# Hailstone sequence

Hailstone sequence
You are encouraged to solve this task according to the task description, using any language you may know.

The Hailstone sequence of numbers can be generated from a starting positive integer, n by:

• If n is 1 then the sequence ends.
• If n is even then the next n of the sequence = n/2
• If n is odd then the next n of the sequence = (3 * n) + 1

The (unproven), Collatz conjecture is that the hailstone sequence for any starting number always terminates.

1. Create a routine to generate the hailstone sequence for a number.
2. Use the routine to show that the hailstone sequence for the number 27 has 112 elements starting with 27, 82, 41, 124 and ending with 8, 4, 2, 1
3. Show the number less than 100,000 which has the longest hailstone sequence together with that sequence's length.
(But don't show the actual sequence!)

## ACL2

(defun hailstone (len)    (loop for x = len              then (if (evenp x)                          (/ x 2)                          (+ 1 (* 3 x)))         collect x until (= x 1))) ;; Must be tail recursive(defun max-hailstone-start (limit mx curr)   (declare (xargs :mode :program))   (if (zp limit)       (mv mx curr)       (let ((new-mx (len (hailstone limit))))          (if (> new-mx mx)              (max-hailstone-start (1- limit) new-mx limit)              (max-hailstone-start (1- limit) mx curr)))))

Output:

> (take 4 (hailstone 27))
(27 82 41 124)
> (nthcdr 108 (hailstone 27))
(8 4 2 1)
> (len (hailstone 27))
112
> (max-hailstone-start 100000 0 0)
(351 77031)

Similar to C method:

with Ada.Text_IO; use Ada.Text_IO;procedure hailstone is	type int_arr is array(Positive range <>) of Integer;	type int_arr_pt is access all int_arr; 	function hailstones(num:Integer; pt:int_arr_pt) return Integer is		stones : Integer := 1;		n : Integer := num;		begin		if pt /= null then pt(1) := num; end if;		while (n/=1) loop			stones := stones + 1;			if n mod 2 = 0 then n := n/2;			else n := (3*n)+1;			end if;			if pt /= null then pt(stones) := n; end if;		end loop;		return stones;	end hailstones; 	nmax,stonemax,stones : Integer := 0;	list : int_arr_pt;begin	stones := hailstones(27,null);	list := new int_arr(1..stones);	stones := hailstones(27,list);	put(" 27: "&Integer'Image(stones)); new_line;	for n in 1..4 loop put(Integer'Image(list(n)));	end loop;	put(" .... ");	for n in stones-3..stones loop put(Integer'Image(list(n))); end loop;	new_line;	for n in 1..100000 loop		stones := hailstones(n,null);		if stones>stonemax then			nmax := n; stonemax := stones;		end if;	end loop;	put_line(Integer'Image(nmax)&" max @ n= "&Integer'Image(stonemax));end hailstone;

Output:

 27:  112
27 82 41 124 ....  8 4 2 1
77031 max @ n=  351


### Alternative method

A method without pointers or dynamic memory allocation, but slower for simply counting. This is also used for the "executable library" task Executable library#Ada.

package Hailstones is   type Integer_Sequence is array(Positive range <>) of Integer;   function Create_Sequence (N : Positive) return Integer_Sequence;end Hailstones;

package body Hailstones is   function Create_Sequence (N : Positive) return Integer_Sequence is   begin      if N = 1 then         -- terminate         return (1 => N);      elsif N mod 2 = 0 then         -- even         return (1 => N) & Create_Sequence (N / 2);      else         -- odd         return (1 => N) & Create_Sequence (3 * N + 1);      end if;   end Create_Sequence;end Hailstones;

with Ada.Text_IO;with Hailstones; procedure Main is   package Integer_IO is new Ada.Text_IO.Integer_IO (Integer);    procedure Print_Sequence (X : Hailstones.Integer_Sequence) is   begin      for I in X'Range loop         Integer_IO.Put (Item => X (I), Width => 0);         if I < X'Last then            Ada.Text_IO.Put (", ");         end if;      end loop;      Ada.Text_IO.New_Line;   end Print_Sequence;    Hailstone_27 : constant Hailstones.Integer_Sequence :=     Hailstones.Create_Sequence (N => 27); begin   Ada.Text_IO.Put_Line ("Length of 27:" & Integer'Image (Hailstone_27'Length));   Ada.Text_IO.Put ("First four: ");   Print_Sequence (Hailstone_27 (Hailstone_27'First .. Hailstone_27'First + 3));   Ada.Text_IO.Put ("Last four: ");   Print_Sequence (Hailstone_27 (Hailstone_27'Last - 3 .. Hailstone_27'Last));    declare      Longest_Length : Natural := 0;      Longest_N      : Positive;      Length         : Natural;   begin      for I in 1 .. 99_999 loop         Length := Hailstones.Create_Sequence (N => I)'Length;         if Length > Longest_Length then            Longest_Length := Length;            Longest_N := I;         end if;      end loop;      Ada.Text_IO.Put_Line ("Longest length is" & Integer'Image (Longest_Length));      Ada.Text_IO.Put_Line ("with N =" & Integer'Image (Longest_N));   end;end Main;

output:

Length of 27: 112
First four: 27, 82, 41, 124
Last four: 8, 4, 2, 1
Longest length is 351
with N = 77031

## ALGOL 68

Translation of: C
- note: This specimen retains the original C coding style.
Works with: ALGOL 68 version Standard - no extensions to language used
Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - using the print routine rather than printf
MODE LINT = # LONG ... # INT; PROC hailstone = (INT in n, REF[]LINT array)INT:(    INT hs := 1;    INT index := 0;    LINT n := in n;     WHILE n /= 1 DO        hs +:= 1;        IF array ISNT REF[]LINT(NIL) THEN array[index +:= 1] := n FI;        n := IF ODD n THEN 3*n+1 ELSE n OVER 2 FI    OD;    IF array ISNT REF[]LINT(NIL) THEN array[index +:= 1] := n FI;    hs); main:(    INT j, hmax := 0;    INT jatmax, n;    INT border = 4;     FOR j TO 100000-1 DO        n := hailstone(j, NIL);       IF hmax < n THEN           hmax := n;           jatmax := j       FI    OD;     [2]INT test := (27, jatmax);    FOR key TO UPB test DO        INT val = test[key];        n := hailstone(val, NIL);        [n]LINT array;        n := hailstone(val, array);         printf(($"[ "n(border)(g(0)", ")" ..."n(border)(", "g(0))"] len="g(0)l$,            array[:border], array[n-border+1:], n))        #;free(array) #    OD;    printf(($"Max "g(0)" at j="g(0)l$, hmax, jatmax))# ELLA Algol68RS:    print(("Max",hmax," at j=",jatmax, new line))#)

Output:

[ 27, 82, 41, 124,  ..., 8, 4, 2, 1] len=112
[ 77031, 231094, 115547, 346642,  ..., 8, 4, 2, 1] len=351
Max 351 at j=77031


## APL

Works with: Dyalog APL
seq←hailstone n;next⍝ Returns the hailstone sequence for a given number seq←n                   ⍝ Init the sequence:While n≠1    next←(n÷2) (1+3×n)  ⍝ Compute both possibilities    n←next[1+2|n]       ⍝ Pick the appropriate next step    seq,←n              ⍝ Append that to the sequence:EndWhile

Output:

 5↑hailstone 2727 82 41 124 62 ¯5↑hailstone 2716 8 4 2 1 ⍴hailstone 27112 1↑{⍵[⍒↑(⍴∘hailstone)¨⍵]}⍳10000077031

## AutoHotkey

; Submitted by MasterFocus --- http://tiny.cc/iTunis ; [1] Generate the Hailstone Seq. for a number List := varNum := 7 ; starting number is 7, not counting elementsWhile ( varNum > 1 )  List .= ", " ( varNum := ( Mod(varNum,2) ? (varNum*3)+1 : varNum//2 ) )MsgBox % List ; [2] Seq. for starting number 27 has 112 elements Count := 1, List := varNum := 27 ; starting number is 27, counting elementsWhile ( varNum > 1 )  Count++ , List .= ", " ( varNum := ( Mod(varNum,2) ? (varNum*3)+1 : varNum//2 ) )MsgBox % "Sequence:n" List "nnCount: " Count ; [3] Find number<100000 with longest seq. and show both values MaxNum := Max := 0 ; reset the Maximum variablesTimesToLoop := 100000 ; limit number here is 100000Offset := 70000 ; offset - use 0 to process from 0 to 100000Loop, %TimesToLoop%{  If ( TimesToLoop < ( varNum := Index := A_Index+Offset ) )    Break  text := "Processing...n-------------------n"  text .= "Current starting number: " Index "n"  text .= "Current sequence count: " Count  text .= "n-------------------n"  text .= "Maximum starting number: " MaxNum "n"  text .= "Maximum sequence count: " Max " <<" ; text split to avoid long code lines  ToolTip, %text%  Count := 1 ; going to count the elements, but no "List" required  While ( varNum > 1 )    Count++ , varNum := ( Mod(varNum,2) ? (varNum*3)+1 : varNum//2 )  If ( Count > Max )    Max := Count , MaxNum := Index ; set the new maximum values, if necessary}ToolTipMsgBox % "Number: " MaxNum "nCount: " Max

## AutoIt

 $Hail = Hailstone(27)ConsoleWrite("Sequence-Lenght: "&$Hail&@CRLF)$Big = -1$Sequenzlenght = -1For $I = 1 To 100000$Hail = Hailstone($i, False) If Number($Hail) > $Sequenzlenght Then$Sequenzlenght = Number($Hail)$Big = $i EndIfNextConsoleWrite("Longest Sequence : "&$Sequenzlenght&" from number "&$Big&@CRLF)Func Hailstone($int, $sequence = True)$Counter = 0	While True		$Counter += 1 If$sequence = True Then ConsoleWrite($int & ",") If$int = 1 Then ExitLoop		If Not Mod($int, 2) Then$int = $int / 2 Else$int = 3 * $int + 1 EndIf If Not Mod($Counter, 25) AND $sequence = True Then ConsoleWrite(@CRLF) WEnd If$sequence = True Then ConsoleWrite(@CRLF)	Return $CounterEndFunc ;==>Hailstone  Output: 27,82,41,124,62,31,94,47,142,71,214,107,322,161,484,242,121,364,182,91,274,137,412,206,103, 310,155,466,233,700,350,175,526,263,790,395,1186,593,1780,890,445,1336,668,334,167,502,251,754,377,1132, 566,283,850,425,1276,638,319,958,479,1438,719,2158,1079,3238,1619,4858,2429,7288,3644,1822,911,2734,1367,4102,2051, 6154,3077,9232,4616,2308,1154,577,1732,866,433,1300,650,325,976,488,244,122,61,184,92,46,23,70,35,106, 53,160,80,40,20,10,5,16,8,4,2,1, Sequence-Lenght: 112 Longest Sequence : 351 from number 77031  ## AWK  #!/usr/bin/awk -ffunction hailstone(v, verbose) { n = 1; u = v; while (1) { if (verbose) printf " "u; if (u==1) return(n); n++; if (u%2 > 0 ) u = 3*u+1; else u = u/2; } } BEGIN { i = 27; printf("hailstone(%i) has %i elements\n",i,hailstone(i,1)); ix=0; m=0; for (i=1; i<100000; i++) { n = hailstone(i,0); if (m<n) { m=n; ix=i; } } printf("longest hailstone sequence is %i and has %i elements\n",ix,m);}  Output: 27 82 41 124 ....... 8 4 2 1 hailstone(27) has 112 elements longest hailstone sequence is 77031 and has 351 elements  ## BASIC ### Applesoft BASIC 10 HOME 100 N = 27110 GOSUB 400"HAILSTONE120 DEF FN L(I) = E(I + 4 * (I < 0))130IFL=112AND(S(0)=27ANDS(1)=82ANDS(2)=41ANDS(3)=124)AND(FNL(M-3)=8ANDFNL(M-2)=4ANDFNL(M-1)=2ANDFNL(M)=1)THENPRINT"THE HAILSTONE SEQUENCE FOR THE NUMBER 27 HAS 112 ELEMENTS STARTING WITH 27, 82, 41, 124 AND ENDING WITH 8, 4, 2, 1"140 PRINT150 V = PEEK(37) + 1 200 N = 1210 GOSUB 400"HAILSTONE220 MN = 1230 ML = L240 FOR I = 2 TO 99999250 N = I260 GOSUB 400"HAILSTONE270 IFL>MLTHENMN=I:ML=L:VTABV:HTAB1:PRINT "THE NUMBER " MN " HAS A HAILSTONE SEQUENCE LENGTH OF "L" WHICH IS THE LONGEST HAILSTONE SEQUENCE OF NUMBERS LESS THAN ";:Y=PEEK(37)+1:X=PEEK(36)+1280 IF Y THEN VTAB Y : HTAB X : PRINTI+1;290 NEXT I 300 END 400 M = 0410 FOR L = 1 TO 1E38420 IF L < 5 THEN S(L-1) = N430 M = (M + 1) * (M < 3)440 E(M) = N450 IF N = 1 THEN RETURN460 EVEN = INT(N/2)=N/2470 IF EVEN THEN N=N/2480 IF NOT EVEN THEN N = (3 * N) + 1490 NEXT L : STOP ### BBC BASIC  seqlen% = FNhailstone(27, TRUE) PRINT '"Sequence length = "; seqlen% maxlen% = 0 FOR number% = 2 TO 100000 seqlen% = FNhailstone(number%, FALSE) IF seqlen% > maxlen% THEN maxlen% = seqlen% maxnum% = number% ENDIF NEXT PRINT "The number with the longest hailstone sequence is " ; maxnum% PRINT "Its sequence length is " ; maxlen% END DEF FNhailstone(N%, S%) LOCAL L% IF S% THEN PRINT N%; WHILE N% <> 1 IF N% AND 1 THEN N% = 3 * N% + 1 ELSE N% DIV= 2 IF S% THEN PRINT N%; L% += 1 ENDWHILE = L% + 1 Output:  27 82 41 124 62 31 94 47 142 71 214 107 322 161 484 242 121 364 182 91 274 137 412 206 103 310 155 466 233 700 350 175 526 263 790 395 1186 593 1780 890 445 1336 668 334 167 502 251 754 377 1132 566 283 850 425 1276 638 319 958 479 1438 719 2158 1079 3238 1619 4858 2429 7288 3644 1822 911 2734 1367 4102 2051 6154 3077 9232 4616 2308 1154 577 1732 866 433 1300 650 325 976 488 244 122 61 184 92 46 23 70 35 106 53 160 80 40 20 10 5 16 8 4 2 1 Sequence length = 112 The number with the longest hailstone sequence is 77031 Its sequence length is 351  ### Liberty BASIC print "Part 1: Create a routine to generate the hailstone sequence for a number."print ""while hailstone < 1 or hailstone <> int(hailstone) input "Please enter a positive integer: "; hailstonewendprint ""print "The following is the 'Hailstone Sequence' for your number..."print ""print hailstonewhile hailstone <> 1 if hailstone / 2 = int(hailstone / 2) then hailstone = hailstone / 2 else hailstone = (3 * hailstone) + 1 print hailstonewendprint ""input "Hit 'Enter' to continue to part 2...";dummy$clsprint "Part 2: Use the routine to show that the hailstone sequence for the number 27 has 112 elements starting with 27, 82, 41, 124 and ending with 8, 4, 2, 1."print ""print "No. in Seq.","Hailstone Sequence Number for 27"print ""c = 1: hailstone = 27print c, hailstonewhile hailstone <> 1    c = c + 1    if hailstone / 2 = int(hailstone / 2) then hailstone = hailstone / 2 else hailstone = (3 * hailstone) + 1    print c, hailstonewendprint ""input "Hit 'Enter' to continue to part 3...";dummy$clsprint "Part 3: Show the number less than 100,000 which has the longest hailstone sequence together with that sequence's length.(But don't show the actual sequence)!"print ""print "Calculating result... Please wait... This could take a little while..."print ""print "Percent Done", "Start Number", "Seq. Length", "Maximum Sequence So Far"print ""for cc = 1 to 99999 hailstone = cc: c = 1 while hailstone <> 1 c = c + 1 if hailstone / 2 = int(hailstone / 2) then hailstone = hailstone / 2 else hailstone = (3 * hailstone) + 1 wend if c > max then max = c: largesthailstone = cc locate 1, 7 print " " locate 1, 7 print using("###.###", cc / 99999 * 100);"%", cc, c, max scannext ccprint ""print "The number less than 100,000 with the longest 'Hailstone Sequence' is "; largesthailstone;". It's sequence length is "; max;"."end ### OxygenBasic  function Hailstone(sys *n)'=========================if n and 1 n=n*3+1else n=n>>1end ifend function function HailstoneSequence(sys n) as sys'=======================================count=1do Hailstone n Count++ if n=1 then exit doend doreturn countend function 'MAIN'==== maxc=0maxn=0e=100000for n=1 to e c=HailstoneSequence n if c>maxc maxc=c maxn=n end ifnext print e ", " maxn ", " maxc 'result 100000, 77031, 351  ### PureBasic NewList Hailstones.i() ; Make a linked list to use as we do not know the numbers of elements needed for an Array Procedure.i FillHailstones(n) ; Fills the list & returns the amount of elements in the list Shared Hailstones() ; Get access to the Hailstones-List ClearList(Hailstones()) ; Remove old data Repeat AddElement(Hailstones()) ; Add an element to the list Hailstones()=n ; Fill current value in the new list element If n=1 ProcedureReturn ListSize(Hailstones()) ElseIf n%2=0 n/2 Else n=(3*n)+1 EndIf ForEverEndProcedure If OpenConsole() Define i, l, maxl, maxi l=FillHailstones(27) Print("#27 has "+Str(l)+" elements and the sequence is: "+#CRLF$)  ForEach Hailstones()    If i=6      Print(#CRLF$) i=0 EndIf i+1 Print(RSet(Str(Hailstones()),5)) If Hailstones()<>1 Print(", ") EndIf Next i=1 Repeat l=FillHailstones(i) If l>maxl maxl=l maxi=i EndIf i+1 Until i>=100000 Print(#CRLF$+#CRLF$+"The longest sequence below 100000 is #"+Str(maxi)+", and it has "+Str(maxl)+" elements.") Print(#CRLF$+#CRLF$+"Press ENTER to exit."): Input() CloseConsole()EndIf Output #27 has 112 elements and the sequence is: 27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, 322, 161, 484, 242, 121, 364, 182, 91, 274, 137, 412, 206, 103, 310, 155, 466, 233, 700, 350, 175, 526, 263, 790, 395, 1186, 593, 1780, 890, 445, 1336, 668, 334, 167, 502, 251, 754, 377, 1132, 566, 283, 850, 425, 1276, 638, 319, 958, 479, 1438, 719, 2158, 1079, 3238, 1619, 4858, 2429, 7288, 3644, 1822, 911, 2734, 1367, 4102, 2051, 6154, 3077, 9232, 4616, 2308, 1154, 577, 1732, 866, 433, 1300, 650, 325, 976, 488, 244, 122, 61, 184, 92, 46, 23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1 The longest sequence found up to 100000 is #77031 which has 351 elements. Press ENTER to exit.  ### Run BASIC print "Part 1: Create a routine to generate the hailstone sequence for a number."print "" while hailstone < 1 or hailstone <> int(hailstone) input "Please enter a positive integer: "; hailstonewendcount = doHailstone(hailstone,"Y") print: print "Part 2: Use the routine to show that the hailstone sequence for the number 27 has 112 elements starting with 27, 82, 41, 124 and ending with 8, 4, 2, 1."count = doHailstone(27,"Y") print: print "Part 3: Show the number less than 100,000 which has the longest hailstone sequence together with that sequence's length.(But don't show the actual sequence)!"print "Calculating result... Please wait... This could take a little while..."print "Stone Percent Count"for i = 1 to 99999 count = doHailstone(i,"N") if count > maxCount then theBigStone = i maxCount = count print using("#####",i);" ";using("###.#", i / 99999 * 100);"% ";using("####",count) end ifnext iend '---------------------------------------------' pass number and print (Y/N)FUNCTION doHailstone(hailstone,prnt$)if prnt$= "Y" then print print "The following is the 'Hailstone Sequence' for number:";hailstoneend ifwhile hailstone <> 1 if (hailstone and 1) then hailstone = (hailstone * 3) + 1 else hailstone = hailstone / 2 doHailstone = doHailstone + 1 if prnt$ = "Y" then     print hailstone;chr$(9); if (doHailstone mod 10) = 0 then print end ifwendEND FUNCTION ### Visual Basic .NET Works with: Visual Basic .NET version 2005+ Module HailstoneSequence Sub Main() ' Checking sequence of 27. Dim l As List(Of Long) = HailstoneSequence(27) Console.WriteLine("27 has {0} elements in sequence:", l.Count()) For i As Integer = 0 To 3 : Console.Write("{0}, ", l(i)) : Next Console.Write("... ") For i As Integer = l.Count - 4 To l.Count - 1 : Console.Write(", {0}", l(i)) : Next Console.WriteLine() ' Finding longest sequence for numbers below 100000. Dim max As Integer = 0 Dim maxCount As Integer = 0 For i = 1 To 99999 l = HailstoneSequence(i) If l.Count > maxCount Then max = i maxCount = l.Count End If Next Console.WriteLine("Max elements in sequence for number below 100k: {0} with {1} elements.", max, maxCount) Console.ReadLine() End Sub Private Function HailstoneSequence(ByVal n As Long) As List(Of Long) Dim valList As New List(Of Long)() valList.Add(n) Do Until n = 1 n = IIf(n Mod 2 = 0, n / 2, (3 * n) + 1) valList.Add(n) Loop Return valList End Function End Module Output: 27 has 112 elements in sequence: 27, 82, 41, 124, ... , 8, 4, 2, 1 Max elements in sequence for number below 100k: 77031 with 351 elements.  ## Batch File 1. Create a routine to generate the hailstone sequence for a number. @echo offsetlocal enabledelayedexpansionif "%1" equ "" goto :eofcall :hailstone %1 seq cntecho %seq%goto :eof :hailstoneset num=%1set %2=%1 :loopif %num% equ 1 goto :eofcall :iseven %num% resif %res% equ T goto divideby2set /a num = (3 * num) + 1set %2=!%2! %num%goto loop:divideby2set /a num = num / 2set %2=!%2! %num%goto loop :isevenset /a tmp = %1 %% 2if %tmp% equ 1 ( set %2=F) else ( set %2=T)goto :eof Demonstration >hailstone.cmd 2020 10 5 16 8 4 2 1 ## Befunge  This example may be incorrect. Calculates the Hailstone sequence but might not complete everything from task description. Please verify it and remove this message. If the example does not match the requirements or does not work, replace this message with Template:incorrect or fix the code yourself. &>:.:1-| >3*^ @ |%2: < v>2/>+ ## Bracmat ( ( hailstone = L len . !arg:?L & whl ' ( !arg:~1 & (!arg*1/2:~/|3*!arg+1):?arg & !arg !L:?L ) & (!L:? [?len&!len.!L) )& ( reverse = L e . :?L & whl'(!arg:%?e ?arg&!e !L:?L) & !L )& hailstone$27:(?len.?list)& reverse$!list:?first4 [4 ? [-5 ?last4& put$"Hailstone sequence starting with "& put$!first4& put$(str$(" has " !len " elements and ends with "))& put$(!last4 \n)& 1:?N& 0:?max:?Nmax&   whl  ' ( !N+1:<100000:?N    &   hailstone$!N : ( >!max:?max&!N:?Nmax | ? . ? ) )& out$ ( str    $( "The number <100000 with the longest hailstone sequence is " !Nmax " with " !max " elements." ) )); ## Brainf***  This example is incomplete. Please ensure that it meets all task requirements and remove this message. Prints the number of terms required to map the input to 1. Does not count the first term of the sequence. >,[ [ ----------[ >>>[>>>>]+[[-]+<[->>>>++>>>>+[>>>>]++[->+<<<<<]]<<<] ++++++[>------<-]>--[>>[->>>>]+>+[<<<<]>-],< ]> ]>>>++>+>>[ <<[>>>>[-]+++++++++<[>-<-]+++++++++>[-[<->-]+[<<<<]]<[>+<-]>] >[>[>>>>]+[[-]<[+[->>>>]>+<]>[<+>[<<<<]]+<<<<]>>>[->>>>]+>+[<<<<]] >[[>+>>[<<<<+>>>>-]>]<<<<[-]>[-<<<<]]>>>>>>> ]>>+[[-]++++++>>>>]<<<<[[<++++++++>-]<.[-]<[-]<[-]<]<,] 27111 ## Brat hailstone = { num | sequence = [num] while { num != 1 } { true? num % 2 == 0 { num = num / 2 } { num = num * 3 + 1 } sequence << num } sequence} #Check sequence for 27seq = hailstone 27true? (seq[0,3] == [27 82 41 124] && seq[-1, -4] == [8 4 2 1]) { p "Sequence for 27 is correct" } { p "Sequence for 27 is not correct!" } #Find longest sequence for numbers < 100,000longest = [number: 0 length: 0] 1.to 99999 { index | seq = hailstone index true? seq.length > longest[:length] { longest[:length] = seq.length longest[:number] = index p "Longest so far: #{index} @ #{longest[:length]} elements" } index = index + 1 } p "Longest was starting from #{longest[:number]} and was of length #{longest[:length]}" Output: Sequence for 27 is correct Longest so far: 1 @ 1 elements Longest so far: 2 @ 2 elements Longest so far: 3 @ 8 elements ... Longest so far: 52527 @ 340 elements Longest so far: 77031 @ 351 elements Longest was starting from 77031 and was of length 351 ## Burlesque  blsq ) 27{^^^^2.%{3.*1.+}\/{2./}\/ie}{1!=}w!bx{\/+]}{\/isn!}w!L[112  ## C #include <stdio.h>#include <stdlib.h> int hailstone(int n, int *arry){ int hs = 1; while (n!=1) { hs++; if (arry) *arry++ = n; n = (n&1) ? (3*n+1) : (n/2); } if (arry) *arry++ = n; return hs;} int main(){ int j, hmax = 0; int jatmax, n; int *arry; for (j=1; j<100000; j++) { n = hailstone(j, NULL); if (hmax < n) { hmax = n; jatmax = j; } } n = hailstone(27, NULL); arry = malloc(n*sizeof(int)); n = hailstone(27, arry); printf("[ %d, %d, %d, %d, ...., %d, %d, %d, %d] len=%d\n", arry[0],arry[1],arry[2],arry[3], arry[n-4], arry[n-3], arry[n-2], arry[n-1], n); printf("Max %d at j= %d\n", hmax, jatmax); free(arry); return 0;} Output [ 27, 82, 41, 124, ...., 8, 4, 2, 1] len= 112 Max 351 at j= 77031 ### With caching Much faster if you want to go over a million or so. #include <stdio.h> #define N 10000000#define CS N /* cache size */ typedef unsigned long ulong;ulong cache[CS] = {0}; ulong hailstone(ulong n){ int x; if (n == 1) return 1; if (n < CS && cache[n]) return cache[n]; x = 1 + hailstone((n & 1) ? 3 * n + 1 : n / 2); if (n < CS) cache[n] = x; return x;} int main(){ int i, l, max = 0, mi; for (i = 1; i < N; i++) { if ((l = hailstone(i)) > max) { max = l; mi = i; } } printf("max below %d: %d, length %d\n", N, mi, max); return 0;} ## C# using System;using System.Collections.Generic;using System.Linq;using System.Text; namespace Hailstone{ class Program { public static List<int> hs(int n,List<int> seq) { List<int> sequence = seq; sequence.Add(n); if (n == 1) { return sequence; }else{ int newn = (n % 2 == 0) ? n / 2 : (3 * n) + 1; return hs(newn, sequence); } } static void Main(string[] args) { int n = 27; List<int> sequence = hs(n,new List<int>()); Console.WriteLine(sequence.Count + " Elements"); List<int> start = sequence.GetRange(0, 4); List<int> end = sequence.GetRange(sequence.Count - 4, 4); Console.WriteLine("Starting with : " + string.Join(",", start) + " and ending with : " + string.Join(",", end)); int number = 0, longest = 0; for (int i = 1; i < 100000; i++) { int count = (hs(i, new List<int>())).Count; if (count > longest) { longest = count; number = i; } } Console.WriteLine("Number < 100000 with longest Hailstone seq.: " + number + " with length of " + longest); } }} 112 Elements Starting with : 27,82,41,124 and ending with : 8,4,2,1 Number < 100000 with longest Hailstone seq.: 77031 with length of 351  ### With caching As with the C example, much faster if you want to go over a million or so. using System;using System.Collections.Generic; namespace ConsoleApplication1{ class Program { public static void Main() { int longestChain = 0, longestNumber = 0; var recursiveLengths = new Dictionary<int, int>(); const int maxNumber = 100000; for (var i = 1; i <= maxNumber; i++) { var chainLength = Hailstone(i, recursiveLengths); if (longestChain >= chainLength) continue; longestChain = chainLength; longestNumber = i; } Console.WriteLine("max below {0}: {1} ({2} steps)", maxNumber, longestNumber, longestChain); } private static int Hailstone(int num, Dictionary<int, int> lengths) { if (num == 1) return 1; while (true) { if (lengths.ContainsKey(num)) return lengths[num]; lengths[num] = 1 + ((num%2 == 0) ? Hailstone(num/2, lengths) : Hailstone((3*num) + 1, lengths)); } } }} max below 100000: 77031 (351 steps)  ## C++ #include <iostream>#include <vector>#include <utility> std::vector<int> hailstone(int i){ std::vector<int> v; while(true){ v.push_back(i); if (1 == i) break; i = (i % 2) ? (3 * i + 1) : (i / 2); } return v;} std::pair<int,int> find_longest_hailstone_seq(int n){ std::pair<int, int> maxseq(0, 0); int l; for(int i = 1; i < n; ++i){ l = hailstone(i).size(); if (l > maxseq.second) maxseq = std::make_pair(i, l); } return maxseq;} int main () { // Use the routine to show that the hailstone sequence for the number 27 std::vector<int> h27; h27 = hailstone(27); // has 112 elements int l = h27.size(); std::cout << "length of hailstone(27) is " << l;// starting with 27, 82, 41, 124 and std::cout << " first four elements of hailstone(27) are "; std::cout << h27[0] << " " << h27[1] << " " << h27[2] << " " << h27[3] << std::endl;// ending with 8, 4, 2, 1 std::cout << " last four elements of hailstone(27) are " << h27[l-4] << " " << h27[l-3] << " " << h27[l-2] << " " << h27[l-1] << std::endl; std::pair<int,int> m = find_longest_hailstone_seq(100000); std::cout << "the longest hailstone sequence under 100,000 is " << m.first << " with " << m.second << " elements." <<std::endl; return 0;} output: length of hailstone(27) is 112 first four elements of hailstone(27) are 27 82 41 124 last four elements of hailstone(27) are 8 4 2 1 the longest hailstone sequence under 100,000 is 77031 with 351 elements.  ## CLIPS (deftemplate longest (slot bound) ; upper bound for the range of values to check (slot next (default 2)) ; next value that needs to be checked (slot start (default 1)) ; starting value of longest sequence (slot len (default 1)) ; length of longest sequence) (deffacts startup (query 27) (longest (bound 100000))) (deffunction hailstone-next (?n) (if (evenp ?n) then (div ?n 2) else (+ (* 3 ?n) 1) )) (defrule extend-sequence ?hail <- (hailstone$?sequence ?tail&:(> ?tail 1))  =>  (retract ?hail)  (assert (hailstone ?sequence ?tail (hailstone-next ?tail)))) (defrule start-query  (query ?num)  =>  (assert (hailstone ?num))) (defrule result-query  (query ?num)  (hailstone ?num $?sequence 1) => (bind ?sequence (create$ ?num ?sequence 1))  (printout t "Hailstone sequence starting with " ?num ":" crlf)  (bind ?len (length ?sequence))  (printout t "  Length: " ?len crlf)  (printout t "  First four: " (implode$(subseq$ ?sequence 1 4)) crlf)  (printout t "  Last four: " (implode$(subseq$ ?sequence (- ?len 3) ?len)) crlf)  (printout t crlf)) (defrule longest-create-next-hailstone  (longest (bound ?bound) (next ?next))  (test (<= ?next ?bound))  (not (hailstone ?next $?)) => (assert (hailstone ?next))) (defrule longest-check-next-hailstone ?longest <- (longest (bound ?bound) (next ?next) (start ?start) (len ?len)) (test (<= ?next ?bound)) ?hailstone <- (hailstone ?next$?sequence 1)  =>  (retract ?hailstone)  (bind ?thislen (+ 2 (length ?sequence)))  (if (> ?thislen ?len) then    (modify ?longest (start ?next) (len ?thislen) (next (+ ?next 1)))    else    (modify ?longest (next (+ ?next 1)))  )) (defrule longest-finished  (longest (bound ?bound) (next ?next) (start ?start) (len ?len))  (test (> ?next ?bound))  =>  (printout t "The number less than " ?bound " that has the largest hailstone" crlf)  (printout t "sequence is " ?start " with a length of " ?len "." crlf)  (printout t crlf))

Output:

The number less than 100000 that has the largest hailstone
sequence is 77031 with a length of 351.

Hailstone sequence starting with 27:
Length: 112
First four: 27 82 41 124
Last four: 8 4 2 1

## Clojure

(defn hailstone-seq [n]  (:pre [(pos? n)])  (lazy-seq    (cond (= n 1)   '(1)         (even? n) (cons n (hailstone-seq (/ n 2)))         :else     (cons n (hailstone-seq (+ (* n 3) 1)))))) (def hseq27 (hailstone-seq 27))(assert (= (count hseq27) 112))(assert (= (take 4 hseq27) [27 82 41 124]))(assert (= (drop 108 hseq27) [8 4 2 1])) (let [{max-i :num, max-len :len}      (reduce #(max-key :len %1 %2)              (for [i (range 1 100000)]                {:num i, :len (count (hailstone-seq i))}))]  (println "Maximum length" max-len "was found for hailstone(" max-i ")."))

## CoffeeScript

Recursive version:

hailstone = (n) ->  if n is 1    [n]   else if n % 2 is 0    [n].concat hailstone n/2   else    [n].concat hailstone (3*n) + 1 h27 = hailstone 27console.log "hailstone(27) = #{h27[0..3]} ... #{h27[-4..]} (length: #{h27.length})" maxlength = 0maxnums = [] for i in [1..100000]  seq = hailstone i   if seq.length is maxlength    maxnums.push i  else if seq.length > maxlength    maxlength = seq.length    maxnums = [i] console.log "Max length: #{maxlength}; numbers generating sequences of this length: #{maxnums}"
hailstone(27) = 27,82,41,124 ... 8,4,2,1 (length: 112)
Max length: 351; numbers generating sequences of this length: 77031

## Common Lisp

(defun hailstone (n)  (cond ((= n 1) '(1))	((evenp n) (cons n (hailstone (/ n 2))))	(t (cons n (hailstone (+ (* 3 n) 1)))))) (defun longest (n)  (let ((k 0) (l 0))    (loop for i from 1 below n do	 (let ((len (length (hailstone i))))	   (when (> len l) (setq l len k i)))	 finally (format t "Longest hailstone sequence under ~A for ~A, having length ~A." n k l))))

Sample session:

ROSETTA> (length (hailstone 27))
112
ROSETTA> (subseq (hailstone 27) 0 4)
(27 82 41 124)
ROSETTA> (last (hailstone 27) 4)
(8 4 2 1)
ROSETTA> (longest-hailstone 100000)
Longest hailstone sequence under 100000 for 77031, having length 351.
NIL

## D

### Basic Version

import std.stdio, std.algorithm, std.range, std.typecons; auto hailstone(uint n) pure nothrow {  auto result = [n];  while (n != 1) {    n = n & 1 ? n*3 + 1 : n/2;    result ~= n;  }  return result;} void main() {  enum M = 27;  immutable h = M.hailstone;  writeln("hailstone(", M, ")= ", h[0 .. 4], " ... " , h[$- 4 ..$]);  writeln("Length hailstone(", M, ")= ", h.length);   enum N = 100_000;  immutable p = iota(1, N)                .map!(i => tuple(i.hailstone.length, i))                .reduce!max;  writeln("Longest sequence in [1,", N, "]= ",p[1]," with len ",p[0]);}
Output:
hailstone(27)= [27, 82, 41, 124] ... [8, 4, 2, 1]
Length hailstone(27)= 112
Longest sequence in [1,100000]= 77031 with len 351

### Faster Lazy Version

Same output.

import std.stdio, std.algorithm, std.range, std.typecons; struct Hailstone {  uint n;  bool empty() const pure nothrow { return n == 0; }  uint front() const pure nothrow { return n; }  void popFront() pure nothrow {    n = n == 1 ? 0 : (n & 1 ? n*3 + 1 : n/2);  }} void main() {  enum M = 27;  immutable h = M.Hailstone.array;  writeln("hailstone(", M, ")= ", h[0 .. 4], " ... " , h[$- 4 ..$]);  writeln("Length hailstone(", M, ")= ", h.length);   enum N = 100_000;  immutable p = iota(1, N)                .map!(i => tuple(i.Hailstone.walkLength, i))                .reduce!max;  writeln("Longest sequence in [1,", N, "]= ",p[1]," with len ",p[0]);}

### Lazy Version With Caching

Faster, same output.

import std.stdio, std.algorithm, std.range, std.typecons; struct Hailstone(size_t cacheSize = 500_000) {  size_t n;  __gshared static size_t[cacheSize] cache;   bool empty() const pure nothrow { return n == 0; }  size_t front() const pure nothrow { return n; }   void popFront() nothrow {    if (n >= cacheSize) {      n = n == 1 ? 0 : (n & 1 ? n*3 + 1 : n/2);    } else if (cache[n]) {      n = cache[n];    } else {      immutable n2 = n == 1 ? 0 : (n & 1 ? n*3 + 1 : n/2);      n = cache[n] = n2;    }  }} void main() {  enum M = 27;  const h = M.Hailstone!().array;  writeln("hailstone(", M, ")= ", h[0 .. 4], " ... " , h[$- 4 ..$]);  writeln("Length hailstone(", M, ")= ", h.length);   enum N = 100_000;  immutable p = iota(1, N)                .map!(i => tuple(i.Hailstone!().walkLength, i))                .reduce!max;  writeln("Longest sequence in [1,", N, "]= ",p[1]," with len ",p[0]);}

## Déjà Vu

local hailstone:	swap [ over ]	while < 1 dup:		if % over 2:			#odd			++ * 3		else:			#even			/ swap 2		swap push-through rot dup	drop if = (name) :(main):	local :h27 hailstone 27	!. = 112 len h27	!. = 27 h27! 0	!. = 82 h27! 1	!. = 41 h27! 2	!. = 124 h27! 3	!. = 8 h27! 108	!. = 4 h27! 109	!. = 2 h27! 110	!. = 1 h27! 111 	local :max 0	local :maxlen 0	for i range 1 99999:		dup len hailstone i		if < maxlen:			set :maxlen			set :max i		else:			drop	!print( "number: " to-str max ", length: " to-str maxlen )else:	@hailstone
Output:
true
true
true
true
true
true
true
true
true
number: 77031, length: 351

List<int> hailstone(int n) {  if(n<=0) {    throw new IllegalArgumentException("start value must be >=1)");  }  Queue<int> seq=new Queue<int>();  seq.add(n);  while(n!=1) {    n=n%2==0?(n/2).toInt():3*n+1;    seq.add(n);  }  return new List<int>.from(seq);} // apparently List is missing toString()String iterableToString(Iterable seq) {  String str="[";  Iterator i=seq.iterator();  while(i.hasNext()) {    str+=i.next();    if(i.hasNext()) {      str+=",";    }  }  return str+"]";} main() {  for(int i=1;i<=10;i++) {    print("h($i)="+iterableToString(hailstone(i))); } List<int> h27=hailstone(27); List<int> first4=h27.getRange(0,4); print("first 4 elements of h(27): "+iterableToString(first4)); Expect.listEquals([27,82,41,124],first4); List<int> last4=h27.getRange(h27.length-4,4); print("last 4 elements of h(27): "+iterableToString(last4)); Expect.listEquals([8,4,2,1],last4); print("length of sequence h(27): "+h27.length); Expect.equals(112,h27.length); int seq,max=0; for(int i=1;i<=100000;i++) { List<int> h=hailstone(i); if(h.length>max) { max=h.length; seq=i; } } print("up to 100000 the sequence h($seq) has the largest length ($max)");} Output h(1)=[1] h(2)=[2,1] h(3)=[3,10,5,16,8,4,2,1] h(4)=[4,2,1] h(5)=[5,16,8,4,2,1] h(6)=[6,3,10,5,16,8,4,2,1] h(7)=[7,22,11,34,17,52,26,13,40,20,10,5,16,8,4,2,1] h(8)=[8,4,2,1] h(9)=[9,28,14,7,22,11,34,17,52,26,13,40,20,10,5,16,8,4,2,1] h(10)=[10,5,16,8,4,2,1] first 4 elements of h(27): [27,82,41,124] last 4 elements of h(27): [8,4,2,1] length of sequence h(27): 112 up to 100000 the sequence h(77031) has the largest length (351) ## Dc Firstly, this code takes the value from the stack, computes and prints the corresponding Hailstone sequence, and the length of the sequence. The q procedure is for counting the length of the sequence. The e and o procedure is for even and odd number respectively. The x procedure is for overall control. 27[[--: ]nzpq]sq[d 2/ p]se[d 3*1+ p]so[d2% 0=e d1=q d2% 1=o d1=q lxx]dsxx Output 82 41 124 62 (omitted) 8 4 2 1 --: 112  Then we could wrap the procedure x with a new procedure s, and call it with l which is loops the value of t from 1 to 100000, and cleaning up the stack after each time we finish up with a number. Register L for the length of the longest sequence and T for the corresponding number. Also, procedure q is slightly modified for storing L and T if needed, and all printouts in procedure e and o are muted. 0dsLsT1st[dsLltsT]sM[[zdlL<M q]sq[d 2/]se[d 3*1+ ]so[d2% 0=e d1=q d2% 1=o d1=q lxx]dsxx]ss[lt1+dstlsxc lt100000>l]dslxlTn[:]nlLp  Output (Takes quite some time on a decent machine) 77031:351 ## Delphi program ShowHailstoneSequence; {$APPTYPE CONSOLE} uses SysUtils, Generics.Collections; procedure GetHailstoneSequence(aStartingNumber: Integer; aHailstoneList: TList<Integer>);var  n: Integer;begin  aHailstoneList.Clear;  aHailstoneList.Add(aStartingNumber);  n := aStartingNumber;   while n <> 1 do  begin    if Odd(n) then      n := (3 * n) + 1    else      n := n div 2;    aHailstoneList.Add(n);  end;end; var  i: Integer;  lList: TList<Integer>;  lMaxSequence: Integer;  lMaxLength: Integer;begin  lList := TList<Integer>.Create;  try    GetHailstoneSequence(27, lList);    Writeln(Format('27: %d elements', [lList.Count]));    Writeln(Format('[%d,%d,%d,%d ... %d,%d,%d,%d]',      [lList[0], lList[1], lList[2], lList[3],      lList[lList.Count - 4], lList[lList.Count - 3], lList[lList.Count - 2], lList[lList.Count - 1]]));    Writeln;     lMaxSequence := 0;    lMaxLength := 0;    for i := 1 to 100000 do    begin      GetHailstoneSequence(i, lList);      if lList.Count > lMaxLength then      begin        lMaxSequence := i;        lMaxLength := lList.Count;      end;    end;    Writeln(Format('Longest sequence under 100,000: %d with %d elements', [lMaxSequence, lMaxLength]));  finally    lList.Free;  end;   Readln;end.

Output:

27: 112 elements
[27 82 41 124 ... 8 4 2 1]

Longest sequence under 100,000: 77031 with 351 elements

## Elixir

defmodule Hailstone do  def step(1), do: 0  def step(n) when Integer.even?(n), do: div(n,2)  def step(n) when Integer.odd?(n), do: n*3 + 1  def sequence(n) do    Enum.to_list(Stream.take_while(Stream.iterate(n, &step/1), &(&1 > 0)))  end   def run do    seq27 = Hailstone.sequence(27)    len27 = length(seq27)    repr = String.replace(inspect(seq27, limit: 4), "]",                           String.replace(inspect(Enum.drop(seq27,len27-4)), "[", ", "))    IO.puts("Hailstone(27) has #{len27} elements: #{repr}")     {start, len}  = Enum.max_by( Enum.map(1..100_000, fn(n) -> {n, length(Hailstone.sequence(n))} end),                                 fn({_,len}) -> len end )    IO.puts("Longest sequence starting under 100000 begins with #{start} and has #{len} elements.")  endend Hailstone.run
Output:
Hailstone(27) has 112 elements: [27, 82, 41, 124, ..., 8, 4, 2, 1]
Longest sequence starting under 100000 begins with 77031 and has 351 elements.


## Erlang

-module(hailstone).-import(io).-export([main/0]). hailstone(1) -> [1];hailstone(N) when N band 1 == 1 -> [N|hailstone(N * 3 + 1)];hailstone(N) when N band 1 == 0 -> [N|hailstone(N div 2)]. max_length(Start, Stop) ->    F = fun (N) -> {length(hailstone(N)), N} end,    Lengths = lists:map(F, lists:seq(Start, Stop)),    lists:max(Lengths). main() ->    io:format("hailstone(4): ~w~n", [hailstone(4)]),    Seq27 = hailstone(27),    io:format("hailstone(27) length: ~B~n", [length(Seq27)]),    io:format("hailstone(27) first 4: ~w~n",              [lists:sublist(Seq27, 4)]),    io:format("hailstone(27) last 4: ~w~n",              [lists:nthtail(length(Seq27) - 4, Seq27)]),    io:format("finding maximum hailstone(N) length for 1 <= N <= 100000..."),    {Length, N} = max_length(1, 100000),    io:format(" done.~nhailstone(~B) length: ~B~n", [N, Length]).
Output:
Eshell V5.8.4  (abort with ^G)
1> c(hailstone).
{ok,hailstone}
2> hailstone:main().
hailstone(4): [4,2,1]
hailstone(27) length: 112
hailstone(27) first 4: [27,82,41,124]
hailstone(27) last 4: [8,4,2,1]
finding maximum hailstone(N) length for 1 <= N <= 100000... done.
hailstone(77031) length: 351
ok

## Euler Math Toolbox

 >function hailstone (n) ...$v=[n];$  repeat$if mod(n,2) then n=3*n+1;$    else n=n/2;$endif;$    v=v|n;$until n==1;$  end;$return v;$  endfunction>hailstone(27), length(%) [ 27  82  41  124  62  31  94  47  142  71  214  107  322  161  484  242 121  364  182  91  274  137  412  206  103  310  155  466  233  700 350  175  526  263  790  395  1186  593  1780  890  445  1336  668 334  167  502  251  754  377  1132  566  283  850  425  1276  638  319 958  479  1438  719  2158  1079  3238  1619  4858  2429  7288  3644 1822  911  2734  1367  4102  2051  6154  3077  9232  4616  2308  1154 577  1732  866  433  1300  650  325  976  488  244  122  61  184  92 46  23  70  35  106  53  160  80  40  20  10  5  16  8  4  2  1 ] 112>function hailstonelength (n) ...$v=zeros(1,n);$  v[1]=4; v[2]=2;$loop 3 to n;$    count=1;$n=#;$    repeat$if mod(n,2) then n=3*n+1;$      else n=n/2;$endif;$      if n<=cols(v) and v[n] then $v[#]=v[n]+count;$        break;$endif;$      count=count+1;    $end;$  end;$return v;$  endfunction>h=hailstonelength(100000);>ex=extrema(h); ex[3], ex[4] 351 77031

## Euphoria

function hailstone(atom n)    sequence s    s = {n}    while n != 1 do        if remainder(n,2)=0 then            n /= 2        else            n = 3*n + 1        end if        s &= n    end while    return send function function hailstone_count(atom n)    integer count    count = 1    while n != 1 do        if remainder(n,2)=0 then            n /= 2        else            n = 3*n + 1        end if        count += 1    end while    return countend function sequence ss = hailstone(27)puts(1,"hailstone(27) =\n")? sprintf(1,"len = %d\n\n",length(s)) integer max,imax,countmax = 0for i = 2 to 1e5-1 do    count = hailstone_count(i)    if count > max then        max = count        imax = i    end ifend for printf(1,"The longest hailstone sequence under 100,000 is %d with %d elements.\n",    {imax,max})

Output:

hailstone(27) =
{27,82,41,124,62,31,94,47,142,71,214,107,322,161,484,242,121,364,182,
91,274,137,412,206,103,310,155,466,233,700,350,175,526,263,790,395,
1186,593,1780,890,445,1336,668,334,167,502,251,754,377,1132,566,283,
850,425,1276,638,319,958,479,1438,719,2158,1079,3238,1619,4858,2429,
7288,3644,1822,911,2734,1367,4102,2051,6154,3077,9232,4616,2308,1154,
577,1732,866,433,1300,650,325,976,488,244,122,61,184,92,46,23,70,35,
106,53,160,80,40,20,10,5,16,8,4,2,1}
len = 112

The longest hailstone sequence under 100,000 is 77031 with 351 elements.


## Excel

 This example may be incorrect. Calculates the Hailstone sequence but might not complete everything from task description. Please verify it and remove this message. If the example does not match the requirements or does not work, replace this message with Template:incorrect or fix the code yourself.
   In cell A1, place the starting number.
In cell A2 enter this formula =IF(MOD(A1,2)=0,A1/2,A1*3+1)
Drag and copy the formula down until 4, 2, 1


## Ezhil

Ezhil is a Tamil programming language, see | Wikipedia entry.

 நிரல்பாகம்  hailstone ( எண் )           பதிப்பி "=> ",எண் #hailstone seq	    @( எண் == 1 )   ஆனால் 	        	        பின்கொடு எண்	    முடி 	      @( (எண்%2) == 1 )  ஆனால்	      	   hailstone( 3*எண் + 1)              இல்லை	           hailstone( எண்/2 )              முடிமுடி  எண்கள் = [5,17,19,23,37]@(எண்கள் இல் இவ்வெண்) ஒவ்வொன்றாக   பதிப்பி "****** calculating hailstone seq for ",இவ்வெண்," *********"   hailstone( இவ்வெண் )   பதிப்பி "**********************************************"முடி

## Factor

! rosetta/hailstone/hailstone.factorUSING: arrays io kernel math math.ranges prettyprint sequences vectors ;IN: rosetta.hailstone : hailstone ( n -- seq )    [ 1vector ] keep    [ dup 1 number= ]    [        dup even? [ 2 / ] [ 3 * 1 + ] if        2dup swap push    ] until    drop ; <PRIVATE: main ( -- )    27 hailstone dup dup    "The hailstone sequence from 27:" print    "  has length " write length .    "  starts with " write 4 head [ unparse ] map ", " join print    "  ends with " write 4 tail* [ unparse ] map ", " join print     ! Maps n => { length n }, and reduces to longest Hailstone sequence.    1 100000 [a,b)    [ [ hailstone length ] keep 2array ]    [ [ [ first ] bi@ > ] most ] map-reduce    first2    "The hailstone sequence from " write pprint    " has length " write pprint "." print ;PRIVATE> MAIN: main
Output:

## Forth

: hail-next ( n -- n )  dup 1 and if 3 * 1+ else 2/ then ;: .hail ( n -- )  begin dup . dup 1 > while hail-next repeat drop ;: hail-len ( n -- n )  1 begin over 1 > while swap hail-next swap 1+ repeat nip ; 27 hail-len . cr27 .hail cr : longest-hail ( max -- )  0 0 rot 1+ 1 do    ( n length )    i hail-len 2dup < if      nip nip i swap    else drop then  loop  swap . ." has hailstone sequence length " . ; 100000 longest-hail

## Fortran

Works with: Fortran version 95 and later
program Hailstone  implicit none   integer :: i, maxn  integer :: maxseqlen = 0, seqlen  integer, allocatable :: seq(:)   call hs(27, seqlen)  allocate(seq(seqlen))  call hs(27, seqlen, seq)  write(*,"(a,i0,a)") "Hailstone sequence for 27 has ", seqlen, " elements"  write(*,"(a,4(i0,a),3(i0,a),i0)") "Sequence = ", seq(1), ", ", seq(2), ", ", seq(3), ", ", seq(4), " ...., ",  &                                     seq(seqlen-3), ", ", seq(seqlen-2), ", ", seq(seqlen-1), ", ", seq(seqlen)    do i = 1, 99999    call hs(i, seqlen)    if (seqlen > maxseqlen) then      maxseqlen = seqlen      maxn = i    end if  end do  write(*,*)  write(*,"(a,i0,a,i0,a)") "Longest sequence under 100000 is for ", maxn, " with ", maxseqlen, " elements"   deallocate(seq) contains subroutine hs(number, length, seqArray)  integer, intent(in)  :: number  integer, intent(out) :: length    integer, optional, intent(inout) :: seqArray(:)  integer :: n   n = number  length = 1  if(present(seqArray)) seqArray(1) = n  do while(n /= 1)    if(mod(n,2) == 0) then      n = n / 2    else      n = n * 3 + 1    end if    length = length + 1    if(present(seqArray)) seqArray(length) = n  end doend subroutine end program

Output:

Hailstone sequence for 27 has 112 elements
Sequence = 27, 82, 41, 124, ...., 8, 4, 2, 1

Longest sequence under 100000 is for 77031 with 351 elements

## Frege

Works with: Frege version 3.20.113
module Hailstone where import Data.List (maximumBy) hailstone :: Int -> [Int]hailstone 1             = [1]hailstone n | even n    = n : hailstone (n div 2)            | otherwise = n : hailstone (n * 3 + 1) withResult :: (t -> t1) -> t -> (t1, t)withResult f x = (f x, x) main _ = do let h27 = hailstone 27 printStrLn $show$ length h27 let h4 = show $take 4 h27 let t4 = show$ drop (length h27 - 4) h27 printStrLn ("hailstone 27: " ++ h4 ++ " ... " ++ t4) printStrLn $show$ maximumBy (comparing fst) $map (withResult (length . hailstone)) (1..100000) Output: 112 hailstone 27: [27, 82, 41, 124] ... [8, 4, 2, 1] (351, 77031) runtime 4.374 wallclock seconds.  ## F# let rec hailstone n = seq { match n with | 1 -> yield 1 | n when n % 2 = 0 -> yield n; yield! hailstone (n / 2) | n -> yield n; yield! hailstone (n * 3 + 1)} let hailstone27 = hailstone 27 |> Array.ofSeqassert (Array.length hailstone27 = 112)assert (hailstone27.[..3] = [|27;82;41;124|])assert (hailstone27.[108..] = [|8;4;2;1|]) let maxLen, maxI = Seq.max <| seq { for i in 1..99999 -> Seq.length (hailstone i), i}printfn "Maximum length %d was found for hailstone(%d)" maxLen maxI Output: Maximum length 351 was found for hailstone(77031) ## GAP CollatzSequence := function(n) local v; v := [ n ]; while n > 1 do if IsEvenInt(n) then n := QuoInt(n, 2); else n := 3*n + 1; fi; Add(v, n); od; return v;end; CollatzLength := function(n) local m; m := 1; while n > 1 do if IsEvenInt(n) then n := QuoInt(n, 2); else n := 3*n + 1; fi; m := m + 1; od; return m;end; CollatzMax := function(a, b) local n, len, nmax, lmax; lmax := 0; for n in [a .. b] do len := CollatzLength(n); if len > lmax then nmax := n; lmax := len; fi; od; return [ nmax, lmax ];end; CollatzSequence(27);# [ 27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, 322, 161, 484, 242, 121, 364, 182, 91, 274, 137, 412, 206, # 103, 310, 155, 466, 233, 700, 350, 175, 526, 263, 790, 395, 1186, 593, 1780, 890, 445, 1336, 668, 334, 167, 502, # 251, 754, 377, 1132, 566, 283, 850, 425, 1276, 638, 319, 958, 479, 1438, 719, 2158, 1079, 3238, 1619, 4858, 2429, # 7288, 3644, 1822, 911, 2734, 1367, 4102, 2051, 6154, 3077, 9232, 4616, 2308, 1154, 577, 1732, 866, 433, 1300, # 650, 325, 976, 488, 244, 122, 61, 184, 92, 46, 23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1 ]CollatzLength(27); # 112 CollatzMax(1, 100);# [ 97, 119 ]CollatzMax(1, 1000);# [ 871, 179 ]CollatzMax(1, 10000);# [ 6171, 262 ]CollatzMax(1, 100000);# [ 77031, 351 ]CollatzMax(1, 1000000);# [ 837799, 525 ] ## Go package main import "fmt" // 1st arg is the number to generate the sequence for.// 2nd arg is a slice to recycle, to reduce garbage.func hs(n int, recycle []int) []int { s := append(recycle[:0], n) for n > 1 { if n&1 == 0 { n = n / 2 } else { n = 3*n + 1 } s = append(s, n) } return s} func main() { seq := hs(27, nil) fmt.Printf("hs(27): %d elements: [%d %d %d %d ... %d %d %d %d]\n", len(seq), seq[0], seq[1], seq[2], seq[3], seq[len(seq)-4], seq[len(seq)-3], seq[len(seq)-2], seq[len(seq)-1]) var maxN, maxLen int for n := 1; n < 100000; n++ { seq = hs(n, seq) if len(seq) > maxLen { maxN = n maxLen = len(seq) } } fmt.Printf("hs(%d): %d elements\n", maxN, maxLen)} Output: hs(27): 112 elements: [27 82 41 124 ... 8 4 2 1] hs(77031): 351 elements  Alternate solution (inspired both by recent news of a new proof submitted for publication and by recent chat on #rosettacode about generators.) This solution interprets the wording of the task differently, and takes the word "generate" to mean use a generator. This has the advantage of not storing the whole sequence in memory at once. Elements are generated one at a time, counted and discarded. A time optimization added for task 3 is to store the sequence lengths computed so far. Output is the same as version above. package main import "fmt" // Task 1 implemented with a generator. Calling newHg will "create// a routine to generate the hailstone sequence for a number."func newHg(n int) func() int { return func() (n0 int) { n0 = n if n&1 == 0 { n = n / 2 } else { n = 3*n + 1 } return }} func main() { // make generator for sequence starting at 27 hg := newHg(27) // save first four elements for printing later s1, s2, s3, s4 := hg(), hg(), hg(), hg() // load next four elements in variables to use as shift register. e4, e3, e2, e1 := hg(), hg(), hg(), hg() // 4+4= 8 that we've generated so far ec := 8 // until we get to 1, generate another value, shift, and increment. // note that intermediate elements--those shifted off--are not saved. for e1 > 1 { e4, e3, e2, e1 = e3, e2, e1, hg() ec++ } // Complete task 2: fmt.Printf("hs(27): %d elements: [%d %d %d %d ... %d %d %d %d]\n", ec, s1, s2, s3, s4, e4, e3, e2, e1) // Task 3: strategy is to not store sequences, but just the length // of each sequence. as soon as the sequence we're currently working on // dips into the range that we've already computed, we short-circuit // to the end by adding the that known length to whatever length // we've accumulated so far. var nMaxLen int // variable holds n with max length encounted so far // slice holds sequence length for each n as it is computed var computedLen [1e5]int computedLen[1] = 1 for n := 2; n < 1e5; n++ { var ele, lSum int for hg := newHg(n); ; lSum++ { ele = hg() // as soon as we get an element in the range we have already // computed, we're done... if ele < n { break } } // just add the sequence length already computed from this point. lSum += computedLen[ele] // save the sequence length for this n computedLen[n] = lSum // and note if it's the maximum so far if lSum > computedLen[nMaxLen] { nMaxLen = n } } fmt.Printf("hs(%d): %d elements\n", nMaxLen, computedLen[nMaxLen])} ## Groovy def hailstone = { long start -> def sequence = [] while (start != 1) { sequence << start start = (start % 2l == 0l) ? start / 2l : 3l * start + 1l } sequence << start} Test Code def sequence = hailstone(27)assert sequence.size() == 112assert sequence[0..3] == [27, 82, 41, 124]assert sequence[-4..-1] == [8, 4, 2, 1] def results = (1..100000).collect { [n:it, size:hailstone(it).size()] }.max { it.size }println results Output: [n:77031, size:351] ## Haskell import Data.List (maximumBy)import Data.Ord (comparing) hailstone :: Int -> [Int]hailstone 1 = [1]hailstone n | even n = n : hailstone (n div 2) | otherwise = n : hailstone (n * 3 + 1) withResult :: (t -> t1) -> t -> (t1, t)withResult f x = (f x, x) main :: IO ()main = do let h27 = hailstone 27 print$ length h27 let h4 = show $take 4 h27 let t4 = show$ drop (length h27 - 4) h27 putStrLn ("hailstone 27: " ++ h4 ++ " ... " ++ t4) print $maximumBy (comparing fst)$ map (withResult (length . hailstone)) [1..100000]

Output:

112
hailstone 27: [27,82,41,124] ... [8,4,2,1]
(351,77031)

## HicEst

DIMENSION stones(1000) H27 = hailstone(27)ALIAS(stones,1, first4,4)ALIAS(stones,H27-3,  last4,4)WRITE(ClipBoard, Name) H27, first4, "...", last4 longest_sequence = 0DO try = 1, 1E5  elements = hailstone(try)  IF(elements >= longest_sequence) THEN      number = try      longest_sequence = elements      WRITE(StatusBar, Name) number, longest_sequence  ENDIFENDDOWRITE(ClipBoard, Name) number, longest_sequenceEND FUNCTION hailstone( n )   USE : stones    stones(1) = n   DO i = 1, LEN(stones)     IF(stones(i) == 1) THEN         hailstone = i         RETURN     ELSEIF( MOD(stones(i),2) ) THEN       stones(i+1) = 3*stones(i) + 1     ELSE       stones(i+1) = stones(i) / 2     ENDIF   ENDDOEND

H27=112; first4(1)=27; first4(2)=82; first4(3)=41; first4(4)=124; ...; last4(1)=8; last4(2)=4; last4(3)=2; last4(4)=1;
number=77031; longest_sequence=351;

## Icon and Unicon

A simple solution that generates (in the Icon sense) the sequence is:

procedure hailstone(n)    while n > 1 do {        suspend n        n := if n%2 = 0 then n/2 else 3*n+1        }    suspend 1end

and a test program for this solution is:

procedure main(args)    n := integer(!args) | 27    every writes(" ",hailstone(n))end

but this solution is computationally expensive when run repeatedly (task 3).

The following solution uses caching to improve performance on task 3 at the expense of space.

procedure hailstone(n)    static cache    initial {        cache := table()        cache[1] := [1]        }    /cache[n] := [n] ||| hailstone(if n%2 = 0 then n/2 else 3*n+1)    return cache[n]end

A test program is:

procedure main(args)    n := integer(!args) | 27    task2(n)    write()    task3()end procedure task2(n)    count := 0    every writes(" ",right(!(sequence := hailstone(n)),5)) do        if (count +:= 1) % 15 = 0 then write()    write()    write(*sequence," value",(*sequence=1,"")|"s"," in the sequence.")end procedure task3()    maxHS := 0    every n := 1 to 100000 do {        count := *hailstone(n)        if maxHS <:= count then maxN := n        }    write(maxN," has a sequence of ",maxHS," values")end

A sample run is:

->hs
27    82    41   124    62    31    94    47   142    71   214   107   322   161   484
242   121   364   182    91   274   137   412   206   103   310   155   466   233   700
350   175   526   263   790   395  1186   593  1780   890   445  1336   668   334   167
502   251   754   377  1132   566   283   850   425  1276   638   319   958   479  1438
719  2158  1079  3238  1619  4858  2429  7288  3644  1822   911  2734  1367  4102  2051
6154  3077  9232  4616  2308  1154   577  1732   866   433  1300   650   325   976   488
244   122    61   184    92    46    23    70    35   106    53   160    80    40    20
10     5    16     8     4     2     1
112 values in the sequence.

77031 has a sequence of 351 values
->


## Io

Here is a simple, brute-force approach:

 makeItHail := method(n,  stones := list(n)  while (n != 1,    if(n isEven,      n = n / 2,      n = 3 * n + 1    )    stones append(n)  )) out := makeItHail(27)writeln("For the sequence beginning at 27, the number of elements generated is ", out size, ".")write("The first four elements generated are ")for(i, 0, 3,  write(out at(i), " "))writeln(".") write("The last four elements generated are ")for(i, out size - 4, out size - 1,  write(out at(i), " "))writeln(".") numOfElems := 0nn := 3for(x, 3, 100000,  out = makeItHail(x)  if(out size > numOfElems,    numOfElems = out size    nn = x  )) writeln("For numbers less than or equal to 100,000, ", nn," has the longest sequence of ", numOfElems, " elements.")

Output:

For the sequence beginning at 27, the number of elements generated is 112.
The first four elements generated are 27 82 41 124 .
The last four elements generated are 8 4 2 1 .
For numbers less than or equal to 100,000, 77031 has the longest sequence of 351 elements.


## Ioke

 This example may be incorrect. Calculates the Hailstone sequence but might not complete everything from task description. Please verify it and remove this message. If the example does not match the requirements or does not work, replace this message with Template:incorrect or fix the code yourself.
collatz = method(n,  n println  unless(n <= 1,    if(n even?, collatz(n / 2), collatz(n * 3 + 1))))

## Inform 7

This solution uses a cache to speed up the length calculation for larger numbers.

Home is a room. To decide which list of numbers is the hailstone sequence for (N - number):	let result be a list of numbers;	add N to result;	while N is not 1:		if N is even, let N be N / 2;		otherwise let N be (3 * N) + 1;		add N to result;	decide on result. Hailstone length cache relates various numbers to one number. To decide which number is the hailstone sequence length for (N - number):	let ON be N;	let length so far be 0;	while N is not 1:		if N relates to a number by the hailstone length cache relation:			let result be length so far plus the number to which N relates by the hailstone length cache relation;			now the hailstone length cache relation relates ON to result;			decide on result;		if N is even, let N be N / 2;		otherwise let N be (3 * N) + 1;		increment length so far;	let result be length so far plus 1;	now the hailstone length cache relation relates ON to result;	decide on result. To say first and last (N - number) entry/entries in (L - list of values of kind K):	let length be the number of entries in L;	if length <= N * 2:		say L;	else:		repeat with M running from 1 to N:			if M > 1, say ", ";			say entry M in L;		say " ... ";		repeat with M running from length - N + 1 to length:			say entry M in L;			if M < length, say ", ". When play begins:	let H27 be the hailstone sequence for 27;	say "Hailstone sequence for 27 has [number of entries in H27] element[s]: [first and last 4 entries in H27].";	let best length be 0;	let best number be 0;	repeat with N running from 1 to 99999:		let L be the hailstone sequence length for N;		if L > best length:			let best length be L;			let best number be N;	say "The number under 100,000 with the longest hailstone sequence is [best number] with [best length] element[s].";	end the story.

Output:

Hailstone sequence for 27 has 112 elements: 27, 82, 41, 124 ... 8, 4, 2, 1.
The number under 100,000 with the longest hailstone sequence is 77031 with 351 elements.

## J

Solution:

hailseq=: -:(1 3&p.)@.(2&|) ^:(1 ~: ]) ^:a:"0

Usage:

   # hailseq 27                 NB. sequence length112   4 _4 {."0 1 hailseq 27       NB. first & last 4 numbers in sequence27 82 41 124 8  4  2   1   (>:@(i. >./) , >./) #@hailseq }.i. 1e5  NB. number < 100000 with max seq length & its seq length77031 351

## Java

Works with: Java version 1.5+
import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map; class Hailstone {   public static List<Long> getHailstoneSequence(long n) {    if (n <= 0)      throw new IllegalArgumentException("Invalid starting sequence number");    List<Long> list = new ArrayList<Long>();    list.add(Long.valueOf(n));    while (n != 1) {      if ((n & 1) == 0)        n = n / 2;      else        n = 3 * n + 1;      list.add(Long.valueOf(n));    }    return list;  }   public static void main(String[] args) {    List<Long> sequence27 = getHailstoneSequence(27);    System.out.println("Sequence for 27 has " + sequence27.size() + " elements: " + sequence27);     long MAX = 100000;    // Simple way    {      long highestNumber = 1;      int highestCount = 1;      for (long i = 2; i < MAX; i++) {        int count = getHailstoneSequence(i).size();        if (count > highestCount) {          highestCount = count;          highestNumber = i;        }      }      System.out.println("Method 1, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);    }     // More memory efficient way    {      long highestNumber = 1;      int highestCount = 1;      for (long i = 2; i < MAX; i++) {        int count = 1;        long n = i;        while (n != 1) {          if ((n & 1) == 0)            n = n / 2;          else            n = 3 * n + 1;          count++;        }        if (count > highestCount) {          highestCount = count;          highestNumber = i;        }      }      System.out.println("Method 2, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);    }     // Efficient for analyzing all sequences    {      long highestNumber = 1;      long highestCount = 1;      Map<Long, Integer> sequenceMap = new HashMap<Long, Integer>();      sequenceMap.put(Long.valueOf(1), Integer.valueOf(1));       List<Long> currentList = new ArrayList<Long>();      for (long i = 2; i < MAX; i++) {        currentList.clear();        Long n = Long.valueOf(i);        Integer count = null;        while ((count = sequenceMap.get(n)) == null) {          currentList.add(n);          long nValue = n.longValue();          if ((nValue & 1) == 0)            n = Long.valueOf(nValue / 2);          else            n = Long.valueOf(3 * nValue + 1);        }        int curCount = count.intValue();        for (int j = currentList.size() - 1; j >= 0; j--)          sequenceMap.put(currentList.get(j), Integer.valueOf(++curCount));        if (curCount > highestCount) {          highestCount = curCount;          highestNumber = i;        }      }      System.out.println("Method 3, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);    }    return;  }}

Output:

Sequence for 27 has 112 elements: [27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, 322, 161, 484, 242, 121, 364, 182, 91, 274, 137, 412, 206, 103, 310, 155, 466, 233, 700, 350, 175, 526, 263, 790, 395, 1186, 593, 1780, 890, 445, 1336, 668, 334, 167, 502, 251, 754, 377, 1132, 566, 283, 850, 425, 1276, 638, 319, 958, 479, 1438, 719, 2158, 1079, 3238, 1619, 4858, 2429, 7288, 3644, 1822, 911, 2734, 1367, 4102, 2051, 6154, 3077, 9232, 4616, 2308, 1154, 577, 1732, 866, 433, 1300, 650, 325, 976, 488, 244, 122, 61, 184, 92, 46, 23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1]
Method 1, number 77031 has the longest sequence, with a length of 351
Method 2, number 77031 has the longest sequence, with a length of 351
Method 3, number 77031 has the longest sequence, with a length of 351


## JavaScript

function hailstone (n) {    var seq = [n];    while (n > 1) {        n = n % 2 ? 3 * n + 1 : n / 2;        seq.push(n);    }    return seq;} // task 2: verify the sequence for n = 27var h = hailstone(27), hLen = h.length;print("sequence 27 is (" + h.slice(0, 4).join(", ") + " ... "    + h.slice(hLen - 4, hLen).join(", ") + "). length: " + hLen); // task 3: find the longest sequence for n < 100000for (var n, max = 0, i = 100000; --i;) {    var seq = hailstone(i), sLen = seq.length;    if (sLen > max) {        n = i;        max = sLen;    }}print("longest sequence: " + max + " numbers for starting point " + n);

outputs

sequence 27 is (27, 82, 41, 124 ... 8, 4, 2, 1). length: 112
longest sequence: 351 numbers for starting point 77031

## Julia

function hailstone(n)	seq = [n]	while n>1 		n = n % 2 == 0 ? n >> 1 : 3n + 1 		push!(seq,n)	end	return seqend
julia> h = hailstone(27);

julia> @assert length(h) == 112

julia> @assert h[1:4] == [27,82,41,124]

julia> @assert h[end-3:end] == [8,4,2,1]

julia> maximum([(length(hailstone(i)),i) for i in 1:100000])
(351,77031)

## K

  hail: (1<){:[x!2;1+3*x;_ x%2]}\  seqn: hail 27   #seqn112  4#seqn27 82 41 124  -4#seqn8 4 2 1   {m,x@s?m:|/s:{#hail x}'x}{x@&x!2}!:1e5351 77031

## Mercury

The actual calculation (including module ceremony) providing both a function and a predicate implementation:

:- module hailstone. :- interface. :- import_module int, list. :- func hailstone(int) = list(int).:- pred hailstone(int::in, list(int)::out) is det. :- implementation. hailstone(N) = S :- hailstone(N, S). hailstone(N, [N|S]) :-  ( N = 1 ->       S = []  ; N mod 2 = 0 -> hailstone(N/2, S)  ;                hailstone(3 * N + 1, S) ). :- end_module hailstone.

The mainline test driver (making use of unification for more succinct tests):

:- module test_hailstone. :- interface. :- import_module io. :- pred main(io.state::di, io.state::uo) is det. :- implementation. :- import_module int, list.:- import_module hailstone. :- pred longest(int::in, int::out, int::out) is det.:- pred longest(int::in, int::in, int::in, int::out, int::out) is det. longest(M, N, L) :- longest(M, 0, 0, N, L). longest(N, CN, CL, MN, ML) :-  ( N > 1 ->      L = list.length(hailstone(N)),      ( L > CL -> longest(N - 1, N,  L,  MN, ML)      ;           longest(N - 1, CN, CL, MN, ML) )  ;   MN = CN, ML = CL ).  main(!IO) :-  S = hailstone(27),  ( list.length(S) = 112,    list.append([27, 82, 41, 124], _, S),    list.remove_suffix(S, [8, 4, 2, 1], _),    longest(100000, 77031, 351) ->      io.write_string("All tests succeeded.\n", !IO)  ;   io.write_string("At least one test failed.\n", !IO) ). :- end_module test_hailstone.

The output of running this program is:

All tests succeeded.


For those unused to logic programming languages it seems that nothing has been proved in terms of confirming anything, but if you look at the predicate declaration for longest/3

:- pred longest(int::in, int::out, int::out) is det.

… you see that the second and third parameters are output parameters. This by calling longest(100000, 77031, 351) you prove, through unification, that the the longest sequence is with the number 77031 and that it is 351 cycles long.

Similarly, using list.append([27, 82, 41, 124], _, S) automatically proves that the generated sequence begins with the provided sequence, etc. Thus we know that the correct sequences and values were generated without bothering to print them out.

## Modula-2

MODULE hailst; IMPORT  InOut; CONST   maxCard         = MAX (CARDINAL) DIV 3;TYPE    action          = (List, Count, Max);VAR     a               : CARDINAL; PROCEDURE HailStone (start  : CARDINAL;  type  : action) : CARDINAL; VAR     n, max, count           : CARDINAL; BEGIN  count := 1;  n := start;  max := n;  LOOP    IF  type = List  THEN      InOut.WriteCard (n, 12);      IF  count MOD 6 = 0  THEN  InOut.WriteLn  END    END;    IF  n = 1  THEN  EXIT  END;    IF  ODD (n)  THEN      IF  n < maxCard  THEN        n := 3 * n + 1;        IF   n > max  THEN  max := n  END      ELSE        InOut.WriteString ("Exceeding max value for type CARDINAL at count = ");        InOut.WriteCard (count, 10);        InOut.WriteString (" for intermediate value ");        InOut.WriteCard (n, 10);        InOut.WriteString (". Aborting.");        HALT      END    ELSE      n := n DIV 2    END;    INC (count)  END;  IF  type = Max  THEN  RETURN  max  ELSE  RETURN  count  ENDEND HailStone; PROCEDURE FindMax (num   : CARDINAL); VAR     val, maxCount, maxVal, cnt      : CARDINAL; BEGIN  maxCount := 0;  maxVal := 0;  FOR  val := 2 TO num  DO   cnt := HailStone (val, Count);    IF  cnt > maxCount  THEN      maxVal := val;      maxCount := cnt    END  END;  InOut.WriteString ("Longest sequence below ");        InOut.WriteCard (num, 1);  InOut.WriteString (" is ");           InOut.WriteCard (HailStone (maxVal, Count), 1);  InOut.WriteString (" for n = ");      InOut.WriteCard (maxVal, 1);  InOut.WriteString (" with an intermediate maximum of ");  InOut.WriteCard (HailStone (maxVal, Max), 1);  InOut.WriteLnEND FindMax; BEGIN  a := HailStone (27, List);  InOut.WriteLn;  InOut.WriteString ("Iterations total = ");    InOut.WriteCard (HailStone (27, Count), 12);  InOut.WriteString (" max value = ");          InOut.WriteCard (HailStone (27, Max)  , 12);  InOut.WriteLn;  FindMax (100000);  InOut.WriteString ("Done.");          InOut.WriteLnEND hailst.

Producing:

## NetRexx

/* NetRexx */ options replace format comments java crossref savelog symbols binary do  start = 27  hs = hailstone(start)  hsCount = hs.words  say 'The number' start 'has a hailstone sequence comprising' hsCount 'elements'  say '  its first four elements are:' hs.subword(1, 4)  say '   and last four elements are:' hs.subword(hsCount - 3)   hsMax = 0  hsCountMax = 0  llimit = 100000  loop x_ = 1 to llimit - 1    hs = hailstone(x_)    hsCount = hs.words    if hsCount > hsCountMax then do      hsMax = x_      hsCountMax = hsCount      end    end x_   say 'The number' hsMax 'has the longest hailstone sequence in the range 1 to' llimit - 1 'with a sequence length of' hsCountMaxcatch ex = Exception  ex.printStackTraceend return method hailstone(hn = long) public static returns Rexx signals IllegalArgumentException   hs = Rexx('')  if hn <= 0 then signal IllegalArgumentException('Invalid start point.  Must be a positive integer greater than 0')   loop label n_ while hn > 1    hs = hs' 'hn    if hn // 2 \= 0 then hn = hn * 3 + 1                    else hn = hn % 2    end n_  hs = hs' 'hn   return hs.strip
Output
The number 27 has a hailstone sequence comprising 112 elements
its first four elements are: 27 82 41 124
and last four elements are: 8 4 2 1
The number 77031 has the longest hailstone sequence in the range 1 to 99999 with a sequence length of 351


## Oberon-2

MODULE hailst; IMPORT  Out; CONST   maxCard         = MAX (INTEGER) DIV 3;        List            = 1;        Count           = 2;        Max             = 3; VAR     a               : INTEGER; PROCEDURE HailStone (start, type  : INTEGER) : INTEGER; VAR     n, max, count           : INTEGER; BEGIN  count := 1;  n := start;  max := n;  LOOP    IF  type = List  THEN      Out.Int (n, 12);      IF  count MOD 6 = 0  THEN  Out.Ln  END    END;    IF  n = 1  THEN  EXIT  END;    IF  ODD (n)  THEN      IF  n < maxCard  THEN        n := 3 * n + 1;        IF   n > max  THEN  max := n  END      ELSE        Out.String ("Exceeding max value for type INTEGER at: ");        Out.String (" n = ");           Out.Int (start, 12);        Out.String (" , count = ");     Out.Int (count, 12);        Out.String (" and intermediate value ");        Out.Int (n, 1);        Out.String (". Aborting.");        Out.Ln;        HALT (2)      END    ELSE      n := n DIV 2    END;    INC (count)  END;  IF  type = Max  THEN  RETURN  max  ELSE  RETURN  count  ENDEND HailStone;  PROCEDURE FindMax (num   : INTEGER); VAR     val, maxCount, maxVal, cnt      : INTEGER; BEGIN  maxCount := 0;  maxVal := 0;  FOR  val := 2 TO num  DO   cnt := HailStone (val, Count);    IF  cnt > maxCount  THEN      maxVal := val;      maxCount := cnt    END  END;  Out.String ("Longest sequence below ");       Out.Int (num, 1);  Out.String (" is ");                          Out.Int (HailStone (maxVal, Count), 1);  Out.String (" for n = ");                     Out.Int (maxVal, 1);  Out.String (" with an intermediate maximum of ");  Out.Int (HailStone (maxVal, Max), 1);  Out.LnEND FindMax; BEGIN  a := HailStone (27, List);  Out.Ln;  Out.String ("Iterations total = ");   Out.Int (HailStone (27, Count), 12);  Out.String (" max value = ");         Out.Int (HailStone (27, Max)  , 12);  Out.Ln;  FindMax (1000000);  Out.String ("Done.");  Out.LnEND hailst.

Producing

          27          82          41         124          62          31
94          47         142          71         214         107
322         161         484         242         121         364
182          91         274         137         412         206
103         310         155         466         233         700
350         175         526         263         790         395
1186         593        1780         890         445        1336
668         334         167         502         251         754
377        1132         566         283         850         425
1276         638         319         958         479        1438
719        2158        1079        3238        1619        4858
2429        7288        3644        1822         911        2734
1367        4102        2051        6154        3077        9232
4616        2308        1154         577        1732         866
433        1300         650         325         976         488
244         122          61         184          92          46
23          70          35         106          53         160
80          40          20          10           5          16
8           4           2           1

Iterations total = 112 max value =  9232

Exceeding max value for type INTEGER at:  n = 113383 , count = 120 and intermediate value 827370449. Aborting.

## OCaml

#load "nums.cma";;open Num;; (* generate Hailstone sequence *)let hailstone n =  let one = Int 1  and two = Int 2  and three = Int 3 in  let rec g s x =    if x =/ one    then x::s    else g (x::s) (if mod_num x two =/ one                   then three */ x +/ one                   else x // two)  in  g [] (Int n);; (* compute only sequence length *)let haillen n =  let one = Int 1  and two = Int 2  and three = Int 3 in  let rec g s x =    if x =/ one    then s+1    else g (s+1) (if mod_num x two =/ one                  then three */ x +/ one                  else x // two)  in  g 0 (Int n);; (* max length for starting values in 1..n *)let hailmax =  let rec g idx len = function  | 0 -> (idx, len)  | i ->       let a = haillen i in      if a > len      then g i a (i-1)      else g idx len (i-1)  in  g 0 0;; hailmax 100000 ;;(* - : int * int = (77031, 351) *) List.rev_map string_of_num (hailstone 27) ;; (* - : string list =["27"; "82"; "41"; "124"; "62"; "31"; "94"; "47"; "142"; "71"; "214"; "107"; "322"; "161"; "484"; "242"; "121"; "364"; "182"; "91"; "274"; "137"; "412"; "206"; "103"; "310"; "155"; "466"; "233"; "700"; "350"; "175"; "526"; "263"; "790"; "395"; "1186"; "593"; "1780"; "890"; "445"; "1336"; "668"; "334"; "167"; "502"; "251"; "754"; "377"; "1132"; "566"; "283"; "850"; "425"; "1276"; "638"; "319"; "958"; "479"; "1438"; "719"; "2158"; "1079"; "3238"; "1619"; "4858"; "2429"; "7288"; "3644"; "1822"; "911"; "2734"; "1367"; "4102"; "2051"; "6154"; "3077"; "9232"; "4616"; "2308"; "1154"; "577"; "1732"; "866"; "433"; "1300"; "650"; "325"; "976"; "488"; "244"; "122"; "61"; "184"; "92"; "46"; "23"; "70"; "35"; "106"; "53"; "160"; "80"; "40"; "20"; "10"; "5"; "16"; "8"; "4"; "2"; "1"] *)

## ooRexx

 sequence = hailstone(27)say "Hailstone sequence for 27 has" sequence~items "elements and is ["sequence~toString('l', ", ")"]" highestNumber = 1highestCount = 1 loop i = 2 to 100000    sequence = hailstone(i)    count = sequence~items    if count > highestCount then do        highestNumber = i        highestCount = count    endendsay "Number" highestNumber "has the longest sequence with" highestCount "elements" -- short routine to generate a hailstone sequence::routine hailstone  use arg n   sequence = .array~of(n)  loop while n \= 1      if n // 2 == 0 then n = n / 2      else n = 3 * n + 1      sequence~append(n)  end  return sequence 

Output:

Hailstone sequence for 27 has 112 elements and is [27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, 322, 161, 484, 242, 121, 364, 182, 91, 274, 137, 412, 206, 103, 310, 155, 466, 233, 700, 350, 175, 526, 263, 790, 395, 1186, 593, 1780, 890, 445, 1336, 668, 334, 167, 502, 251, 754, 77, 1132, 566, 283, 850, 425, 1276, 638, 319, 958, 479, 1438, 719, 2158, 1079, 3238, 1619, 4858, 2429, 7288, 3644, 1822, 911, 2734, 1367, 102, 051, 6154, 3077, 9232, 4616, 2308, 1154, 577, 1732, 866, 433, 1300, 650, 325, 976, 488, 244, 122, 61, 184, 92, 46, 23, 70, 35, 106, 53, 160, 0, 40, 20, 10, 5, 16, 8, 4, 2, 1]
Number 77031 has the longest sequence with 351 elements


## Order

To display the length, and first and last elements, of the hailstone sequence for 27, we could do this:

#include <order/interpreter.h> #define ORDER_PP_DEF_8hailstone ORDER_PP_FN(                  \8fn(8N,                                                       \    8cond((8equal(8N, 1), 8seq(1))                            \          (8is_0(8remainder(8N, 2)),                          \           8seq_push_front(8N, 8hailstone(8quotient(8N, 2)))) \          (8else,                                             \           8seq_push_front(8N, 8hailstone(8inc(8times(8N, 3))))))) ) ORDER_PP(  8lets((8H, 8seq_map(8to_lit, 8hailstone(27)))        (8S, 8seq_size(8H)),        8print(8(h(27) - length:) 8to_lit(8S) 8comma 8space               8(starts with:) 8seq_take(4, 8H) 8comma 8space               8(ends with:) 8seq_drop(8minus(8S, 4), 8H))        ) )
Output:
h(27) - length:112, starts with:(27)(82)(41)(124), ends with:(8)(4)(2)(1)

Unfortunately, the C preprocessor not really being designed with large amounts of garbage collection in mind, trying to compute the hailstone sequences up to 100000 is almost guaranteed to run out of memory (and take a very, very long time). If we wanted to try, we could add this to the program, which in most languages would use relatively little memory:

#define ORDER_PP_DEF_8h_longest ORDER_PP_FN( \8fn(8M, 8P, \    8if(8is_0(8M), \        8P, \        8let((8L, 8seq_size(8hailstone(8M))), \             8h_longest(8dec(8M), \                        8if(8greater(8L, 8tuple_at_1(8P)), \                            8pair(8M, 8L), 8P))))) ) ORDER_PP(  8let((8P, 8h_longest(8nat(1,0,0,0,0,0), 8pair(0, 0))),       8pair(8to_lit(8tuple_at_0(8P)), 8to_lit(8tuple_at_1(8P)))))

...or even this "more elegant" version, which will run out of memory very quickly indeed (but in practice seems to work better for smaller ranges):

ORDER_PP(  8let((8P,        8seq_head(          8seq_sort(8fn(8P, 8Q, 8greater(8tuple_at_1(8P),                                         8tuple_at_1(8Q))),                    8seq_map(8fn(8N,                                 8pair(8N, 8seq_size(8hailstone(8N)))),                             8seq_iota(1, 8nat(1,0,0,0,0,0)))))),       8pair(8to_lit(8tuple_at_0(8P)), 8to_lit(8tuple_at_1(8P)))) )

Notice that large numbers (>100) must be entered as digit sequences with 8nat. 8to_lit converts a digit sequence back to a readable number.

## Oz

declare  fun {HailstoneSeq N}     N > 0 = true %% assert     if N == 1 then         [1]     elseif {IsEven N} then N|{HailstoneSeq N div 2}     else                   N|{HailstoneSeq 3*N+1}     end  end   HSeq27 = {HailstoneSeq 27}  {Length HSeq27} = 112  {List.take HSeq27 4} = [27 82 41 124]  {List.drop HSeq27 108} = [8 4 2 1]   fun {MaxBy2nd A=A1#A2 B=B1#B2}     if B2 > A2 then B else A end  end   Pairs = {Map {List.number 1 99999 1}           fun {$I} I#{Length {HailstoneSeq I}} end} MaxI#MaxLen = {List.foldL Pairs MaxBy2nd 0#0} {System.showInfo "Maximum length "#MaxLen#" was found for hailstone("#MaxI#")"} Output: Maximum length 351 was found for hailstone(77031)  ## PARI/GP show(n)={ my(t=1); while(n>1, print1(n","); n=if(n%2, 3*n+1 , n/2 ); t++ ); print(1); t}; len(n)={ my(t=1); while(n>1, if(n%2, t+=2; n+=(n>>1)+1 , t++; n>>=1 ) ); t}; show(27)r=0;for(n=1,1e5,t=len(n);if(t>r,r=t;ra=n));print(ra"\t"r) Output: 27,82,41,124,62,31,94,47,142,71,214,107,322,161,484,242,121,364,182,91,274,137,4 12,206,103,310,155,466,233,700,350,175,526,263,790,395,1186,593,1780,890,445,133 6,668,334,167,502,251,754,377,1132,566,283,850,425,1276,638,319,958,479,1438,719 ,2158,1079,3238,1619,4858,2429,7288,3644,1822,911,2734,1367,4102,2051,6154,3077, 9232,4616,2308,1154,577,1732,866,433,1300,650,325,976,488,244,122,61,184,92,46,2 3,70,35,106,53,160,80,40,20,10,5,16,8,4,2,1 and 77031 351 ## Pascal See Delphi ## Perl ###  Straightforward #!/usr/bin/perl use warnings;use strict; my @h = hailstone(27);print "Length of hailstone(27) = " . scalar @h . "\n";print "[" . join(", ", @h[0 .. 3], "...", @h[-4 .. -1]) . "]\n"; my ($max, $n) = (0, 0);for my$x (1 .. 99_999) {    @h = hailstone($x); if (scalar @h >$max) {        ($max,$n) = (scalar @h, $x); }} print "Max length$max was found for hailstone($n) for numbers < 100_000\n"; sub hailstone { my ($n) = @_;     my @sequence = ($n); while ($n > 1) {        if ($n % 2 == 0) {$n = int($n / 2); } else {$n = $n * 3 + 1; } push @sequence,$n;    }     return @sequence;}

Output:

Length of hailstone(27) = 112
[27, 82, 41, 124, ..., 8, 4, 2, 1]
Max length 351 was found for hailstone(77031) for numbers < 100_000


###  Compact

A more compact version:

#!/usr/bin/perluse strict; sub hailstone {    @_ = local $_ = shift; push @_,$_ = $_ % 2 ? 3 *$_ + 1 : $_ / 2 while$_ > 1;    @_;} my @h = hailstone($_ = 27);print "$_: @h[0 .. 3] ... @h[-4 .. -1] (".@h.")\n"; @h = ();for (1 .. 99_999) { @h = ($_,$h[2]) if ($h[2] = hailstone($_)) > $h[1] }printf "%d: (%d)\n", @h; The same approach as in the compact version above, obfuscated: sub _{my$_=$_[''];push@_,$_&1?$_+=$_++<<1:($_>>=1)while$_^1;@_}@_=_($_=031^2);print "$_: @_[0..3] ... @_[-4..-1] (".@_.")\n";$_[1]<($_[2]=_($_))and@_=($_,$_[2])for 1..1e5-1;printf "%d: (%d)\n", @_; Output in either case: 27: 27 82 41 124 ... 8 4 2 1 (112) 77031: (351)  ## Perl 6 sub hailstone($n) { $n, {$_ %% 2 ?? $_ div 2 !!$_ * 3 + 1 } ... 1 } my @h = hailstone(27);say "Length of hailstone(27) = {+@h}";say ~@h; my $m max= +hailstone($_) => $_ for 1..99_999;say "Max length$m.key() was found for hailstone($m.value()) for numbers < 100_000"; ## PHP function hailstone($n,$seq=array()){$sequence = $seq;$sequence[] = $n; if($n == 1){		return $sequence; }else{$n = ($n%2==0) ?$n/2 : (3*$n)+1; return hailstone($n, $sequence); }}$result = hailstone(27);echo count($result) . ' Elements.<br>';echo 'Starting with : ' . implode(",",array_slice($result,0,4)) .' and ending with : ' . implode(",",array_slice($result,count($result)-4)) . '<br>'; $maxResult = array(0); for($i=1;$i<=100000;$i++){		$result = count(hailstone($i));		if($result > max($maxResult)){			$maxResult = array($i=>$result); }}foreach($maxResult as $key =>$val){echo 'Number < 100000 with longest Hailstone seq.: ' . $key . ' with length of ' .$val;}
112 Elements.
Starting with : 27,82,41,124 and ending with : 8,4,2,1
Number < 100000 with longest Hailstone seq.: 77031 with length of 351


## PicoLisp

(de hailstone (N)   (make      (until (= 1 (link N))         (setq N            (if (bit? 1 N)               (inc (* N 3))               (/ N 2) ) ) ) ) ) (let L (hailstone 27)   (println 27 (length L) (head 4 L) '- (tail 4 L)) ) (let N (maxi '((N) (length (hailstone N))) (range 1 100000))   (println N (length (hailstone N))) )

Output:

27 112 (27 82 41 124) - (8 4 2 1)
77031 351

## Pike

#!/usr/bin/env pike int next(int n){    if (n==1)        return 0;    if (n%2)        return 3*n+1;    else        return n/2;} array(int) hailstone(int n){    array seq = ({ n });    while (n=next(n))        seq += ({ n });    return seq;} void main(){    array(int) two = hailstone(27);    if (equal(two[0..3], ({ 27, 82, 41, 124 })) && equal(two[<3..], ({ 8,4,2,1 })))        write("sizeof(({ %{%d, %}, ... %{%d, %} }) == %d\n", two[0..3], two[<3..], sizeof(two));     mapping longest = ([ "length":0, "start":0 ]);     foreach(allocate(100000); int start; )    {        int length = sizeof(hailstone(start));        if (length > longest->length)        {            longest->length = length;            longest->start = start;        }    }    write("longest sequence starting at %d has %d elements\n", longest->start, longest->length);}

Output:

sizeof(({ 27, 82, 41, 124, , ... 8, 4, 2, 1,  }) == 112
longest sequence starting at 77031 has 351 elements


## PL/I

test: proc options (main);   declare (longest, n) fixed (15);   declare flag bit (1);   declare (i, value) fixed (15);    /* Task 1: */   flag = '1'b;   put skip list ('The sequence for 27 is');   i = hailstones(27);    /* Task 2: */   flag = '0'b;   longest = 0;   do i = 1 to 99999;      if longest < hailstones(i) then         do; longest = hailstones(i); value = i; end;   end;   put skip edit (value, ' has the longest sequence of ', longest) (a); hailstones: procedure (n) returns ( fixed (15));   declare n fixed (15) nonassignable;   declare (m, p) fixed (15);    m = n;   p = 1;   if flag then put skip list (m);   do p = 1 by 1 while (m > 1);      if iand(m, 1) = 0 then         m = m/2;      else         m = 3*m + 1;      if flag then put skip list (m);   end;   if flag then put skip list ('The hailstone sequence has length' || p);   return (p);end hailstones; end test;

Output:

The sequence for 27 is
27
82
41
124
62
31
94
47
142
71
214
107
322
161
484
242
121
364
182
91
274
137
412
206
103
310
155
466
233
700
350
175
526
263
790
395
1186
593
1780
890
445
1336
668
334
167
502
251
754
377
1132
566
283
850
425
1276
638
319
958
479
1438
719
2158
1079
3238
1619
4858
2429
7288
3644
1822
911
2734
1367
4102
2051
6154
3077
9232
4616
2308
1154
577
1732
866
433
1300
650
325
976
488
244
122
61
184
92
46
23
70
35
106
53
160
80
40
20
10
5
16
8
4
2
1
The hailstone sequence has length               112
77031 has the longest sequence of                351


## PowerShell

 This example does not show the output mentioned in the task description on this page (or a page linked to from here). Please ensure that it meets all task requirements and remove this message. Note that phrases in task descriptions such as "print and display" and "print and show" for example, indicate that (reasonable length) output be a part of a language's solution.
Works with: PowerShell version 3.0
 function Get-HailStone ($n){ switch($n)   {      1 {$n;return} {$n % 2 -eq 0}{$n; return Get-Hailstone ($n = $n / 2)} {$n % 2 -ne 0}{$n; return Get-Hailstone ($n = ($n * 3) +1)} }} function Get-HailStoneBelowLimit($UpperLimit){    begin {$Counts = @()} process { for ($i = 1; $i -lt$UpperLimit; $i++) {$Object =             [pscustomobject]@{                'Number' = $i 'Count' = (Get-HailStone$i).count            }             $Counts +=$Object        }    }     end {$Counts}}Get-HailStoneBelowLimit 100000 | Sort-Object count -descending | Select-Object number -first 1  ## Prolog 1. Create a routine to generate the hailstone sequence for a number. hailstone(1,[1]) :- !.hailstone(N,[N|S]) :- 0 is N mod 2, N1 is N / 2, hailstone(N1,S).hailstone(N,[N|S]) :- 1 is N mod 2, N1 is (3 * N) + 1, hailstone(N1, S). 2. Use the routine to show that the hailstone sequence for the number 27 has 112 elements starting with 27, 82, 41, 124 and ending with 8, 4, 2, 1. The following query performs the test. hailstone(27,X),length(X,112),append([27, 82, 41, 124], _, X),append(_, [8, 4, 2, 1], X). 3. Show the number less than 100,000 which has the longest hailstone sequence together with that sequences length. longestHailstoneSequence(M, Seq, Len) :- longesthailstone(M, 1, 1, Seq, Len).longesthailstone(1, Cn, Cl, Mn, Ml):- Mn = Cn, Ml = Cl.longesthailstone(N, _, Cl, Mn, Ml) :- hailstone(N, X), length(X, L), Cl < L, N1 is N-1, longesthailstone(N1, N, L, Mn, Ml).longesthailstone(N, Cn, Cl, Mn, Ml) :- N1 is N-1, longesthailstone(N1, Cn, Cl, Mn, Ml). run this query. longestHailstoneSequence(100000, Seq, Len). to get the following result Seq = 77031, Len = 351  ### Constraint Handling Rules CHR is a programming language created by Professor Thom Frühwirth. Works with SWI-Prolog and module chr written by Tom Schrijvers and Jan Wielemaker :- use_module(library(chr)).:- chr_option(debug, off).:- chr_option(optimize, full). :- chr_constraint collatz/2, hailstone/1, clean/0. % to remove all constraints hailstone/1 after computationclean @ clean \ hailstone(_) <=> true.clean @ clean <=> true. % compute Collatz numberinit @ collatz(1,X) <=> X = 1 | true.collatz @ collatz(N, C) <=> (N mod 2 =:= 0 -> C is N / 2; C is 3 * N + 1). % Hailstone loophailstone(1) ==> true.hailstone(N) ==> N \= 1 | collatz(N, H), hailstone(H). Code for task one : task1 :- hailstone(27), findall(X, find_chr_constraint(hailstone(X)), L), clean, % check the requirements ( (length(L, 112), append([27, 82, 41, 124 | _], [8,4,2,1], L)) -> writeln(ok); writeln(ko)). Output :  ?- task1. ok true. Code for task two : longest_sequence :- seq(2, 100000, 1-[1], Len-V), format('For ~w sequence has ~w len ! ~n', [V, Len]). % walk through 2 to 100000 and compute the length of the sequences% memorize the longestseq(N, Max, Len-V, Len-V) :- N is Max + 1, !.seq(N, Max, CLen - CV, FLen - FV) :- len_seq(N, Len - N), ( Len > CLen -> Len1 = Len, V1 = [N] ; Len = CLen -> Len1 = Len, V1 = [N | CV] ; Len1 = CLen, V1 = CV), N1 is N+1, seq(N1, Max, Len1 - V1, FLen - FV). % compute the len of the Hailstone sequence for a numberlen_seq(N, Len - N) :- hailstone(N), findall(hailstone(X), find_chr_constraint(hailstone(X)), L), length(L, Len), clean. Output :  ?- longest_sequence. For [77031] sequence has 351 len ! true.  ## Pure // 1. Create a routine to generate the hailstone sequence for a number. type odd x::int = x mod 2;type even x::int = ~odd x;odd x = typep odd x;even x = typep even x; hailstone 1 = [1];hailstone n::even = n:hailstone (n div 2);hailstone n::odd = n:hailstone (3*n + 1); // 2. Use the routine to show that the hailstone sequence for the number 27 // has 112 elements starting with 27, 82, 41, 124 and ending with 8, 4, 2, 1n = 27;hs = hailstone n;l = # hs;using system; printf ("the hailstone sequence for the number %d has %d elements " + "starting with %s and ending with %s\n") (n, l, __str__ (hs!!(0..3)), __str__ ( hs!!((l-4)..l))); // 3. Show the number less than 100,000 which has the longest hailstone // sequence together with that sequences length.printf ("the number under 100,000 with the longest sequence is %d " + "with a sequence length of %d\n") (foldr (\ (a,b) (c,d) -> if (b > d) then (a,b) else (c,d)) (0,0) (map (\ x -> (x, # hailstone x)) (1..100000))); Output: the hailstone sequence for the number 27 has 112 elements starting with [27,82,41,124] and ending with [8,4,2,1] the number under 100,000 with the longest sequence is 77031 with a sequence length of 351  ## Python def hailstone(n): seq = [n] while n>1: n = 3*n + 1 if n & 1 else n//2 seq.append(n) return seq if __name__ == '__main__': h = hailstone(27) assert len(h)==112 and h[:4]==[27, 82, 41, 124] and h[-4:]==[8, 4, 2, 1] print("Maximum length %i was found for hailstone(%i) for numbers <100,000" % max((len(hailstone(i)), i) for i in range(1,100000))) Sample Output Maximum length 351 was found for hailstone(77031) for numbers <100,000 ## R ### PART 1:makeHailstone <- function(n){ hseq <- n while (hseq[length(hseq)] > 1){ current.value <- hseq[length(hseq)] if (current.value %% 2 == 0){ next.value <- current.value / 2 } else { next.value <- (3 * current.value) + 1 } hseq <- append(hseq, next.value) } return(list(hseq=hseq, seq.length=length(hseq)))} ### PART 2:twenty.seven <- makeHailstone(27)twenty.seven$hseqtwenty.seven$seq.length ### PART 3:max.length <- 0; lower.bound <- 1; upper.bound <- 100000 for (index in lower.bound:upper.bound){ current.hseq <- makeHailstone(index) if (current.hseq$seq.length > max.length){    max.length <- current.hseq$seq.length max.index <- index }} cat("Between ", lower.bound, " and ", upper.bound, ", the input of ", max.index, " gives the longest hailstone sequence, which has length ", max.length, ". \n", sep="") Output: > twenty.seven$hseq  [1]   27   82   41  124   62   31   94   47  142   71  214  107  322  161  484 [16]  242  121  364  182   91  274  137  412  206  103  310  155  466  233  700 [31]  350  175  526  263  790  395 1186  593 1780  890  445 1336  668  334  167 [46]  502  251  754  377 1132  566  283  850  425 1276  638  319  958  479 1438 [61]  719 2158 1079 3238 1619 4858 2429 7288 3644 1822  911 2734 1367 4102 2051 [76] 6154 3077 9232 4616 2308 1154  577 1732  866  433 1300  650  325  976  488 [91]  244  122   61  184   92   46   23   70   35  106   53  160   80   40   20[106]   10    5   16    8    4    2    1 > twenty.seven$seq.length[1] 112 Between 1 and 1e+05, the input of 77031 gives the longest hailstone sequence, which has length 351. ## Racket  #lang racket (define hailstone (let ([t (make-hasheq)]) (hash-set! t 1 '(1)) (λ(n) (hash-ref! t n (λ() (cons n (hailstone (if (even? n) (/ n 2) (+ (* 3 n) 1))))))))) (define h27 (hailstone 27))(printf "h(27) = ~s, ~s items\n" (,@(take h27 4) ... ,@(take-right h27 4)) (length h27)) (define N 100000)(define longest (for/fold ([m #f]) ([i (in-range 1 (add1 N))]) (define h (hailstone i)) (if (and m (> (cdr m) (length h))) m (cons i (length h)))))(printf "for x<=~s, ~s has the longest sequence with ~s items\n" N (car longest) (cdr longest))  Output: h(27) = (27 82 41 124 ... 8 4 2 1), 112 items for x<=100000, 77031 has the longest sequence with 351 items  ## REXX ### non-optimized /*REXX pgm tests a number and a range for hailstone (Collatz) sequences.*/parse arg x y . /*get optional arguments from CL.*/if x=='' | x==',' then x=27 /*Any 1st argument? Use default.*/if y=='' | y==',' then y=100000-1 /*Any 2nd argument? Use default.*/numeric digits 20; @.=0 /*handle big #s; initialize array*/$=hailstone(x)      /*═══════════════════task 1═════════════════════════*/say x   ' has a hailstone sequence of '    words($)say ' and starts with: ' subword($, 1, 4)    " ∙∙∙"say '     and  ends  with:  ∙∙∙'   subword($, max(1, words($)-3))sayif y==0 then exit   /*═══════════════════task 2═════════════════════════*/w=0;         do j=1  for y             /*traipse through the numbers.   */             call hailstone j          /*compute the hailstone sequence.*/             if #hs<=w  then iterate   /*Not big 'nuff? Then keep going.*/             bigJ=j;    w=#hs          /*remember what # has biggest HS.*/             end   /*j*/say '(between 1──►'y") "   bigJ   ' has the longest hailstone sequence:' wexit                                   /*stick a fork in it, we're done.*//*──────────────────────────────────HAILSTONE subroutine────────────────*/hailstone: procedure expose #hs; parse arg n 1 s  /*N & S set to 1st arg*/            do #hs=1  while  n\==1      /*loop while  N  isn't  unity.   */           if n//2  then n=n*3+1       /*if  N  is odd,  calc:   3*n +1 */                    else n=n%2         /* "  "   " even, perform fast ÷ */           s=s n                       /*build a sequence list (append).*/           end   /*#hs*/return s

output

27  has a hailstone sequence of  112
and starts with:  27 82 41 124  ∙∙∙
and  ends  with:  ∙∙∙ 8 4 2 1

(between 1──►99999)  77031  has the longest hailstone sequence: 351


### optimized

This optimized version is about seven times faster than the unoptimized version.

/*REXX pgm tests a number and a range for hailstone (Collatz) sequences.*/parse arg x y .                        /*get optional arguments from CL.*/if x=='' | x==','   then x=27          /*Any 1st argument?  Use default.*/if y=='' | y==','   then y=99999       /*Any 2nd argument?  Use default.*/numeric digits 20;  @.=0               /*handle big #s; initialize array*/$=hailstone(x) /*═══════════════════task 1═════════════════════════*/say x ' has a hailstone sequence of ' words($)say '     and starts with: '       subword($, 1, 4) " ∙∙∙"say ' and ends with: ∙∙∙' subword($, max(1, words($)-3))sayif y==0 then exit /*═══════════════════task 2═════════════════════════*/w=0; do j=1 for y /*loop through all numbers <100k.*/$=hailstone(j)            /*compute the hailstone sequence.*/             #hs=words($) /*find the length of the sequence*/ if #hs<=w then iterate /*Not big 'nuff? Then keep going.*/ bigJ=j; w=#hs /*remember what # has biggest HS.*/ end /*j*/say '(between 1──►'y") " bigJ ' has the longest hailstone sequence:' wexit /*stick a fork in it, we're done.*//*──────────────────────────────────HAILSTONE subroutine────────────────*/hailstone: procedure expose @.; parse arg n 1 s 1 o /*N,S,O = 1st arg.*/@.1= /*special case for unity. */ do forever /*loop while N isn't unity. */ if @.n\==0 then do; s=s @.n; leave; end /*been here before?*/ if n//2 then n=n*3+1 /*if N is odd, calc: 3*n +1 */ else n=n%2 /* " " " even, perform fast ÷ */ s=s n /*build a sequence list (append).*/ end /*forever*/@.o=subword(s,2) /*memoization for this hailstone.*/return s output is the same as the non-optimized version. ## Ruby This program uses new methods (Integer#even? and Enumerable#max_by) from Ruby 1.8.7. Works with: Ruby version 1.8.7 def hailstone n seq = [n] until n == 1 n = (n.even?) ? (n / 2) : (3 * n + 1) seq << n end seqend # for n = 27, show sequence length and first and last 4 elementshs27 = hailstone 27p [hs27.length, hs27[0..3], hs27[-4..-1]] # find the longest sequence among n less than 100,000n, len = (1 ... 100_000) .collect {|n| [n, hailstone(n).length]} .max_by {|n, len| len}puts "#{n} has a hailstone sequence length of #{len}"puts "the largest number in that sequence is #{hailstone(n).max}" Output: [112, [27, 82, 41, 124], [8, 4, 2, 1]] 77031 has a hailstone sequence length of 351 the largest number in that sequence is 21933016 ###  With shared structure This version builds some linked lists with shared structure. Hailstone::ListNode is an adaptation of ListNode from Singly-linked list/Element definition#Ruby. When two sequences contain the same value, those two lists share a tail. This avoids recomputing the end of the sequence. Works with: Ruby version 1.8.7 module Hailstone class ListNode include Enumerable attr_reader :value, :size, :succ def initialize(value, size, succ=nil) @value, @size, @succ = value, size, succ end def each node = self while node yield node.value node = node.succ end end end @@sequence = {1 => ListNode.new(1, 1)} module_function def sequence(n) unless @@sequence[n] ary = [] m = n until succ = @@sequence[m] ary << m m = (m.even?) ? (m / 2) : (3 * m + 1) end ary.reverse_each do |m| @@sequence[m] = succ = ListNode.new(m, succ.size + 1, succ) end end @@sequence[n] endend # for n = 27, show sequence length and first and last 4 elementshs27 = Hailstone.sequence(27).to_ap [hs27.length, hs27[0..3], hs27[-4..-1]] # find the longest sequence among n less than 100,000hs_big = (1 ... 100_000) .collect {|n| Hailstone.sequence n}.max_by {|hs| hs.size}puts "#{hs_big.first} has a hailstone sequence length of #{hs_big.size}"puts "the largest number in that sequence is #{hs_big.max}" ## Scala Library: Scala Works with: Scala version 2.10.2 object HailstoneSequence extends App { def hailstone(n: Int): Stream[Int] = n #:: (if (n == 1) Stream.empty else hailstone(if (n % 2 == 0) n / 2 else n * 3 + 1)) val nr = args.headOption.map(_.toInt).getOrElse(27) val collatz = hailstone(nr) println(s"Use the routine to show that the hailstone sequence for the number:$nr.")  println(collatz.toList)  println(s"It has ${collatz.length} elements.") println println( "Compute the number < 100,000, which has the longest hailstone sequence with that sequence's length.") val (n, len) = (1 until 100000).map(n => (n, hailstone(n).length)).maxBy(_._2) println(s"Longest hailstone sequence length=$len occurring with number $n.")} Output: Use the routine to show that the hailstone sequence for the number: 27. List(27, 82, 41, 124, 62, 31, 94, 47, 142, 71, 214, 107, 322, 161, 484, 242, 121, 364, 182, 91, 274, 137, 412, 206, 103, 310, 155, 466, 233, 700, 350, 175, 526, 263, 790, 395, 1186, 593, 1780, 890, 445, 1336, 668, 334, 167, 502, 251, 754, 377, 1132, 566, 283, 850, 425, 1276, 638, 319, 958, 479, 1438, 719, 2158, 1079, 3238, 1619, 4858, 2429, 7288, 3644, 1822, 911, 2734, 1367, 4102, 2051, 6154, 3077, 9232, 4616, 2308, 1154, 577, 1732, 866, 433, 1300, 650, 325, 976, 488, 244, 122, 61, 184, 92, 46, 23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1) It has 112 elements. Compute the number < 100,000, which has the longest hailstone sequence with that sequence's length. Longest hailstone sequence length= 351 occurring with number 77031. ## Scheme (define (collatz n)(if (= n 1) '(1)(cons n (collatz (if (even? n) (/ n 2) (+ 1 (* 3 n))))))) (define (collatz-length n)(let aux ((n n) (r 1)) (if (= n 1) r(aux (if (even? n) (/ n 2) (+ 1 (* 3 n))) (+ r 1))))) (define (collatz-max a b)(let aux ((i a) (j 0) (k 0))(if (> i b) (list j k)(let ((h (collatz-length i)))(if (> h k) (aux (+ i 1) i h) (aux (+ i 1) j k)))))) (collatz 27); (27 82 41 124 62 31 94 47 142 71 214 107 322 161 484 242 121 364 182; 91 274 137 412 206 103 310 155 466 233 700 350 175 526 263 790 395; 1186 593 1780 890 445 1336 668 334 167 502 251 754 377 1132 566 283; 850 425 1276 638 319 958 479 1438 719 2158 1079 3238 1619 4858 2429; 7288 3644 1822 911 2734 1367 4102 2051 6154 3077 9232 4616 2308 1154; 577 1732 866 433 1300 650 325 976 488 244 122 61 184 92 46 23 70 35; 106 53 160 80 40 20 10 5 16 8 4 2 1) (collatz-length 27); 112 (collatz-max 1 100000); (77031 351) ## Seed7 $ include "seed7_05.s7i"; const func array integer: hailstone (in var integer: n) is func  result    var array integer: hSequence is 0 times 0;  begin    while n <> 1 do      hSequence &:= n;      if odd(n) then        n := 3 * n + 1;      else        n := n div 2;      end if;    end while;    hSequence &:= n;  end func; const func integer: hailstoneSequenceLength (in var integer: n) is func  result    var integer: sequenceLength is 1;  begin    while n <> 1 do      incr(sequenceLength);      if odd(n) then        n := 3 * n + 1;      else        n := n div 2;      end if;    end while;  end func; const proc: main is func  local    var integer: number is 0;    var integer: length is 0;    var integer: maxLength is 0;    var integer: numberOfMaxLength is 0;    var array integer: h27 is 0 times 0;  begin    for number range 1 to 99999 do      length := hailstoneSequenceLength(number);      if length > maxLength then        maxLength := length;        numberOfMaxLength := number;      end if;    end for;    h27 := hailstone(27);    writeln("hailstone(27):");    for number range 1 to 4 do      write(h27[number] <& ", ");    end for;    write("....");    for number range length(h27) -3 to length(h27) do      write(", " <& h27[number]);    end for;    writeln("  length=" <& length(h27));    writeln("Maximum length " <& maxLength <& " at number=" <& numberOfMaxLength);  end func;

Output:

hailstone(27):
27, 82, 41, 124, ...., 8, 4, 2, 1  length=112
Maximum length 351 at number=77031


## Smalltalk

Works with: GNU Smalltalk
Object subclass: Sequences [  Sequences class >> hailstone: n [      |seq|       seq := OrderedCollection new.      seq add: n.      (n = 1) ifTrue: [ ^seq ].      (n even) ifTrue: [ seq addAll: (Sequences hailstone: (n / 2)) ]               ifFalse: [ seq addAll: (Sequences hailstone: ( (3*n) + 1 ) ) ].      ^seq.  ]   Sequences class >> hailstoneCount: n [      ^ (Sequences hailstoneCount: n num: 1)  ]   "this 'version' avoids storing the sequence, it just counts   its length - no memoization anyway"  Sequences class >> hailstoneCount: n num: m [      (n = 1) ifTrue: [ ^m ].      (n even) ifTrue: [ ^ Sequences hailstoneCount: (n / 2) num: (m + 1) ]               ifFalse: [ ^ Sequences hailstoneCount: ( (3*n) + 1) num: (m + 1) ].  ]].
|r|r := Sequences hailstone: 27.  "hailstone 'from' 27"(r size) displayNl.            "its length" "test 'head' ..."( (r first: 4) = #( 27  82  41  124 ) asOrderedCollection ) displayNl. "... and 'tail'"( ( (r last: 4 ) ) = #( 8 4 2 1 ) asOrderedCollection) displayNl. |longest|longest := OrderedCollection from: #( 1 1 ).2 to: 100000 do: [ :c |  |l|  l := Sequences hailstoneCount: c.  (l > (longest at: 2) ) ifTrue: [ longest replaceFrom: 1 to: 2 with: { c . l }  ].]. ('Sequence generator %1, sequence length %2' % { (longest at: 1) . (longest at: 2) })   displayNl.

## SNUSP

   /@+@@@+++# 27
|    halve odd   /===count<<\    /recurse\    #/?\ zero
$>@/===!/===-?\==>?!/-<+++\ \!/=!\@\>?!\@/<@\.!\-/ /+<-\!>\?-<+>/++++<\?>+++/*6+4 | | \=/ \=itoa=@@@+@+++++# \=>?/<=!=\ | | ! /+ !/+ !/+ !/+ \ mod10 |//!==/========\ | /<+> -\!?-\!?-\!?-\!?-\! /=>?\<=/\<+>!\->+>+<<?/>>=print@/\ln \?!\-?!\-?!\-?!\-?!\-?/\ div10 \+<-/!< ----------.++++++++++/ # +/! +/! +/! +/! +/  ## Tcl The core looping structure is an example of an n-plus-one-half loop, except the loop is officially infinite here. proc hailstone n { while 1 { lappend seq$n	if {$n == 1} {return$seq}	set n [expr {$n & 1 ?$n*3+1 : $n/2}] }} set h27 [hailstone 27]puts "h27 len=[llength$h27]"puts "head4 = [lrange $h27 0 3]"puts "tail4 = [lrange$h27 end-3 end]" set maxlen [set max 0]for {set i 1} {$i<100000} {incr i} { set l [llength [hailstone$i]]    if {$l>$maxlen} {set maxlen $l;set max$i}}puts "max is $max, with length$maxlen"

Output:

h27 len=112
head4 = 27 82 41 124
tail4 = 8 4 2 1
max is 77031, with length 351


## TXR

@(do (defun hailstone (n)       (cons n              (gen (not (eq n 1))                  (set n (if (evenp n)                           (trunc n 2)                           (+ (* 3 n) 1)))))))@(next :list @(mapcar* (fun tostring) (hailstone 27)))278241124@(skip)8421@(eof)@(do (let ((max 0) maxi)       (each* ((i (range 1 99999))               (h (mapcar* (fun hailstone) i))               (len (mapcar* (fun length) h)))         (if (> len max)           (progn             (set max len)             (set maxi i))))       (format t "longest sequence is ~a for n = ~a\n" max maxi)))
$txr -l hailstone.txr longest sequence is 351 for n = 77031 ## UNIX Shell The best way is to use a shell with built-in arrays and arithmetic, such as Bash. Works with: Bash #!/bin/bash# seq is the array genereated by hailstone# index is used for seqdeclare -a seqdeclare -i index # Create a routine to generate the hailstone sequence for a numberhailstone () { unset seq index seq[$((index++))]=$((n=$1))  while [ $n -ne 1 ]; do [$((n % 2)) -eq 1 ] && ((n=n*3+1)) || ((n=n/2))    seq[$((index++))]=$n  done} # Use the routine to show that the hailstone sequence for the number 27# has 112 elements starting with 27, 82, 41, 124 and ending with 8, 4, 2, 1i=27hailstone $iecho "$i: ${#seq[@]}"echo "${seq[@]:0:4} ... ${seq[@]:(-4):4}" # Show the number less than 100,000 which has the longest hailstone# sequence together with that sequences length.# (But don't show the actual sequence)!max=0maxlen=0for ((i=1;i<100000;i++)); do hailstone$i  if [ $((len=${#seq[@]})) -gt $maxlen ]; then max=$i    maxlen=$len fidone echo "${max} has a hailstone sequence length of ${maxlen}" output 27: 112 27 82 41 124 ... 8 4 2 1 77031 has a hailstone sequence of 351 ### Bourne Shell This script follows tradition for the Bourne Shell; its hailstone() function writes the sequence to standard output, so the shell can capture or pipe this output. This script is very slow because it forks many processes. Each command substitution forks a subshell, and each expr(1) command forks a process. • Therefore, this script only examines sequences from 1 to 1000, not 100000. A fast computer might run this script in 45 to 120 seconds, using most time to run system calls in kernel mode. If the script went to 100000, it would need several hours. Works with: Bourne Shell # Outputs a hailstone sequence from$1, with one element per line.# Clobbers $n.hailstone() { n=expr "$1" + 0	eval "test $? -lt 2 || return$?"  # $n must be integer. echo$n	while test $n -ne 1; do if expr$n % 2 >/dev/null; then			n=expr 3 \* $n + 1 else n=expr$n / 2		fi		echo $n done} set -- hailstone 27echo "Hailstone sequence from 27 has$# elements:"first="$1,$2, $3,$4"shift expr $# - 4echo "$first, ..., $1,$2, $3,$4" i=1 max=0 maxlen=0while test $i -lt 1000; do len=hailstone$i | wc -l | tr -d ' '	test $len -gt$maxlen && max=$i maxlen=$len	i=expr $i + 1doneecho "Hailstone sequence from$max has $maxlen elements." ### C Shell This script is several times faster than the previous Bourne Shell script, because it uses C Shell expressions, not the expr(1) command. This script is slow, but it can reach 100000, and a fast computer might run it in less than 15 minutes. # Outputs a hailstone sequence from !:1, with one element per line.# Clobbers$n.alias hailstone eval \''@ n = \!:1:q		\\	echo $n \\ while ($n != 1 )			\\		if ( $n % 2 ) then \\ @ n = 3 *$n + 1	\\		else				\\			@ n /= 2		\\		endif				\\		echo $n \\ end \\'\' set sequence=(hailstone 27)echo "Hailstone sequence from 27 has$#sequence elements:"@ i = $#sequence - 3echo "$sequence[1-4] ... $sequence[$i-]" # hailstone-length $i# acts like# @ len = hailstone$i | wc -l | tr -d ' '#   but without forking any subshells.alias hailstone-length eval \''@ n = \!:1:q	\\	@ len = 1				\\	while ( $n != 1 ) \\ if ($n % 2 ) then		\\			@ n = 3 * $n + 1 \\ else \\ @ n /= 2 \\ endif \\ @ len += 1 \\ end \\'\' @ i = 1@ max = 0@ maxlen = 0while ($i < 100000)	# XXX - I must run hailstone-length in a subshell, because my	# C Shell has a bug when it runs hailstone-length inside this	# while ($i < 1000) loop: it forgets about this loop, and # reports an error <<end: Not in while/foreach.>> @ len = hailstone-length$i; echo $len if ($len > $maxlen) then @ max =$i		@ maxlen = $len endif @ i += 1endecho "Hailstone sequence from$max has $maxlen elements." $ csh -f hailstone.csh
Hailstone sequence from 27 has 112 elements:
27 82 41 124 ... 8 4 2 1
Hailstone sequence from 77031 has 351 elements.

## Ursala

#import std#import nat hail = @iNC ~&h~=1->x ^C\~& @h ~&h?\~&t successor+ sum@iNiCX #show+ main = <   ^T(@ixX take/$4; %nLP~~lrxPX; ^|TL/~& :/'...',' has length '--@h+ %nP+ length) hail 27, ^|TL(~&,:/' has sequence length ') %nP~~ nleq$^&r ^(~&,length+ hail)* nrange/1 100000>

The hail function computes the sequence as follows.

• Given a number as an argument, @iNC makes a list containing only that number before passing it to the rest of the function. The i in the expression stands for the identity function, N for the constant null function, and C for the cons operator.
• The iteration combinator (->) is used with a predicate of ~&h~=l which tests the condition that the head (~&h) of its argument is not equal (~=) to 1. Iteration of the rest of the function continues while this predicate holds.
• The x suffix says to return the reversal of the list after the iteration finishes.
• The function being iterated builds a list using the cons operator (^C) with the identity function (~&) of the argument for the tail, and the result of the rest of the line for the head.
• The @h operator says that the function following will be applied to the head of the list.
• The conditional operator (?) has the head function (~&h) as its predicate, which tests whether the head of its argument is non-null.
• In this case, the argument is a natural number, but naturals are represented as lists of booleans, so taking the head of a number is the same as testing the least significant bit.
• If the condition is not met, the number has a 0 least significant bit, and therefore is even. In this case, the conditional predicate calls for taking its tail (~&t), effectively dividing it by 2 using a bit shift.
• If the condition is met, the number is odd, so the rest of the function computes the successor of the number multiplied by three.
• Rather than multiplying the hard way, the function sum@iNiCX computes the sum of the pair (X) of numbers given by the identity function (i) of the argument, and the doubling of the argument (NiC), also obtained by a bit shift, with a zero bit (N) consed (C) with the identity (i).

Most of the main expression pertains to less interesting printing and formatting, but the part that searches for the longest sequence in the range is nleq$^&r ^(~&,length+ hail)* nrange/1 100000. • The expression nrange/1 100000 evaluates to the list of the first 100000 positive integers. • The map operator (*) causes a list to be made of the results of its operand applied to each number. • The operand to the map operator, applied to an individual number in the list, constructs a pair (^) with the identity function (~&) of the number on the left, and the length of the hail sequence on the right. • The maximizing operator ($^) with respect to the natural less or equal relation (nleq) applied to the right sides (&r) of its pair of arguments extracts the number with the maximum length sequence.

output:

<27,82,41,124>...<8,4,2,1> has length 112
77031 has sequence length 351

## XPL0

include c:\cxpl\codes;  \intrinsic 'code' declarationsint Seq(1000);          \more than enough for longest sequence func Hailstone(N);      \Return length of Hailstone sequence starting at Nint  N;                 \ also fills Seq array with sequenceint  I;[I:= 0;loop [Seq(I):= N;  I:= I+1;     if N=1 then return I;     N:= if N&1 then N*3+1 else N/2;     ];]; int N, SN, Len, MaxLen;[Len:= Hailstone(27);Text(0, "27's Hailstone length = ");  IntOut(0, Len);  CrLf(0); Text(0, "Sequence = ");for N:= 0 to 3 do [IntOut(0, Seq(N));  ChOut(0, ^ )];Text(0, "... ");for N:= Len-4 to Len-1 do [IntOut(0, Seq(N));  ChOut(0, ^ )];CrLf(0); MaxLen:= 0;for N:= 1 to 100_000-1 do    [Len:= Hailstone(N);    if Len > MaxLen then [MaxLen:= Len;  SN:= N];       \save N with max length    ];IntOut(0, SN);  Text(0, "'s Hailstone length = ");  IntOut(0, MaxLen);]

Output:

27's Hailstone length = 112
Sequence = 27 82 41 124 ... 8 4 2 1
77031's Hailstone length = 351


## zkl

fcn collatz(n,z=L()){ z.append(n); if(n==1) return(z);   if(n.isEven) return(self.fcn(n/2,z)); return(self.fcn(n*3+1,z)) }

This uses tail recursion and thus is stack efficient.

Output:
var n=collatz(27)
n.len()
112
n[0,4]
L(27,82,41,124)
n[-4,*]
L(8,4,2,1)


Rather than write a function that calculates the length, just roll through all 100,000 sequences and save the largest (length,sequence start) pair. Creating all those Collatz lists isn't quick. This works by using a [mutable] list to hold state as the pump does the basic looping.

[2..0d100_000].pump(Void,  // loop n from 2 to 100,000   collatz,              // generate Collatz sequence(n)   fcn(c,n){           // if new longest sequence, save length/C, return longest      if(c.len()>n[0]) n.clear(c.len(),c[0]); n}.fp1(L(0,0)))
Output:
L(351,77031)  // length, hailstone