Collatz conjecture

Collatz conjecture is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Note: There is an ongoing discussion on removing this draft task in favour of this one. Please give your opinion, but DO NOT add to this page as it is likely to go away.

These programs calculate the Collatz sequence for a given integer, and stop if 1 is reached.

These programs were written before this task was created; as such, the specific actual requirements of this task have not yet been determined.

bash

<lang sh>collatz () { n=$1; echo $n; if [ "$n" -gt 1 ]; then

  #if odd
  if [ "$(expr $n % 2)" -eq 1 ] ;then 
     collatz $(($n*3+1)) 
  else
     collatz $(($n/2))
  fi

fi }</lang>

Befunge

<lang befunge>&>:.:1-|

 >3*^ @
 |%2: <
v>2/>+</lang>

C

<lang c>#include <stdio.h>

1. include <stdlib.h>

int collatz( int n ) {

   printf( "%i\n", n );

   if( n == 1 )
       return 1;
   else
   {
       if( (n % 2) == 0 )
           return collatz( n / 2 );
       else
           return collatz(1 + (n * 3));
   }
   return 1;

}

int main( int argc, char** argv ) {

   int n = atoi( argv[1] );
   int i = collatz( n );
   printf( "%i\n", i );

   return 0;

}</lang>

C++

<lang cpp>#include <iostream> using namespace std;

int main() {

   int n;
   cin >> n;

   while (n > 1) {

if (n%2 != 0) { n = n*3 + 1; } else { n = n/2; } cout << n << endl;

   }

}</lang>

Clojure

<lang lisp>(defn collatz [n]

 (println n) 
 (if (> n 1) (if (even? n) 

(recur (/ n 2)) (recur (inc (* 3 n))))))

(collatz (read-string (read-line)))</lang>

Common Lisp

<lang lisp>(defun collatz (n)

 (format t "~a~%" n)
 (cond ((= n 1) 1)
       ((= (mod n 2) 0) (collatz (/ n 2)))
       (t (collatz (+ 1 (* n 3))))))

(collatz (read))</lang>

Excel

   In cell A1, place the starting number.
   In cell A2 enter this formula =IF(A1/2=ROUND(A1/2,0),A1/2,A1*3+1)
   Drag and copy the formula down until 4, 2, 1

Ioke

<lang ioke>collatz = method(n,

 n println
 unless(n <= 1,
   if(n even?, collatz(n / 2), collatz(n * 3 + 1)))

)</lang>

Java

<lang java>public class Collatz {

 public static void main(String[] args) {
   for (int x = Integer.parseInt(args[0]); x > 1; x = (x % 2 == 0) ? x / 2 : x * 3 + 1) {
     System.out.println(x);
   }
 }

}</lang>

Fortran

Works with: Fortran version 90

<lang fortran>program main

 integer(4) :: n,i

 write(6,*) "Input a natural number: "
 read (6,*) n

 do while(n.gt.1)
    if(mod(n,2).eq.0) then ! number is even
       n=n/2
    else                   ! number is odd
       n=3*n +1
    endif
    write(6,*) n     
 enddo

end program main</lang>

Haskell

<lang haskell>collatz :: Integer -> Integer collatz n

   | n == 1    = 1
   | even n    = collatz (n `div` 2)
   | otherwise = collatz (3 * n + 1)</lang>

Oz

<lang oz>declare

 fun lazy {CollatzSeq N}
    N > 0 = true %% assert
    if N == 1 then         [1]
    elseif {IsEven N} then N|{CollatzSeq N div 2}
    else                   N|{CollatzSeq 3*N+1}
    end
 end

in

 {ForAll {CollatzSeq 42} Show}</lang>

Perl

<lang perl>#!/usr/bin/perl

die("Usage: $0 n\n") if (!$ARGV[0] || !int($ARGV[0]));

my $n = int($ARGV[0]);

while ($n > 1) {

 $n = ($n % 2 != 0) ? $n * 3 + 1 : $n / 2;
 print "$n\n";

} </lang>

PHP

<lang php>while($n > 1) {

  if($n % 2 != 0)
  {
     $n = $n*3 + 1;
  }
  else
  {
     $n = $n/2;
  }
  echo $n . '
';

}</lang>

PureBasic

<lang PureBasic>While n>1

 If n%2
   n=(3*n)+1
 Else
   n/2
 EndIf
 PrintN(Str(n))

Wend</lang>

Python

<lang python>def collatz(n):

   while n > 1:
       print (n)
       if n & 1:
           n = (n*3)+1
       else:
           n >> 1
   print (n)</lang>

Scala

<lang scala>def collatz(n:BigInt):Stream[BigInt] =

   if (n == 1) {
       Stream(1);
   } else {
       def next(n:BigInt):BigInt = if ((n % 2) == 0) (n / 2) else (n * 3 + 1);
       Stream.cons(n, collatz(next(n)));
   }</lang>