // trim(adjustl(d)) // trim(x)
          c = 1
          x = in(i:i)
       end if
    end do
    write(d, "(I2)") c
    out = trim(out) // trim(adjustl(d)) // trim(x)
  end subroutine look_and_say

end module LookAndSay

program LookAndSayTest
  use LookAndSay
  implicit none
 
  integer :: i
  character(len=200) :: t, r
  t = "1"
  print *,trim(t)
  call look_and_say(t, r)
  print *, trim(r)
  do i = 1, 10
     call look_and_say(r, t)
     r = t
     print *, trim(r)
  end do
 
end program LookAndSayTest

FreeBASIC

Translation of: BASIC256

Dim As Integer n, j, k, k0, r
Dim As String X(2)
Dim As Integer i = 0  ' índice de cadena de entrada
X(0) = "1"

Input "Indica cuantas repeticiones: ", r
Print Chr(10) & "Secuencia:"

Print X(i)
For n = 1 To r-1
    j = 1 - i  ' índice de cadena de salida
    X(j) = ""
    k = 1
    While k <= Len(X(i))
        k0 = k + 1
        While ((k0 <= Len(X(i))) And (Mid(X(i), k, 1) = Mid(X(i), k0, 1)))
            k0 += 1
        Wend
        X(j) += Str(k0 - k) + Mid(X(i), k, 1)
        k = k0
    Wend
    i = j
    Print X(j)
Next n
End

Output:

Indica cuantas repeticiones: 10

Secuencia:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Frink

LookAndSay = "12211123"
println["Starting Value: " + LookAndSay]

LASStr = { |LaS|
   length[LaS@0] + length[LaS@1] + LaS@0
   /*
      The returned results from the Regex are divided between a distinct matching
      character and any following identical characters. For example, a string
      of 2222 would be returned from this function as [2,222].
      The function adds the length of both elements (1 and 3 in the example
      above) and returns that value with the matching character.
      i.e. Length of 1st element = 1, Length of 2nd element = 3, value of 1st element = 2
      1 + 3 = 4 & value 2. Returned result is "42" i.e. "Four 2s."
   */
}

// Calculate the next 10 Look and Say Sequence Values

for i = 1 to 10
{
   LookAndSayReg = LookAndSay =~ %r/(\d)(\1{0,})/g
   LookAndSay = join["",mapList[LASStr,LookAndSayReg]]
   println["$i - $LookAndSay"]
}

Output:

Starting Value: 1
1 - 11
2 - 21
3 - 1211
4 - 111221
5 - 312211
6 - 13112221
7 - 1113213211
8 - 31131211131221
9 - 13211311123113112211
10 - 11131221133112132113212221

FutureBasic

include "NSLog.incl"

local fn LookAndSay( testWord as CFStringRef ) as CFStringRef
  NSUInteger         i, length, times
  CFMutableStringRef result = fn MutableStringWithCapacity(0)
  
  unichar repeat = fn StringCharacterAtIndex( testWord, 0 )
  times    = 1
  testWord = fn StringWithFormat( @"%@ ", fn StringSubstringFromIndex( testWord, 1 ) )
  length   = len(testWord)
  
  for i = 0 to length - 1
    unichar actual = fn StringCharacterAtIndex( testWord, i )
    if ( actual != repeat )
      MutableStringAppendFormat( result, @"%d%c", times, repeat )
      times = 1
      repeat = actual
    else
      times++
    end if
  next
end fn = fn StringWithString( result )

void local fn DoIt
  NSUInteger  i
  CFStringRef numStr = @"1"
  
  for i = 1 to i <= 15
    NSLog( @"%@", numStr )
    numStr = fn LookAndSay( numStr )
  next
end fn

fn DoIt

HandleEvents

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

Gambas

Code is modified from the [PureBasic] example

Click this link to run this code

Public Sub Main()
Dim i, j, cnt As Integer
Dim txt$, curr$, result$ As String

txt$ = "1211"
i = 1

Print "Sequence: " & txt$ & " = ";
  Repeat
    j = 1
    result$ = ""
      Repeat
        curr$ = Mid(txt$, j, 1)
        cnt = 0
          Repeat
            Inc cnt 
            Inc j 
          Until Mid(txt$, j, 1) <> curr$
        result$ &= Str(cnt) & curr$
      Until j > Len(txt$)
    Print result$
    txt$ = result$
    Dec i 
  Until i <= 0
End

Output:

Sequence: 1211 = 111221

GAP

LookAndSay := function(s)
  local c, r, cur, ncur, v;
  v := "123";
  r := "";
  cur := 0;
  ncur := 0;
  for c in s do
    if c = cur then
      ncur := ncur + 1;
    else
      if ncur > 0 then
        Add(r, v[ncur]);
        Add(r, cur);
      fi;
      cur := c;
      ncur := 1;
    fi;
  od;
  Add(r, v[ncur]);
  Add(r, cur);
  return r;
end;

LookAndSay("1");     # "11"
LookAndSay(last);    # "21"
LookAndSay(last);    # "1211"
LookAndSay(last);    # "111221"
LookAndSay(last);    # "312211"
LookAndSay(last);    # "13112221"
LookAndSay(last);    # "1113213211"
LookAndSay(last);    # "31131211131221"
LookAndSay(last);    # "13211311123113112211"
LookAndSay(last);    # "11131221133112132113212221"
LookAndSay(last);    # "3113112221232112111312211312113211"
LookAndSay(last);    # "1321132132111213122112311311222113111221131221"
LookAndSay(last);    # "11131221131211131231121113112221121321132132211331222113112211"
LookAndSay(last);    # "311311222113111231131112132112311321322112111312211312111322212311322113212221"

Go

package main

import (
    "fmt"
    "strconv"
)

func lss(s string) (r string) {
    c := s[0]
    nc := 1
    for i := 1; i < len(s); i++ {
        d := s[i]
        if d == c {
            nc++
            continue
        }
        r += strconv.Itoa(nc) + string(c)
        c = d
        nc = 1
    }
    return r + strconv.Itoa(nc) + string(c)
}

func main() {
    s := "1"
    fmt.Println(s)
    for i := 0; i < 8; i++ {
        s = lss(s)
        fmt.Println(s)
    }
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221

Groovy

def lookAndSay(sequence) {
    def encoded = new StringBuilder()
    (sequence.toString() =~ /(([0-9])\2*)/).each { matcher ->
        encoded.append(matcher[1].size()).append(matcher[2])
    }
    encoded.toString()
}

Test Code

def sequence = "1"
(1..12).each {
    println sequence
    sequence = lookAndSay(sequence)
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

Haskell

import Control.Monad (liftM2)
import Data.List (group)

-- this function is composed out of many functions; data flows from the bottom up
lookAndSay :: Integer -> Integer
lookAndSay = read                                   -- convert digits to integer
           . concatMap                              -- concatenate for each run,
               (liftM2 (++) (show . length)         --    the length of it
                            (take 1))               --    and an example member
           . group                                  -- collect runs of the same digit
           . show                                   -- convert integer to digits

-- less comments
lookAndSay2 :: Integer -> Integer
lookAndSay2 = read . concatMap (liftM2 (++) (show . length)
                                            (take 1))
            . group . show


-- same thing with more variable names
lookAndSay3 :: Integer -> Integer
lookAndSay3 n = read (concatMap describe (group (show n)))
  where describe run = show (length run) ++ take 1 run

main = mapM_ print (iterate lookAndSay 1)           -- display sequence until interrupted

Haxe

using Std;

class Main 
{
	
	static function main() 
	{
		var test = "1";
		for (i in 0...11) {
			Sys.println(test);
			test = lookAndSay(test);
		}
	}

	static function lookAndSay(s:String)
	{
		if (s == null || s == "") return "";
		
		var results = "";
		var repeat = s.charAt(0);
		var amount = 1;
		for (i in 1...s.length)
		{
			var actual = s.charAt(i);
			if (actual != repeat)
			{
				results += amount.string();
				results += repeat;
				repeat = actual;
				amount = 0;
			}
			amount++;
		}
		results += amount.string();
		results += repeat;
		
		return results;
	}
}

Icon and Unicon

procedure main()
every 1 to 10 do 
   write(n := nextlooknsayseq(\n | 1))
end

procedure nextlooknsayseq(n)  #: return next element in look and say sequence
n2 := ""
n ? until pos(0) do {
   i := tab(any(&digits)) | fail  # or fail if not digits
   move(-1) 
   n2 ||:= *tab(many(i)) || i     # accumulate count+digit
   }
return n2                                      
end

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Insitux

(function look-and-say n x
  (return-when (empty? n) x)
  (let digit (0 n)
       [before after] (part-before (!= digit) n))
  (recur after (strn x (len before) digit)))

(var result "1")
(loop 10 i
  (print (var! result look-and-say)))

J

Solution:

las=: ,@((# , {.);.1~ 1 , 2 ~:/\ ])&.(10x&#.inv)@]^:(1+i.@[)

Example:

   10 las 1
1 11 21 1211 111221 312211 13112221 1113213211 31131211131221 13211311123113112211 11131221133112132113212221

Note the result is an actual numeric sequence (cf. the textual solutions given in other languages).

Java

Translation of: C#

Works with: Java version 1.5+

public static String lookandsay(String number){
	StringBuilder result= new StringBuilder();

	char repeat= number.charAt(0);
	number= number.substring(1) + " ";
	int times= 1;

	for(char actual: number.toCharArray()){
		if(actual != repeat){
			result.append(times + "" + repeat);
			times= 1;
			repeat= actual;
		}else{
			times+= 1;
		}
	}
	return result.toString();
}

Testing:

public static void main(String[] args){
	String num = "1"; 
	 
	for (int i=1;i<=10;i++) {
		System.out.println(num);
		num = lookandsay(num);             
	}
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

JavaScript

With RegExp

Translation of: Perl

function lookandsay(str) {
    return str.replace(/(.)\1*/g, function(seq, p1){return seq.length.toString() + p1})
}

var num = "1";
for (var i = 10; i > 0; i--) {
    alert(num);
    num = lookandsay(num);
}

Imperative version

function lookAndSay( s="" ){
  var tokens=[]
  var i=0, j=1
  while( i<s.length ) {
    while( j<s.length && s[j]===s[i] ) j++
    tokens.push( `${j-i}${s[i]}` )
    i=j++
  }
  return tokens.join("")
}
var phrase="1"
for(var n=0; n<10; n++ ) 
  console.log( phrase = lookAndSay( phrase ) )

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

jq

Works with: jq version 1.4

def look_and_say:
  def head(c; n): if .[n:n+1] == c then head(c; n+1) else n end;
  tostring
  | if length == 0 then ""
    else head(.[0:1]; 1) as $len
      | .[0:$len] as $head
      | ($len | tostring) + $head[0:1] + (.[$len:] | look_and_say)
    end ;

# look and say n times
def look_and_say(n):
  if n == 0 then empty
  else look_and_say as $lns
       | $lns, ($lns|look_and_say(n-1))
  end ;

Example

1 | look_and_say(10)

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Julia

Works with: Julia version 1.1

function lookandsay(s::String)
    rst = IOBuffer()
    c = 1
    for i in 1:length(s)
        if i != length(s) && s[i] == s[i+1]
            c += 1
        else
            print(rst, c, s[i])
            c = 1
        end
    end
    String(take!(rst))
end


function lookandsayseq(n::Integer)
    rst = Vector{String}(undef, n)
    rst[1] = "1"
    for i in 2:n
        rst[i] = lookandsay(rst[i-1])
    end
    rst
end

println(lookandsayseq(10))

Output:

String["1", "11", "21", "1211", "111221", "312211", "13112221", "1113213211", "31131211131221", "13211311123113112211"]

K

  las: {x{0$,//$(#:'n),'*:'n:(&1,~=':x)_ x:0$'$x}\1}
  las 8
1 11 21 1211 111221 312211 13112221 1113213211 31131211131221

Kotlin

// version 1.0.6

fun lookAndSay(s: String): String {
    val sb = StringBuilder()
    var digit = s[0]
    var count = 1
    for (i in 1 until s.length) {
        if (s[i] == digit)
            count++
        else {
            sb.append("$count$digit")
            digit = s[i]
            count = 1
        }
    }
    return sb.append("$count$digit").toString()
}

fun main(args: Array<String>) {
    var las = "1"
    for (i in 1..15) {
        println(las)
        las = lookAndSay(las)
    }
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

Lasso

The Look-and-say sequence is a recursive RLE, so the solution can leverage the same method as used for RLE.

define rle(str::string)::string => {
	local(orig = #str->values->asCopy,newi=array, newc=array, compiled=string)
	while(#orig->size) => {
		if(not #newi->size) => {
			#newi->insert(1)
			#newc->insert(#orig->first)
			#orig->remove(1)
		else
			if(#orig->first == #newc->last) => {
				#newi->get(#newi->size) += 1
			else
				#newi->insert(1)
				#newc->insert(#orig->first)
			}
			#orig->remove(1)
		} 
	}
	loop(#newi->size) => {
		#compiled->append(#newi->get(loop_count)+#newc->get(loop_count))
	}
	return #compiled
}
define las(n::integer,run::integer) => {
	local(str = #n->asString)
	loop(#run) => { #str = rle(#str) }
	return #str
}
loop(15) => {^ las(1,loop_count) + '\r' ^}

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221
132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211

LiveCode

This function takes a string and returns the next Look-And-Say iteration of it:

function lookAndSay S
   put 0 into C
   put char 1 of S into lastChar
   repeat with i = 2 to length(S)
      add 1 to C
      if char i of S is lastChar then next repeat
      put C & lastChar after R
      put 0 into C
      put char i of S into lastChar
   end repeat
   return R & C + 1 & lastChar
end lookAndSay

on demoLookAndSay
   put 1 into x
   repeat 10
      put x & cr after message
      put lookAndSay(x) into x
   end repeat
   put x after message
end demoLookAndSay

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Logo

to look.and.say.loop :in :run :c :out
  if empty? :in [output (word :out :run :c)]
  if equal? first :in :c [output look.and.say.loop bf :in :run+1 :c :out]
  output look.and.say.loop bf :in 1 first :in (word :out :run :c)
end
to look.and.say :in
  if empty? :in [output :in]
  output look.and.say.loop bf :in 1 first :in "||
end

show cascade 10 [print ? look.and.say ?] 1

Lua

--returns an iterator over the first n copies of the look-and-say sequence
function lookandsayseq(n)
  local t = {1}
  return function()
    local ret = {}
    for i, v in ipairs(t) do
      if t[i-1] and v == t[i-1] then
        ret[#ret - 1] = ret[#ret - 1] + 1
      else
        ret[#ret + 1] = 1
        ret[#ret + 1] = v
      end
    end
    t = ret
    n = n - 1
    if n > 0 then return table.concat(ret) end
  end
end
for i in lookandsayseq(10) do print(i) end

Alternative solution, using LPeg:

require "lpeg"
local P, C, Cf, Cc = lpeg.P, lpeg.C, lpeg.Cf, lpeg.Cc
lookandsay = Cf(Cc"" * C(P"1"^1 + P"2"^1 + P"3"^1)^1, function (a, b) return a .. #b .. string.sub(b,1,1) end)
t = "1"
for i = 1, 10 do
  print(t)
  t = lookandsay:match(t)
end

Alternative solution, using Lua Pattern:

function lookandsay(t)
  return t:gsub("(1*)(2*)(3*)", function (...)
    local ret = {}
    for i = 1, select("#", ...) do
      local v = select(i, ...)
      if #v > 0 then
        ret[#ret + 1] = #v
        ret[#ret + 1] = v:sub(1,1)
      end
    end
    return table.concat(ret)
  end)
end

local t = "1"
for i = 1, 10 do
  print(t)
  t = lookandsay(t)
end

function lookandsay2(t)
  return t:gsub("(1*)(2*)(3*)", function (x, y, z)
    return (x == "" and x or (#x .. x:sub(1, 1))) ..
      (y == "" and y or (#y .. y:sub(1, 1))) ..
      (z == "" and z or (#z .. z:sub(1, 1)))
  end)
end

local t = "1"
for i = 1, 10 do
  print(t)
  t = lookandsay2(t)
end

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

M4

Using regular expressions:

Translation of: Perl

divert(-1)
define(`for',
   `ifelse($#,0,``$0'',
   `ifelse(eval($2<=$3),1,
   `pushdef(`$1',$2)$4`'popdef(`$1')$0(`$1',incr($2),$3,`$4')')')')

define(`las',
   `patsubst(`$1',`\(\(.\)\2*\)',`len(\1)`'\2')')


define(`v',1)
divert
for(`x',1,10,
   `v
define(`v',las(v))')dnl
v

MACRO-11

        .TITLE  LOKSAY
        .MCALL  .TTYOUT,.EXIT
LOKSAY::MOV     #START,R0
        MOV     #BUFR1,R1
        JSR     PC,COPY
        MOV     #^D14,R5
$1:     MOV     #BUFR1,R1
        JSR     PC,PRINT
        MOV     #NEWLIN,R1
        JSR     PC,PRINT
        JSR     PC,STEP
        SOB     R5,$1
        .EXIT
STEP:   MOV     #BUFR1,R0
        MOV     #BUFR2,R1
        BR      2$
1$:     INC     R3
        CMPB    (R0)+,R4
        BEQ     1$
        ADD     #60,R3
        MOVB    R3,(R1)+
        MOVB    R4,(R1)+
        DEC     R0
2$:     CLR     R3
        MOVB    (R0)+,R4
        BNE     1$
        MOV     #BUFR2,R0
        MOV     #BUFR1,R1
COPY:   MOVB    (R0)+,(R1)+
        BNE     COPY
        RTS     PC
PRINT:  MOVB    (R1)+,R0
        .TTYOUT
        BNE     PRINT
        RTS     PC
NEWLIN: .BYTE   15,12,0,0
START:  .ASCIZ  /1/
BUFR1:  .BLKB   400
BUFR2:  .BLKB   400
        .END    LOKSAY

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211

Maple

generate_seq := proc(s)
	local times, output, i;
	times := 1;
	output := "";
	for i from 2 to StringTools:-Length(s) do
		if (s[i] <> s[i-1]) then
			output := cat(output, times, s[i-1]);
			times := 1; # re-assign
		else 
			times ++;
		end if;
	end do;
	cat(output, times, s[i - 1]);
end proc:

Look_and_Say :=proc(n)
	local value, i;
	value := "1";
	print(value);
	for i from 2 to n do
		value := generate_seq(value);
		print(value);
	end do;
end proc:

#Test:
Look_and_Say(10);

Output:

                              "1"
                              "11"
                              "21"
                             "1211"
                            "111221"
                            "312211"
                           "13112221"
                          "1113213211"
                        "31131211131221"
                     "13211311123113112211"

Mathematica/Wolfram Language

The function:

 LookAndSay[n_Integer?Positive]:= Reverse @@@ Tally /@ Split @ IntegerDigits @ n // Flatten // FromDigits

takes as input an arbitrary positive integer and generates the next member of the ‘Look and Say’ sequence.

The first example returns the next 12 numbers of the sequence starting with 1:

NestList[LookAndSay, 1, 12] // Column

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221

The second example returns the next 12 numbers of the sequence starting with 7:

NestList[LookAndSay, 7, 12] // Column

7
17
1117
3117
132117
1113122117
311311222117
13211321322117
1113122113121113222117
31131122211311123113322117
132113213221133112132123222117
11131221131211132221232112111312111213322117
31131122211311123113321112131221123113111231121123222117 

Maxima

collect(a) := block(
   [n: length(a), b: [ ], x: a[1], m: 1],
   for i from 2 thru n do
      (if a[i] = x then m: m + 1 else (b: endcons([x, m], b), x: a[i], m: 1)),
   b: endcons([x, m], b)
)$

look_and_say(s) := apply(sconcat, map(lambda([p], sconcat(string(p[2]), p[1])), collect(charlist(s))))$

block([s: "1"], for i from 1 thru 10 do (disp(s), s: look_and_say(s)));
/* "1"
   "11"
   "21"
   "1211"
   "111221"
   "312211"
   "13112221"
   "1113213211"
   "31131211131221"
   "13211311123113112211" */

Implementation treating the sequence as numbers

ciphers(n):=block(makelist(floor(mod(n, 10^(k+1)) / 10^k), k, 0,floor(log(n)/log(10))),reverse(%%));

collect(a) := block(
	   [n: length(ciphers(a)), b: [ ], x: ciphers(a)[1], m: 1],
	   for i from 2 thru n do
	      (if ciphers(a)[i] = x then m: m + 1 else (b: endcons([x, m], b), x: ciphers(a)[i], m: 1)),
	   b: endcons([x, m], b),
	    map(reverse,%%),
	    flatten(%%),
	    at(sum((part(%%,k))*y^(length(%%)-k),k,1,length(%%)),y=10)
	)$

block(i:1,append([i],makelist(i:collect(i),9)),table_form(%%));
/* matrix(
		[1],
		[11],
		[21],
		[1211],
		[111221],
		[312211],
		[13112221],
		[1113213211],
		[31131211131221],
		[13211311123113112211]
	) */

MAXScript

fn lookAndSay num =
(
    local result = ""
    num += " "
    local current = num[1]
    local numReps = 1

    for digit in 2 to num.count do
    (
        if num[digit] != current then
        (
            result += (numReps as string) + current
            numReps = 1
            current = num[digit]
        )
        else
        (
            numReps += 1
        )
    )
    result
)

local num = "1"

for i in 1 to 10 do
(
    print num
    num = lookAndSay num
)

Metafont

vardef lookandsay(expr s) =
string r; r := "";
if string s:
  i := 0;
  forever: exitif not (i < length(s));
    c := i+1;
    forever: exitif ( (substring(c,c+1) of s) <> (substring(i,i+1) of s) );
      c := c + 1;
    endfor
    r := r & decimal (c-i) & substring(i,i+1) of s;
    i := c;
  endfor
fi
r
enddef;

string p; p := "1";
for el := 1 upto 10:
  message p;
  p := lookandsay(p);
endfor

end

Miranda

main :: [sys_message]
main = [Stdout (lay (take 15 (iterate looksay "1")))]

looksay :: [char]->[char]
looksay = concat . map f . split
          where f xs = show (#xs) ++ [hd xs]

split :: [*]->[[*]]
split = foldr f []
        where f x []       = [[x]]
              f x (ys:yss) = (x:ys):yss, if x = hd ys
                           = [x]:ys:yss, otherwise

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

MiniScript

// Look and Say Sequence
repeats = function(digit, string)
	count = 0
	for c in string
		if c != digit then break
		count = count + 1
	end for
	return str(count)
end function
 
numbers = "1"
print numbers
for i in range(1,10) // warning, loop size > 15 gets long numbers very quickly
	number = ""
	position = 0
	while position < numbers.len
		repeatCount = repeats(numbers[position], numbers[position:])
		number = number + repeatCount + numbers[position]
		position = position + repeatCount.val
	end while
	print number
	numbers = number
end for

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Modula-2

MODULE LookAndSay;
FROM InOut IMPORT WriteString, WriteLn;
FROM Strings IMPORT Assign, Length;

CONST 
    MaxSize = 128;
    Steps = 14;

VAR 
    buf1, buf2: ARRAY [0..MaxSize-1] OF CHAR;
    step: CARDINAL;

PROCEDURE LookSay(in: ARRAY OF CHAR; VAR out: ARRAY OF CHAR);
    VAR count, inIdx, outIdx: CARDINAL;
        curChar: CHAR;
BEGIN
    inIdx := 0;
    outIdx := 0;
    WHILE in[inIdx] # CHR(0) DO
        curChar := in[inIdx];
        count := 0;
        REPEAT
            INC(inIdx);
            INC(count);
        UNTIL in[inIdx] # curChar;
        out[outIdx] := CHR(ORD('0') + count);
        out[outIdx+1] := curChar;
        outIdx := outIdx + 2;
    END;
    out[outIdx] := CHR(0);
END LookSay;

BEGIN
    Assign("1", buf1);
    FOR step := 1 TO Steps DO
        WriteString(buf1);
        WriteLn();
        LookSay(buf1, buf2);
        Assign(buf2, buf1);
    END;
END LookAndSay.

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
1113122113121113123112111311222112132113213221133122211311221

NewLisp

;;;	Compute the following number in the sequence
(define (next-number s)
	(let (n 0 c (first s) res "")
		(dostring (x s)
			(if (= (char x) c)	; the iteration variable is the ASCII code
				(++ n)
				(begin
					(setq res (string res n c))
					(setq n 1 c (char x)))))
		(setq res (string res n c))
		res))
;
;;;	Print out the first n+1 numbers, starting from 1
(define (go n)
	(let (s "1")
		(println s)
		(dotimes (x n)
			(setq s (next-number s))
			(println s))))
;
(go 10)

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Nim

iterator lookAndSay(n: int): string =
  var current = "1"
  yield current
  for round in 2..n:
    var ch = current[0]
    var count = 1
    var next = ""
    for i in 1..current.high:
      if current[i] == ch:
        inc count
      else:
        next.add $count & ch
        ch = current[i]
        count = 1
    current = next & $count & ch
    yield current

for s in lookAndSay(12):
  echo s

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

Objective-C

#import <Foundation/Foundation.h>

-(NSString*)lookAndSay:(NSString *)word{
    if (!word) {
        return nil;
    }
    NSMutableString *result = [NSMutableString new];
    
    char repeat = [word characterAtIndex:0];
    int times = 1;
    word = [NSString stringWithFormat:@"%@ ",[word substringFromIndex:1] ];
    
    for (NSInteger index = 0; index < word.length; index++) {
        char actual = [word characterAtIndex:index];
        if (actual != repeat) {
            [result appendFormat:@"%d%c", times, repeat];
            times = 1;
            repeat = actual;
        } else {
            times ++;
        }
    }

    return [result copy];
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    NSString *num = @"1";
    for (int i=1;i<=10;i++) {
        NSLog(@"%@", num);

        num = [self lookAndSay:num];
    }
}

OCaml

Functional

This function computes a see-and-say sequence from the previous one:

let rec seeAndSay = function
  | [], nys -> List.rev nys
  | x::xs, [] -> seeAndSay(xs, [x; 1])
  | x::xs, y::n::nys when x=y -> seeAndSay(xs, y::1+n::nys)
  | x::xs, nys -> seeAndSay(xs, x::1::nys)

It can be used like this:

> let gen n =
    let xs = Array.create n [1] in
    for i=1 to n-1 do
      xs.(i) <- seeAndSay(xs.(i-1), [])
    done;
    xs;;
val gen : int -> int list array = <fun>

> gen 10;;
- : int list array =
  [|[1]; [1; 1]; [2; 1]; [1; 2; 1; 1]; [1; 1; 1; 2; 2; 1]; [3; 1; 2; 2; 1; 1];
    [1; 3; 1; 1; 2; 2; 2; 1]; [1; 1; 1; 3; 2; 1; 3; 2; 1; 1];
    [3; 1; 1; 3; 1; 2; 1; 1; 1; 3; 1; 2; 2; 1];
    [1; 3; 2; 1; 1; 3; 1; 1; 1; 2; 3; 1; 1; 3; 1; 1; 2; 2; 1; 1]|]

With regular expressions in the Str library

#load "str.cma";;

let lookandsay =
  Str.global_substitute (Str.regexp "\\(.\\)\\1*")
                        (fun s -> string_of_int (String.length (Str.matched_string s)) ^
                                  Str.matched_group 1 s)

let () =
  let num = ref "1" in
  print_endline !num;
  for i = 1 to 10 do
    num := lookandsay !num;
    print_endline !num;
  done

With regular expressions in the Pcre library

open Pcre

let lookandsay str =
  let rex = regexp "(.)\\1*" in
  let subs = exec_all ~rex str in
  let ar = Array.map (fun sub -> get_substring sub 0) subs in
  let ar = Array.map (fun s -> String.length s, s.[0]) ar in
  let ar = Array.map (fun (n,c) -> (string_of_int n) ^ (String.make 1 c)) ar in
  let res = String.concat "" (Array.to_list ar) in
  (res)

let () =
  let num = ref(string_of_int 1) in
  for i = 1 to 10 do
    num := lookandsay !num;
    print_endline !num;
  done

run this example with 'ocaml -I +pcre pcre.cma script.ml'

Imperative

(* see http://oeis.org/A005150 *)

let look_and_say s =
let n = String.length s
and buf = Buffer.create 0
and prev = ref s.[0]
and count = ref 0 in
let append () = Buffer.add_char buf (char_of_int (48 + !count));
                Buffer.add_char buf !prev in
String.iter (fun c ->
   if c = !prev then incr count else
   begin
      append ();
      prev := c;
      count := 1
   end
) s;
append ();
Buffer.contents buf;;

(* what about length of successive strings ? *)
let iter f a n =
let rec aux r n v = if n = 0
                    then List.rev(r::v)
                    else aux (f r) (n - 1) (r::v) in
aux a n [];;

let las = iter look_and_say "1";;

(* the first sixty terms *)

List.map (String.length) (las 59);;
(*
   [1; 2; 2; 4; 6; 6; 8; 10; 14; 20; 26; 34; 46; 62; 78; 102; 134; 176; 226;
    302; 408; 528; 678; 904; 1182; 1540; 2012; 2606; 3410; 4462; 5808; 7586;
    9898; 12884; 16774; 21890; 28528; 37158; 48410; 63138; 82350; 107312;
    139984; 182376; 237746; 310036; 403966; 526646; 686646; 894810; 1166642;
    1520986; 1982710; 2584304; 3369156; 4391702; 5724486; 7462860; 9727930;
    12680852]
*)

(* see http://oeis.org/A005341 *)

Oforth

import: mapping

: lookAndSay ( n -- )  
   [ 1 ] #[ dup .cr group map( [#size, #first] ) expand ] times( n ) ;

Output:

for n = 10

[1]
[1, 1]
[2, 1]
[1, 2, 1, 1]
[1, 1, 1, 2, 2, 1]
[3, 1, 2, 2, 1, 1]
[1, 3, 1, 1, 2, 2, 2, 1]
[1, 1, 1, 3, 2, 1, 3, 2, 1, 1]
[3, 1, 1, 3, 1, 2, 1, 1, 1, 3, 1, 2, 2, 1]
[1, 3, 2, 1, 1, 3, 1, 1, 1, 2, 3, 1, 1, 3, 1, 1, 2, 2, 1, 1]

Oz

declare
  %% e.g. "21" -> "1211"
  fun {LookAndSayString S}
     for DigitGroup in {Group S} append:Add do
        {Add {Int.toString {Length DigitGroup}}}
        {Add [DigitGroup.1]}
     end
  end

  %% lazy sequence of integers starting with N
  fun {LookAndSay N}
     fun lazy {Loop S}
        {String.toInt S}|{Loop {LookAndSayString S}}
     end
  in
     {Loop {Int.toString N}}
  end

  %% like Haskell's "group"
  fun {Group Xs}
     case Xs of nil then nil
     [] X|Xr then
	Ys Zs
        {List.takeDropWhile Xr fun {$ W} W==X end ?Ys ?Zs}
     in
        (X|Ys) | {Group Zs}
     end
  end
in
  {ForAll {List.take {LookAndSay 1} 10} Show}

PARI/GP

step(n)={
  my(v=eval(Vec(Str(n))),cur=v[1],ct=1,out="");
  v=concat(v,99);
  for(i=2,#v,
    if(v[i]==cur,
      ct++
    ,
      out=Str(out,ct,cur);
      cur=v[i];
      ct=1
    )
  );
  eval(out)
};
n=1;for(i=1,20,print(n);n=step(n))

Pascal

Works with: Free_Pascal

Works with: Delphi

Library: SysUtils

program LookAndSayDemo(input, output);

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}

uses
  SysUtils;

function LookAndSay(s: string): string;
var
  item: char;
  index: integer;
  count: integer;
begin
  Result := '';
  item := s[1];
  count := 1;
  for index := 2 to length(s) do
    if item = s[index] then
      inc(count)
    else
    begin
      Result := Result + intTostr(count) + item;
      item := s[index];
      count := 1;
    end;
  Result := Result + intTostr(count) + item;
end;

var
  number: string;

begin
  writeln('Press RETURN to continue and ^C to stop.');
  number := '1';
  while not eof(input) do
  begin
   write(number);
   readln;
   number := LookAndSay(number);
  end;
end.

Output:

% ./LookAndSay 
Press RETURN to continue and ^C to stop.

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211^C

Even faster imperative Version Improvement: setlength of result,no inttoStr and using pChar But the Code Alignment is very important.

Works with: Free_Pascal

Library: SysUtils

program LookAndSayDemo(input, output);
{$IFDEF FPC}
  {$Mode Delphi}  // using result
  {$optimization ON}
// i3-4330 3.5 Ghz
//  {$CodeAlign proc=16,loop=8} //2,6 secs
  {$CodeAlign proc=16,loop=1}  //1,6 secs so much faster ???
{$ENDIF}

uses
  SysUtils;
const
  cntChar : array[1..9] of char =
           ('1','2','3','4','5','6','7','8','9');

function LookAndSay2 (const s: string): string;
//using pChar for result
var
  source,
  destin : pChar;
  len,
  idxFrom,
  idxTo :  integer;
  cnt: integer;

  item: char;
begin
  idxFrom := length(s);
  source := @s[1];

  //adjust length of result
  len := round(length(s)* 1.306+10);
  setlength(result,len);
  destin := @result[1];
  dec(destin);

  idxto := 1;
  item := source^;
  inc(source);
  cnt := 1;
  for idxFrom := idxFrom downto 2 do
  begin
    if item <> source^ then
    begin
      destin[idxTo]  := cntChar[cnt];
      destin[idxTo+1]:= item;
      item := source^;
      cnt := 1;
      inc(idxto,2);
    end
    else
      inc(cnt);
    inc(source);
  end;
  destin[idxTo] := cntChar[cnt];
  destin[idxTo+1]:= item;
  setlength(result,idxto+1);
end;

var
  number: string;
  l1,l2,
  i : integer;
begin
  number := '1';
  writeln(number);
  writeln(1:4,length(number):16,1/1:10:6);

  For i := 2 to 70 do
  begin
    l1 := length(number);
    number := LookAndSay2(number);
    l2 := length(number);
    IF i <10 then
      writeln(number);
    writeln(i:4,length(number):16,l2/l1:10:6);
  end;
end.

Output:

1
   1               1  1.000000
11
   2               2  2.000000
21
   3               2  1.000000
1211
   4               4  2.000000
111221
   5               6  1.500000
312211
   6               6  1.000000
13112221
   7               8  1.333333
1113213211
   8              10  1.250000
31131211131221
   9              14  1.400000
  10              20  1.428571
  11              26  1.300000
  12              34  1.307692
  13              46  1.352941
  14              62  1.347826
  15              78  1.258065
  16             102  1.307692
........
  67        81117366  1.303580
  68       105745224  1.303608
  69       137842560  1.303535
  70       179691598  1.303600

real	0m1.639s
user	0m1.593s
sys	0m0.043s

Perl

sub lookandsay {
  my $str = shift;
  $str =~ s/((.)\2*)/length($1) . $2/ge;
  return $str;
}

my $num = "1";
foreach (1..10) {
  print "$num\n";
  $num = lookandsay($num);
}

Using string as a cyclic buffer:

for (local $_ = "1\n"; s/((.)\2*)//s;) {
	print $1;
	$_ .= ($1 ne "\n" and length($1)).$2 
}

Phix

function lookandsay(string s)
    string res = ""
    integer p = s[1], c = 1
    for i=2 to length(s) do
        if p=s[i] then
            c += 1
        else
            res &= sprintf("%d%s",{c,p})
            p = s[i]
            c = 1
        end if
    end for
    res &= sprintf("%d%s",{c,p})
    return res
end function
 
string s = "1"
?s
for i=1 to 10 do
    s = lookandsay(s)
    ?s
end for

Output:

"1"
"11"
"21"
"1211"
"111221"
"312211"
"13112221"
"1113213211"
"31131211131221"
"13211311123113112211"
"11131221133112132113212221"

PHP

<?php

function lookAndSay($str) {

	return preg_replace_callback('#(.)\1*#', function($matches) {
	
		return strlen($matches[0]).$matches[1];
	}, $str);
}

$num = "1";

foreach(range(1,10) as $i) {

	echo $num."<br/>";
	$num = lookAndSay($num);
}

?>

Picat

go => 
   S1 = "1",
   foreach(_ in 1..11) 
      println(S1),
      S1 := runs(S1)
   end,
   println(S1),
   nl.

runs(X) = V =>
  S = "",
  Last = X[1], 
  C = 1,
  foreach(I in 2..X.length)
    if X[I] == Last then
       C := C + 1
    else 
       S := S ++ C.to_string() ++ [X[I-1]],
       C := 1,  
       Last := X[I]
    end
  end,
  V = S ++ C.to_string() ++ [Last].

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

PicoLisp

(de las (Lst)
   (make
      (while Lst
         (let (N 1  C)
            (while (= (setq C (pop 'Lst)) (car Lst))
               (inc 'N) )
            (link N C) ) ) ) )

Usage:

: (las (1))
-> (1 1)
: (las @)
-> (2 1)
: (las @)
-> (1 2 1 1)
: (las @)
-> (1 1 1 2 2 1)
: (las @)
-> (3 1 2 2 1 1)
: (las @)
-> (1 3 1 1 2 2 2 1)
: (las @)
-> (1 1 1 3 2 1 3 2 1 1)
: (las @)
-> (3 1 1 3 1 2 1 1 1 3 1 2 2 1)

PL/M

100H:
BDOS: PROCEDURE (FN, ARG); DECLARE FN BYTE, ARG ADDRESS; GO TO 5; END BDOS;
EXIT: PROCEDURE; CALL BDOS(0,0); END EXIT;
PRINT: PROCEDURE (S); DECLARE S ADDRESS; CALL BDOS(9,S); END PRINT;

COPY$STRING: PROCEDURE (SRC, DEST);
    DECLARE (SRC, DEST, I) ADDRESS;
    DECLARE (S BASED SRC, D BASED DEST) BYTE;
    I = 0;
    DO WHILE S(I) <> '$';
        D(I) = S(I);
        I = I + 1;
    END;
    D(I) = '$';
END COPY$STRING;

COUNT: PROCEDURE (POS) BYTE;
    DECLARE POS ADDRESS, (I, P BASED POS) BYTE;
    I = 1;
    DO WHILE P(I) = P;
        I = I + 1; 
    END;
    RETURN I;
END COUNT;

LOOK$SAY: PROCEDURE (OLD, NEW);
    DECLARE (OLD, NEW) ADDRESS;
    DECLARE (O BASED OLD, N BASED NEW, C) BYTE;
    DO WHILE O <> '$';
        C = COUNT(OLD);
        N = O;
        N(1) = C + '0';
        NEW = NEW + 2;
        OLD = OLD + C;
    END;
    N = '$';
END LOOK$SAY;
    
DECLARE STEPS LITERALLY '15';
DECLARE BUF$SIZE LITERALLY '128';

DECLARE BUFR1 (BUF$SIZE) BYTE INITIAL ('1$');
DECLARE BUFR2 (BUF$SIZE) BYTE;
DECLARE I BYTE;

DO I=1 TO STEPS;
    CALL PRINT(.BUFR1);
    CALL PRINT(.(13,10,'$'));
    CALL LOOK$SAY(.BUFR1, .BUFR2);
    CALL COPY$STRING(.BUFR2, .BUFR1);
END;
CALL EXIT;
EOF

Output:

1
11
12
1121
122111
112213
12221131
1123123111
12213111213113
11221131132111311231
12221231123121133112213111
1123112131122131112112321222113113
1221311221113112221131132112213121112312311231
11221131122213311223123112312112221131112113213111213112213111
122212311223113212223111213112213111211223123113211231211131132111311222113113

PowerBASIC

This uses the RLEncode function from the PowerBASIC Run-length encoding entry.

FUNCTION RLEncode (i AS STRING) AS STRING
    DIM tmp1 AS STRING, tmp2 AS STRING, outP AS STRING
    DIM Loop0 AS LONG, count AS LONG

    FOR Loop0 = 1 TO LEN(i)
        tmp1 = MID$(i, Loop0, 1)
        IF tmp1 <> tmp2 THEN
            IF count > 1 THEN
                outP = outP & TRIM$(STR$(count)) & tmp2
                tmp2 = tmp1
                count = 1
            ELSEIF 0 = count THEN
                tmp2 = tmp1
                count = 1
            ELSE
                outP = outP & "1" & tmp2
                tmp2 = tmp1
            END IF
        ELSE
            INCR count
        END IF
    NEXT

    outP = outP & TRIM$(STR$(count)) & tmp2
    FUNCTION = outP
END FUNCTION

FUNCTION lookAndSay(BYVAL count AS LONG) AS STRING
    DIM iii AS STRING, tmp AS STRING

    IF count > 1 THEN
        iii = lookAndSay(count - 1)
    ELSEIF count < 2 THEN
        iii = "1"
    END IF

    tmp = RLEncode(iii)
    lookAndSay = tmp
END FUNCTION

FUNCTION PBMAIN () AS LONG
    DIM v AS LONG
    v = VAL(INPUTBOX$("Enter a number."))
    MSGBOX lookAndSay(v)
END FUNCTION

PowerShell

function Get-LookAndSay ($n = 1) {
    $re = [regex] '(.)\1*'
    $ret = ""
    foreach ($m in $re.Matches($n)) {
        $ret += [string] $m.Length + $m.Value[0]
    }
    return $ret
}

function Get-MultipleLookAndSay ($n) {
    if ($n -eq 0) {
        return @()
    } else {
        $a = 1
        $a
        for ($i = 1; $i -lt $n; $i++) {
            $a = Get-LookAndSay $a
            $a
        }
    }
}

Output:

PS> Get-MultipleLookAndSay 8
1
11
21
1211
111221
312211
13112221
1113213211

Prolog

Works with SWI-Prolog.

look_and_say(L) :-
	maplist(write, L), nl,
	encode(L, L1),
	look_and_say(L1).

% This code is almost identical to the code of "run-length-encoding" 
encode(In, Out) :-
	packList(In, R1),
	append(R1,Out).


% use of library clpfd allows packList(?In, ?Out) to works
% in both ways In --> Out and In <-- Out.

:- use_module(library(clpfd)).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% ?- packList([a,a,a,b,c,c,c,d,d,e], L).
%  L = [[3,a],[1,b],[3,c],[2,d],[1,e]] .
% ?- packList(R,  [[3,a],[1,b],[3,c],[2,d],[1,e]]).
% R = [a,a,a,b,c,c,c,d,d,e] .
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
packList([],[]).

packList([X],[[1,X]]) :- !.


packList([X|Rest],[XRun|Packed]):-
    run(X,Rest, XRun,RRest),
    packList(RRest,Packed).


run(Var,[],[1,Var],[]).

run(Var,[Var|LRest],[N1, Var],RRest):-
    N #> 0,
    N1 #= N + 1,
    run(Var,LRest,[N, Var],RRest).


run(Var,[Other|RRest], [1,Var],[Other|RRest]):-
    dif(Var,Other).

Output:

 ?- look_and_say([1]).
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
..........................

Pure

using system;

// Remove the trailing "L" from the string representation of bigints.
__show__ x::bigint = init (str x);

say x = val $ strcat $ map (sprintf "%d%s") $ look $ chars $ str x with
  look [] = [];
  look xs@(x:_) = (#takewhile (==x) xs,x) : look (dropwhile (==x) xs);
end;

iteraten 5 say 1; // [1,11,21,1211,111221]

// This prints the entire sequence, press Ctrl-C to abort.
do (puts.str) (iterate say 1);

PureBasic

If OpenConsole()
  Define i, j, cnt, txt$, curr$, result$
  Print("Enter start sequence: "): txt$=Input()
  Print("How many repetitions: "): i=Val(Input())
  ;
  PrintN(#CRLF$+"Sequence:"+#CRLF$+txt$)
  Repeat
    j=1
    result$=""
    Repeat
      curr$=Mid(txt$,j,1)
      cnt=0
      Repeat
        cnt+1
        j+1
      Until Mid(txt$,j,1)<>curr$
      result$+Str(cnt)+curr$
    Until j>Len(txt$)    
    PrintN(result$)
    txt$=result$
    i-1
  Until i<=0
  ;
  PrintN(#CRLF$+"Press ENTER to exit."): Input()
  CloseConsole()
EndIf

Output:

 Enter start sequence: 1
 How many repetitions: 7
 
 Sequence:
 1
 11
 21
 1211
 111221
 312211
 13112221
 1113213211

Python

Translation of: C sharp – C#

def lookandsay(number):
    result = ""

    repeat = number[0]
    number = number[1:]+" "
    times = 1

    for actual in number:
        if actual != repeat:
            result += str(times)+repeat
            times = 1
            repeat = actual
        else:
            times += 1

    return result

num = "1"

for i in range(10):
    print num
    num = lookandsay(num)

Functional

Works with: Python version 2.4+

>>> from itertools import groupby
>>> def lookandsay(number):
	return ''.join( str(len(list(g))) + k
		        for k,g in groupby(number) )

>>> numberstring='1'
>>> for i in range(10):
	print numberstring
	numberstring = lookandsay(numberstring)

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

As a generator

>>> from itertools import groupby, islice
>>> 
>>> def lookandsay(number='1'):
	while True:
		yield number
		number = ''.join( str(len(list(g))) + k
		                  for k,g in groupby(number) )

		
>>> print('\n'.join(islice(lookandsay(), 10)))
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Using regular expressions

Translation of: Perl

import re

def lookandsay(str):
    return re.sub(r'(.)\1*', lambda m: str(len(m.group(0))) + m.group(1), str)

num = "1"
for i in range(10):
    print num
    num = lookandsay(num)

Q

las:{{raze string[count@'x],'@'[;0]x:where[differ x]_x}\[x;1#"1"]}
las 8

Output:

,"1"
"11"
"21"
"1211"
"111221"
"312211"
"13112221"
"1113213211"
"31131211131221"

Quackery

  [ stack ] is instances 

  [ 1 instances put
    $ "" swap
    behead swap space join
    witheach
      [ 2dup != iff
        [ rot instances share
          number$ join 
          rot join swap
          1 instances replace ]
        else 
          [ drop 
            1 instances tally ] ]
    drop instances release ]       is lookandsay ( $ --> $ )

  $ "1"
  15 times 
    [ dup echo$ cr
      lookandsay ]
  echo$ cr

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221
132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211

R

Returning the value as an integer limits how long the sequence can get, so the option for integer or character return values are provided.

look.and.say <- function(x, return.an.int=FALSE)
{
   #convert number to character vector
   xstr <- unlist(strsplit(as.character(x), ""))
   #get run length encoding   
   rlex <- rle(xstr)
   #form new string
   odds <- as.character(rlex$lengths)
   evens <- rlex$values
   newstr <- as.vector(rbind(odds, evens))
   #collapse to scalar
   newstr <- paste(newstr, collapse="")
   #convert to number, if desired
   if(return.an.int) as.integer(newstr) else newstr
}

Example usage:

x <- 1
for(i in 1:10)
{
   x <- look.and.say(x)
   print(x)
}

Racket

#lang racket

(define (encode str)
  (regexp-replace* #px"(.)\\1*" str (lambda (m c) (~a (string-length m) c))))

(define (look-and-say-sequence n)
  (reverse (for/fold ([r '("1")]) ([n n]) (cons (encode (car r)) r))))

(for-each displayln (look-and-say-sequence 10))

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

Raku

(formerly Perl 6)

Works with: rakudo version 2018.03

In Raku it is natural to avoid explicit loops; rather we use the sequence operator to define a lazy infinite sequence. We'll print the first 15 values here.

.say for ('1', *.subst(/(.)$0*/, { .chars ~ .[0] }, :g) ... *)[^15];

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

REXX

Programming note:   this version works with any string   (a null is assumed, which causes   1   to be used).

If a negative number is specified (the number of iterations to be used for the calculations), only the length of
the number (or character string) is shown.

simple version

/*REXX program displays the sequence (and/or lengths) for the    look and say    series.*/
parse arg N ! .                                  /*obtain optional arguments from the CL*/
if N=='' | N==","  then N= 20                    /*Not specified?  Then use the default.*/
if !=='' | !==","  then !=  1                    /* "      "         "   "   "     "    */

     do j=1  for abs(N)                          /*repeat a number of times to show NUMS*/
     if j\==1  then != lookNsay(!)               /*invoke function to calculate next #. */
     if N<0    then say 'length['j"]:" length(!) /*Also,  display the sequence's length.*/
               else say '['j"]:"      !          /*display the number to the terminal.  */
     end   /*j*/
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
lookNsay: procedure; parse arg x,,$              /*obtain the (passed) argument  {X}.   */
          fin = '0'x                             /*use unique character to end scanning.*/
          x= x || fin                            /*append the  FIN  character to string.*/
                      do k=1  by 0               /*now,  process the given sequence.    */
                      y=  substr(x, k, 1)        /*pick off one character to examine.   */
                      if y== fin  then return $  /*if we're at the end, then we're done.*/
                      _= verify(x, y, , k) - k   /*see how many characters we have of Y.*/
                      $= $  ||  _  ||  y         /*build the  "look and say"  sequence. */
                      k= k + _                   /*now, point to the next character.    */
                      end   /*k*/

output   when using the default input values of:     20   1

[1]: 1
[2]: 11
[3]: 21
[4]: 1211
[5]: 111221
[6]: 312211
[7]: 13112221
[8]: 1113213211
[9]: 31131211131221
[10]: 13211311123113112211
[11]: 11131221133112132113212221
[12]: 3113112221232112111312211312113211
[13]: 1321132132111213122112311311222113111221131221
[14]: 11131221131211131231121113112221121321132132211331222113112211
[15]: 311311222113111231131112132112311321322112111312211312111322212311322113212221
[16]: 132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211
[17]: 11131221131211132221232112111312212321123113112221121113122113111231133221121321132132211331121321231231121113122113322113111221131221
[18]: 31131122211311123113321112131221123113112211121312211213211321322112311311222113311213212322211211131221131211132221232112111312111213111213211231131122212322211331222113112211
[19]: 1321132132211331121321231231121113112221121321132122311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112111331121113122112132113213211121332212311322113212221
[20]: 11131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113112221121321133112132112312321123113112221121113122113121113123112112322111213211322211312113211

output   when using the input values of:     17   ggg

[1]: ggg
[2]: 3g
[3]: 131g
[4]: 1113111g
[5]: 3113311g
[6]: 132123211g
[7]: 11131211121312211g
[8]: 31131112311211131122211g
[9]: 132113311213211231132132211g
[10]: 11131221232112111312211213211312111322211g
[11]: 3113112211121312211231131122211211131221131112311332211g
[12]: 1321132122311211131122211213211321322112311311222113311213212322211g
[13]: 1113122113121122132112311321322112111312211312111322211213211321322123211211131211121332211g
[14]: 31131122211311122122111312211213211312111322211231131122211311123113322112111312211312111322111213122112311311123112112322211g
[15]: 132113213221133122112231131122211211131221131112311332211213211321322113311213212322211231131122211311123113223112111311222112132113311213211221121332211g
[16]: 11131221131211132221231122212213211321322112311311222113311213212322211211131221131211132221232112111312111213322112132113213221133112132113221321123113213221121113122123211211131221222112112322211g
[17]: 31131122211311123113321112132132112211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112112322211211131221131211132221232112111312211322111312211213211312111322211231131122111213122112311311221132211221121332211g

output   when using the input value of:     -60

(Shown at three-quarter size.)

length[1]: 1
length[2]: 2
length[3]: 2
length[4]: 4
length[5]: 6
length[6]: 6
length[7]: 8
length[8]: 10
length[9]: 14
length[10]: 20
length[11]: 26
length[12]: 34
length[13]: 46
length[14]: 62
length[15]: 78
length[16]: 102
length[17]: 134
length[18]: 176
length[19]: 226
length[20]: 302
length[21]: 408
length[22]: 528
length[23]: 678
length[24]: 904
length[25]: 1182
length[26]: 1540
length[27]: 2012
length[28]: 2606
length[29]: 3410
length[30]: 4462
length[31]: 5808
length[32]: 7586
length[33]: 9898
length[34]: 12884
length[35]: 16774
length[36]: 21890
length[37]: 28528
length[38]: 37158
length[39]: 48410
length[40]: 63138
length[41]: 82350
length[42]: 107312
length[43]: 139984
length[44]: 182376
length[45]: 237746
length[46]: 310036
length[47]: 403966
length[48]: 526646
length[49]: 686646
length[50]: 894810
length[51]: 1166642
length[52]: 1520986
length[53]: 1982710
length[54]: 2584304
length[55]: 3369156
length[56]: 4391702
length[57]: 5724486
length[58]: 7462860
length[59]: 9727930
length[60]: 12680852

faster version

This version appends the generated parts of the sequence, and after it gets to a certain size (chunkSize),
it appends the sequence generated (so far) to the primary sequence, and starts with a null sequence.
This avoids appending a small character string to a growing larger and larger character string.

/*REXX program displays the sequence (and/or lengths) for the    look and say    series.*/
parse arg N ! .                                  /*obtain optional arguments from the CL*/
if N=='' | N==","  then N= 20                    /*Not specified?  Then use the default.*/
if !=='' | !==","  then !=  1                    /* "      "         "   "   "     "    */
                                                 /* [↑]  !:   starting char for the seq.*/
     do j=1  for abs(N)                          /*repeat a number of times to show NUMS*/
     if j\==1  then != lookNsay(!)               /*invoke function to calculate next #. */
     if N<0    then say 'length['j"]:" length(!) /*Also,  display the sequence's length.*/
               else say '['j"]:"      !          /*display the number to the terminal.  */
     end   /*j*/
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
lookNsay: procedure; parse arg x,,$ !            /*obtain the (passed) argument  {X}.   */
          chSize= 1000                           /*define a sensible chunk size.        */
          fin = '0'x                             /*use unique character to end scanning.*/
          x= x || fin                            /*append the  FIN  character to string.*/
               do k=1  by 0                      /*now,  process the given sequence.    */
                  y=  substr(x, k, 1)            /*pick off one character to examine.   */
               if y==fin  then return $          /*if we're at the end, then we're done.*/
               _= verify(x, y, , k)  - k         /*see how many characters we have of Y.*/
               $= $  ||  _  ||  y                /*build the  "look and say"  sequence. */
               k= k + _                          /*now, point to the next character.    */
               if length($)<chSize  then iterate /*Less than chunkSize?  Then keep going*/
               != !  ||  $                       /*append   $   to the  !  string.      */
               $=                                /*now,  start   $   from scratch.      */
               chSize= chSize + 100              /*bump the  chunkSize (length) counter.*/
               end   /*k*/
         return ! || $                           /*return the ! string plus the $ string*/

output   is identical to the 1^st REXX version   (the simple version).

Refal

$ENTRY Go {
    = <Prout <Sequence 10 1>>;
};

Sequence {
    0   e.seq = ;
    s.N e.seq = <Prout e.seq>
                <Sequence <- s.N 1> <LookSay e.seq>>;
}

LookSay {
    = ;
    e.1, 
        <First <Group e.1> e.1>: (e.group) e.rest,
        <Lenw e.group>: s.num s.item e.discard =
        s.num s.item <LookSay e.rest>;
}

Group {
    s.1 s.1 e.rest = <+ 1 <Group s.1 e.rest>>;
    s.1 e.rest     = 1;
};

Output:

1
1 1
2 1
1 2 1 1
1 1 1 2 2 1
3 1 2 2 1 1
1 3 1 1 2 2 2 1
1 1 1 3 2 1 3 2 1 1
3 1 1 3 1 2 1 1 1 3 1 2 2 1
1 3 2 1 1 3 1 1 1 2 3 1 1 3 1 1 2 2 1 1

Ring

number = "1"
for nr = 1 to 10
    number = lookSay(number)
    see number + nl
next
 
func lookSay n
     i = 0 j = 0 c="" o=""
     i = 1
     while i <= len(n)
           c = substr(n,i,1)
           j = i + 1
           while substr(n,j,1) = c
                 j += 1
           end
           o += string(j-i) + c
           i = j
      end
      return o

RPL

Works with: Halcyon Calc version 4.2.7

Code	Comments
≪ DUP 1 DUP SUB → str char ≪ 2 WHILE str OVER DUP SUB char == REPEAT 1 + END DUP 1 - →STR char + str ROT OVER SIZE SUB ≫ ≫ 'CountCut' STO ≪ "" 1 CF SWAP WHILE 1 FC? REPEAT CountCut IF DUP "" == THEN 1 SF END ROT ROT + SWAP END DROP ≫ 'LKSAY' STO	( "####" -- "n#" "remainder" ) Count occurences of 1st char Build "n#" Extract remaining string ("M(n)" -- "M(n+1)" ) Initialize M(n+1) and flag set flag if at end of "M(n)" build "M(n+1)" Forget "M(n)"

The following line of code delivers what is required:

≪ {} 1 "1" 12 START SWAP OVER + SWAP LKSAY NEXT DROP ≫

Output:

1: { "1" "11" "21" "1211" "111221" "312211" "13112221" "1113213211" "31131211131221" "13211311123113112211" "11131221133112132113212221" "3113112221232112111312211312113211" }

Ruby

The simplest one:

class String
  def look_and_say
    gsub(/(.)\1*/){|s| s.size.to_s + s[0]}
  end
end

ss = '1'
12.times {puts ss; ss = ss.look_and_say}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

Translation of: Perl

def lookandsay(str)
  str.gsub(/(.)\1*/) {$&.length.to_s + $1}
end

num = "1"
10.times do
  puts num
  num = lookandsay(num)
end

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Using Enumerable#chunk

def lookandsay(str)
  str.chars.chunk{|c| c}.map{|c,x| [x.size, c]}.join
end

puts num = "1"
9.times do
  puts num = lookandsay(num)
end

The output is the same above.

Without regular expression:

# Adding clusterization (http://apidock.com/rails/Enumerable/group_by)
module Enumerable
  # clumps adjacent elements together
  # >> [2,2,2,3,3,4,2,2,1].cluster
  # => [[2, 2, 2], [3, 3], [4], [2, 2], [1]]
  def cluster
    cluster = []
    each do |element|
      if cluster.last && cluster.last.last == element
        cluster.last << element
      else
        cluster << [element]
      end
    end
    cluster
  end
end

Using Array#cluster defined above:

def print_sequence(input_sequence, seq=10)
  return unless seq > 0
  puts input_sequence.join
  result_array = input_sequence.cluster.map do |cluster|
    [cluster.count, cluster.first]
  end
  print_sequence(result_array.flatten, seq-1)
end

print_sequence([1])

The output is the same above.

Rust

fn next_sequence(in_seq: &[i8]) -> Vec<i8> {
    assert!(!in_seq.is_empty());

    let mut result = Vec::new();
    let mut current_number = in_seq[0];
    let mut current_runlength = 1;

    for i in &in_seq[1..] {
        if current_number == *i {
            current_runlength += 1;
        } else {
            result.push(current_runlength);
            result.push(current_number);
            current_runlength = 1;
            current_number = *i;
        }
    }
    result.push(current_runlength);
    result.push(current_number);
    result
}

fn main() {
    let mut seq = vec![1];

    for i in 0..10 {
        println!("Sequence {}: {:?}", i, seq);
        seq = next_sequence(&seq);
    }
}

Output:

Sequence 0: [1]
Sequence 1: [1, 1]
Sequence 2: [2, 1]
Sequence 3: [1, 2, 1, 1]
Sequence 4: [1, 1, 1, 2, 2, 1]
Sequence 5: [3, 1, 2, 2, 1, 1]
Sequence 6: [1, 3, 1, 1, 2, 2, 2, 1]
Sequence 7: [1, 1, 1, 3, 2, 1, 3, 2, 1, 1]
Sequence 8: [3, 1, 1, 3, 1, 2, 1, 1, 1, 3, 1, 2, 2, 1]
Sequence 9: [1, 3, 2, 1, 1, 3, 1, 1, 1, 2, 3, 1, 1, 3, 1, 1, 2, 2, 1, 1]

Sather

class MAIN is
   look_and_say!: STR is
      current ::= "1";
      loop
         yield current;
         buf ::= #FSTR;
         last ::= current[0];
         count ::= 0;
         loop
            ch ::= current.elt!;
            if ch /= last then
               buf := buf + count + last;
               last := ch; count := 1;
            else
               count := count + 1;
            end;
         end;
         current := (buf + count + last).str;
      end;
   end;

   main is
      loop 12.times!;
         #OUT+ look_and_say! + "\n";
      end;
   end;
end;

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

Scala

Recursive

import scala.annotation.tailrec

object LookAndSay extends App {

  loop(10, "1")

  @tailrec
  private def loop(n: Int, num: String): Unit = {
    println(num)
    if (n <= 0) () else loop(n - 1, lookandsay(num))
  }

  private def lookandsay(number: String): String = {
    val result = new StringBuilder

    @tailrec
    def loop(numberString: String, repeat: Char, times: Int): String =
      if (numberString.isEmpty) result.toString()
      else if (numberString.head != repeat) {
        result.append(times).append(repeat)
        loop(numberString.tail, numberString.head, 1)
      } else loop(numberString.tail, numberString.head, times + 1)

    loop(number.tail + " ", number.head, 1)
  }

}

Output:

See it running in your browser by (JavaScript, non JVM) or by Scastie (JVM).

using Iterator

Library: Scala

def lookAndSay(seed: BigInt) = {
  val s = seed.toString
  ( 1 until s.size).foldLeft((1, s(0), new StringBuilder)) {
    case ((len, c, sb), index) if c != s(index) => sb.append(len); sb.append(c); (1, s(index), sb)
    case ((len, c, sb), _) => (len + 1, c, sb)
  } match {
    case (len, c, sb) => sb.append(len); sb.append(c); BigInt(sb.toString)
  }
}

def lookAndSayIterator(seed: BigInt) = Iterator.iterate(seed)(lookAndSay)

using Stream

object Main extends App {

  def lookAndSay(previous: List[BigInt]): Stream[List[BigInt]] = {

    def next(num: List[BigInt]): List[BigInt] = num match {
      case Nil => Nil
      case head :: Nil => 1 :: head :: Nil
      case head :: tail =>
        val size = (num takeWhile (_ == head)).size
        List(BigInt(size), head) ::: next(num.drop(size))
    }
    val x = next(previous)
    x #:: lookAndSay(x)
  }

  (lookAndSay(1 :: Nil) take 10).foreach(s => println(s.mkString("")))
}

Seed7

$ include "seed7_05.s7i";

const func string: lookAndSay (in integer: level, in string: stri) is func
  result
    var string: lookAndSay is "";
  local
    var integer: index is 2;
  begin
    if level = 1 then
      if stri <> "" then
        while index <= length(stri) and stri[index] = stri[1] do
          incr(index);
        end while;
        lookAndSay := str(pred(index)) & stri[1 len 1] & lookAndSay(level, stri[index ..]);
      end if;
    else
      lookAndSay := lookAndSay(1, lookAndSay(pred(level), stri));
    end if;
  end func;

const proc: main is func
  local
    var integer: level is 0;
  begin
    for level range 1 to 14 do
      writeln(lookAndSay(level, "1"));
    end for;
  end func;

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

SETL

program looksay;
    s := "1";
    loop for i in [1..10] do
        print(s);
        s := looksay(s);
    end loop;

    proc looksay(s);
        loop for c in s do;
            if cur /= c then
                if count /= om then
                    r +:= count + cur;
                end if;
                cur := c;
                count := 1;
            else
                count +:= 1;
            end if;
        end loop;
        r +:= count + cur;
        return r;
    end proc;
end looksay;

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Sidef

Translation of: Perl

func lookandsay(str) {
    str.gsub(/((.)\2*)/, {|a,b| a.len.to_s + b });
}

var num = "1";
{
  say num;
  num = lookandsay(num);
} * 10;

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Smalltalk

Works with: GNU Smalltalk

String extend [
  lookAndSay [ |anElement nextElement counter coll newColl|
     coll := (self asOrderedCollection).
     newColl := OrderedCollection new.
     counter := 0.
     anElement := (coll first).
     [ coll size > 0 ]
     whileTrue: [
        nextElement := coll removeFirst.
	( anElement == nextElement ) ifTrue: [
           counter := counter + 1.
        ] ifFalse: [
	  newColl add: (counter displayString).
	  newColl add: (anElement asString).
	  anElement := nextElement.
	  counter := 1.
        ]
     ].
     newColl add: (counter displayString).
     newColl add: (anElement asString).
     ^(newColl join)
  ]
].

|r|
r := '1'.
10 timesRepeat: [
  r displayNl.
  r := r lookAndSay.
]

Works with: Pharo

String compile:
  'lookAndSay |anElement nextElement counter coll newColl|
     coll := (self asOrderedCollection).
     newColl := OrderedCollection new.
     counter := 0.
     anElement := (coll first).
     [ coll size > 0 ]
     whileTrue: [
        nextElement := coll removeFirst.
	( anElement == nextElement ) ifTrue: [
           counter := counter + 1.
        ] ifFalse: [
	  newColl add: (counter displayString).
	  newColl add: (anElement asString).
	  anElement := nextElement.
	  counter := 1.
        ]
     ].
     newColl add: (counter displayString).
     newColl add: (anElement asString).
     ^('''' join: newColl)'
  classified: 'toys'.
 
result := OrderedCollection new.
r := '1'.
result add: r.
result addAll: ((1 to: 10) collect: [ :i |
  r := r lookAndSay.
]).
result.

Output:

   an OrderedCollection('1' '11' '21' '1211' '111221' '312211' '13112221' '1113213211' '31131211131221' '13211311123113112211' '11131221133112132113212221')

SNOBOL4

Works with: Macro Spitbol

Works with: Snobol4+

Works with: CSnobol

The look-and-say sequence is an iterative run-length string encoding. So looksay( ) is just a wrapper around the Run-length Encoding task. This is by far the easiest solution.

*       # Encode RLE
        define('rle(str)c,n') :(rle_end)
rle     str len(1) . c :f(return)
        str span(c) @n =
        rle = rle n c :(rle)
rle_end

*       # First m members of sequence with seed n 
        define('looksay(n,m)') :(looksay_end)
looksay output = n; m = gt(m,1) m - 1 :f(return)
        n = rle(n) :(looksay)
looksay_end        

*       Test and display        
        looksay(1,10)
end

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

SQL

DROP VIEW delta;
CREATE VIEW delta AS
    SELECT sequence1.v AS x,
           (sequence1.v<>sequence2.v)*sequence1.c AS v,
           sequence1.c AS c
      FROM sequence AS sequence1,
           sequence AS sequence2
     WHERE sequence1.c = sequence2.c+1;

DROP VIEW rle0;
CREATE VIEW rle0 AS
    SELECT delta2.x AS x,
           SUM(delta2.v) AS v,
           delta2.c AS c
      FROM delta AS delta1,
           delta as delta2
     WHERE delta1.c >= delta2.c
  GROUP BY delta1.c;

DROP VIEW rle1;
CREATE VIEW rle1 AS
    SELECT sum(x)/x AS a,
           x AS b,
           c AS c
      FROM rle0
  GROUP BY v;

DROP VIEW rle2;
CREATE VIEW rle2 AS
    SELECT a as v, 1 as o, 2*c+0 as c FROM rle1 UNION
    SELECT b as v, 1 as o, 2*c+1 as c FROM rle1;

DROP VIEW normed;
CREATE VIEW normed AS
    SELECT r1.v as v, SUM(r2.o) as c
      FROM rle2 AS r1,
           rle2 AS r2
     WHERE r1.c >= r2.c
  GROUP BY r1.c;

DROP TABLE rle;
CREATE TABLE rle(v int, c int);
INSERT INTO rle SELECT * FROM normed ORDER BY c;

DELETE FROM sequence;
INSERT INTO sequence VALUES(-1,0);
INSERT INTO sequence SELECT * FROM rle;

Usage:

% sqlite3 
SQLite version 3.4.0
Enter ".help" for instructions
sqlite> CREATE TABLE sequence(v int, c int);
sqlite> INSERT INTO sequence VALUES(-1,0);
sqlite> INSERT INTO sequence VALUES(1,1);
sqlite> SELECT * FROM sequence;
-1|0
1|1
sqlite> .read look.sql
sqlite> SELECT * FROM sequence;
-1|0
1|1
1|2
sqlite> .read look.sql
sqlite> SELECT * FROM sequence;
-1|0
2|1
1|2
sqlite> .read look.sql
sqlite> SELECT * FROM sequence;
-1|0
1|1
2|2
1|3
1|4
sqlite> .read look.sql
sqlite> SELECT * FROM sequence;
-1|0
1|1
1|2
1|3
2|4
2|5
1|6

SQL PL

Works with: Db2 LUW

version 9.7 or higher.

With SQL PL:

SET SERVEROUTPUT ON @

BEGIN
 DECLARE NMBR VARCHAR(100) DEFAULT '1';
 DECLARE J SMALLINT DEFAULT 1;

 CALL DBMS_OUTPUT.PUT_LINE(NMBR);
 WHILE (J < 10) DO
  BEGIN
  DECLARE I SMALLINT;
  DECLARE SIZE SMALLINT;
  DECLARE ACTUAL CHAR(1);
  DECLARE REPEAT CHAR(1);
  DECLARE RESULT VARCHAR(100);
  DECLARE TIMES SMALLINT;

  SET REPEAT = SUBSTR(NMBR, 1, 1);
  SET NMBR = SUBSTR(NMBR, 2) || ' ';
  SET TIMES = 1;
  SET I = 1;
  SET SIZE = LENGTH(NMBR);

  WHILE (I <= SIZE) DO
   SET ACTUAL = SUBSTR(NMBR, I, 1);
   IF (ACTUAL <> REPEAT) THEN
    SET RESULT = COALESCE(RESULT, '') || TIMES || '' || REPEAT;
    SET TIMES = 1;
    SET REPEAT = ACTUAL;
   ELSE
    SET TIMES = TIMES + 1;
   END IF;
   SET I = I + 1;
  END WHILE;

  CALL DBMS_OUTPUT.PUT_LINE(RESULT);
  SET NMBR = RESULT;
  END ;
  SET J = J + 1;
 END WHILE;
END @

Output:

db2 => BEGIN
...
db2 (cont.) => END @
DB20000I  The SQL command completed successfully.

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Swift

Translation of: Rust

func lookAndSay(_ seq: [Int]) -> [Int] {
  var result = [Int]()
  var cur = seq[0]
  var curRunLength = 1

  for i in seq.dropFirst() {
    if cur == i {
      curRunLength += 1
    } else {
      result.append(curRunLength)
      result.append(cur)
      curRunLength = 1
      cur = i
    }
  }

  result.append(curRunLength)
  result.append(cur)

  return result
}

var seq = [1]

for i in 0..<10 {
  print("Seq \(i): \(seq)")
  seq = lookAndSay(seq)
}

Output:

Seq 0: [1]
Seq 1: [1, 1]
Seq 2: [2, 1]
Seq 3: [1, 2, 1, 1]
Seq 4: [1, 1, 1, 2, 2, 1]
Seq 5: [3, 1, 2, 2, 1, 1]
Seq 6: [1, 3, 1, 1, 2, 2, 2, 1]
Seq 7: [1, 1, 1, 3, 2, 1, 3, 2, 1, 1]
Seq 8: [3, 1, 1, 3, 1, 2, 1, 1, 1, 3, 1, 2, 2, 1]
Seq 9: [1, 3, 2, 1, 1, 3, 1, 1, 1, 2, 3, 1, 1, 3, 1, 1, 2, 2, 1, 1]

Tcl

proc lookandsay n {
    set new ""
    while {[string length $n] > 0} {
        set char [string index $n 0]
        for {set count 1} {[string index $n $count] eq $char} {incr count} {}
        append new $count $char
        set n [string range $n $count end]
    }
    interp alias {} next_lookandsay {} lookandsay $new
    return $new
}

puts 1                 ;# ==> 1
puts [lookandsay 1]    ;# ==> 11
puts [next_lookandsay] ;# ==> 21
puts [next_lookandsay] ;# ==> 1211
puts [next_lookandsay] ;# ==> 111221
puts [next_lookandsay] ;# ==> 312211

Alternatively, with coroutines:

Works with: Tcl version 8.6

proc seq_lookandsay {n {coroName next_lookandsay}} {
    coroutine $coroName apply {n {
        for {} {[yield $n] ne "stop"} {set n $new} {
            set new ""
            foreach subseq [regexp -all -inline {0+|1+|2+|3+|4+|5+|6+|7+|8+|9+} $n] {
                append new [string length $subseq] [string index $subseq 0]
            }
        }
    }} $n
}

puts [seq_lookandsay 1]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

TUSCRIPT

$$ MODE TUSCRIPT,{}
num=1,say=""
 LOOP look
  digits=STRINGS (num," ? ")
  digitgrouped=ACCUMULATE (digits,howmany)
   LOOP/CLEAR  h=howmany,digit=digitgrouped
    say=JOIN (say,"",h,digit)
   ENDLOOP
  PRINT say
  num=VALUE(say),say=""
  IF (look==14) EXIT
 ENDLOOP

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221  

UNIX Shell

Works with: bash

lookandsay() {
    local num=$1 char seq i
    for ((i=0; i<=${#num}; i++)); do
        char=${num:i:1}
        if [[ $char == ${seq:0:1} ]]; then
            seq+=$char
        else
            [[ -n $seq ]] && printf "%d%s" ${#seq} ${seq:0:1}
            seq=$char
        fi
    done
}

for ((num=1, i=1; i<=10; i++)); do
    echo $num
    num=$( lookandsay $num )
done

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Ursala

The look_and_say function returns the first n results by iterating the function that maps a given sequence to its successor.

#import std
#import nat

look_and_say "n" = ~&H\'1' next"n" rlc~&E; *= ^lhPrT\~&hNC %nP+ length

#show+

main = look_and_say 10

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

VBA

Public Sub LookAndSay(Optional Niter As Integer = 10)
'generate "Niter" members of the look-and-say sequence
'(argument is optional; default is 10)

Dim s As String            'look-and-say number
Dim news As String         'next number in sequence
Dim curdigit As String     'current digit in s
Dim newdigit As String     'next digit in s
Dim curlength As Integer   'length of current run
Dim p As Integer           'position in s
Dim L As Integer           'length of s

On Error GoTo Oops          'to catch overflow, i.e. number too long

'start with "1"
s = "1"
For i = 1 To Niter
  'initialise
  L = Len(s)
  p = 1
  curdigit = Left$(s, 1)
  curlength = 1
  news = ""
  For p = 2 To L
    'check next digit in s
    newdigit = Mid$(s, p, 1)
    If curdigit = newdigit Then 'extend current run
      curlength = curlength + 1
    Else ' "output" run and start new run
      news = news & CStr(curlength) & curdigit
      curdigit = newdigit
      curlength = 1
    End If
  Next p
  ' "output" last run
  news = news & CStr(curlength) & curdigit
  Debug.Print news
  s = news
Next i
Exit Sub

Oops:
  Debug.Print
  If Err.Number = 6 Then 'overflow
    Debug.Print "Oops - number too long!"
  Else
    Debug.Print "Error: "; Err.Number, Err.Description
  End If
End Sub

Output:

LookAndSay 7
11
21
1211
111221
312211
13112221
1113213211

(Note: overflow occurs at 38th iteration!)

VBScript

Implementation

function looksay( n )
	dim i
	dim accum
	dim res
	dim c
	res = vbnullstring
	do
		if n = vbnullstring then exit do
		accum = 0
		c = left( n,1 )
		do while left( n, 1 ) = c
			accum = accum + 1
			n = mid(n,2)
		loop
		if accum > 0 then 
			res = res & accum & c
		end if
	loop
	looksay = res
end function

Invocation

dim m 
m = 1
for i = 0 to 13
	m = looksay(m)
	wscript.echo m
next

Output:

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

Vedit macro language

This implementation generates look-and-say sequence starting from the sequence on cursor line in edit buffer. Each new sequence is inserted as a new line. 10 sequences are created in this example.

Repeat(10) {
  BOL
  Reg_Empty(20)
  While (!At_EOL) {
    Match("(.)\1*", REGEXP+ADVANCE)
    Num_Str(Chars_Matched, 20, LEFT+APPEND)
    Reg_Copy_Block(20, CP-1, CP, APPEND)
  }
  Ins_Newline Reg_Ins(20)
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

V (Vlang)

Translation of: Go

fn lss(s string) string {
    mut r := ''
    mut c := s[0..1]
    mut nc := 1
    for i := 1; i < s.len; i++ {
        d := s[i..i+1]
        if d == c {
            nc++
            continue
        }
        r += nc.str() + c
        c = d
        nc = 1
    }
    return r + nc.str() + c
}
 
fn main() {
    mut s := "1"
    println(s)
    for i := 0; i < 8; i++ {
        s = lss(s)
        println(s)
    }
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221

Wren

Translation of: Kotlin

var lookAndSay = Fn.new { |s|
    var res = ""
    var digit = s[0]
    var count = 1
    for (i in 1...s.count) {
        if (s[i] == digit) {
            count = count + 1
        } else {
            res = res + "%(count)%(digit)"
            digit = s[i]
            count = 1
        }
    }
    return res + "%(count)%(digit)"
}

var las = "1"
for (i in 1..15) {
    System.print(las)
    las = lookAndSay.call(las)
}

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

XPL0

char Seq0(100), Seq1(100);
int  Iter, Digit, Count, I0, I1, T;
string 0;
[Seq0(0):= ^1;  Seq0(1):= 0;
Text(0, Seq0);  CrLf(0);
for Iter:= 2 to 15 do
    [I1:= 0;  I0:= 0;
    repeat  Digit:= Seq0(I0);
            Count:= ^1;
            I0:= I0+1;
            while Seq0(I0) = Digit do
                [Count:= Count+1;
                I0:= I0+1;
                ];
            Seq1(I1):= Count;  I1:= I1+1;
            Seq1(I1):= Digit;  I1:= I1+1;
    until   Seq0(I0) = 0;
    Seq1(I1):= 0;
    T:= Seq0;  Seq0:= Seq1;  Seq1:= T;
    Text(0, Seq0);  CrLf(0);
    ];
]

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

Yabasic

dim X$(2)
i = 0  // índice de cadena de entrada
X$(i) = "1"

input "Indica cuantas repeticiones: " r
print "\nSecuencia:"

print X$(i)
for n = 1 to r-1
    j = 1 - i  // índice de cadena de salida
    X$(j) = ""
    k = 1
    while k <= len(X$(i))
        k0 = k + 1
        while ((k0 <= len(X$(i))) and (mid$(X$(i), k, 1) = mid$(X$(i), k0, 1)))
            k0 = k0 + 1
        wend
        X$(j) = X$(j) + str$(k0 - k) + mid$(X$(i), k, 1)
        k = k0
    wend
    i = j
    print X$(j)
next n
print

Output:

La salida es similar a la de FreeBASIC, mostrada arriba.

Yorick

func looksay(input) {
    // Special case: one digit
    if(strlen(input) == 1)
        return "1" + input;
    // Convert string into an array of digits
    digits = strchar(input)(:-1);
    // Find indices where each run starts
    w = where(digits(dif));
    start = numberof(w) ? grow(1, w+1) : [1];
    // Find length of each run
    len = grow(start, numberof(digits)+1)(dif);
    // Find digits for each run
    run = digits(start);
    // Construct output array
    result = array(string, numberof(start)*2);
    // Fill in lengths
    result(1::2) = swrite(format="%d", len);
    // Fill in digits; first must add trailing nulls to coerce single string
    // into an array of strings.
    run = transpose([run, array(char(0), numberof(run))])(*);
    result(2::2) = strchar(run);
    // Merge string array into single string
    return result(sum);
}

val = "1";
do {
   write, val;
   val = looksay(val);
} while(strlen(val) < 80);

Output:

 1
 11
 21
 1211
 111221
 312211
 13112221
 1113213211
 31131211131221
 13211311123113112211
 11131221133112132113212221
 3113112221232112111312211312113211
 1321132132111213122112311311222113111221131221
 11131221131211131231121113112221121321132132211331222113112211
 311311222113111231131112132112311321322112111312211312111322212311322113212221

zkl

Treating the task as a string manipulation problem.

Translation of: Scala

fcn lookAndSay(seed){ // numeric String --> numeric String
   len,c:=[1..seed.len()-1].reduce(fcn([(len,c)]lc,index,s,sb){
      if(c!=s[index]) { sb.write(len); sb.write(c); lc.clear(1,s[index]) }
      else lc.clear(len+1,c);
   },L(1,seed[0]), seed,sb:=Sink(String));
   sb.write(len); sb.write(c);
   sb.close();
}

Output:

(0).reduce(10,fcn(seed,_){ lookAndSay(seed).println() },"1");
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221