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HadiModarres's solution

to Collatz Conjecture in the PHP Track

Published at Jul 29 2019 · 1 comment
Test suite


This exercise has changed since this solution was written.

The Collatz Conjecture or 3x+1 problem can be summarized as follows:

Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.

Given a number n, return the number of steps required to reach 1.


Starting with n = 12, the steps would be as follows:

  1. 12
  2. 6
  3. 3
  4. 10
  5. 5
  6. 16
  7. 8
  8. 4
  9. 2
  10. 1

Resulting in 9 steps. So for input n = 12, the return value would be 9.

Running the tests

  1. Go to the root of your PHP exercise directory, which is <EXERCISM_WORKSPACE>/php. To find the Exercism workspace run

     % exercism debug | grep Workspace
  2. Get PHPUnit if you don't have it already.

     % wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
     % chmod +x phpunit.phar
  3. Execute the tests:

     % ./phpunit.phar collatz-conjecture/collatz-conjecture_test.php


An unsolved problem in mathematics named after mathematician Lothar Collatz https://en.wikipedia.org/wiki/3x_%2B_1_problem

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.



class CollatzConjecture extends PHPUnit\Framework\TestCase
    public static function setUpBeforeClass() : void
        require 'collatz-conjecture.php';

    public function testZeroStepsForOne()
        $this->assertEquals(0, steps(1));

    public function testDivideIfEven()
        $this->assertEquals(4, steps(16));

    public function testEvenAndOddSteps()
        $this->assertEquals(9, steps(12));

    public function testLargeNumberOfEvenAndOddSteps()
        $this->assertEquals(152, steps(1000000));

    public function testZeroIsAnError()
        $this->expectExceptionMessage('Only positive numbers are allowed');


    public function testNegativeValueIsAnError()
        $this->expectExceptionMessage('Only positive numbers are allowed');


function steps(int $num):int{
    if ($num===1){
        return 0;
    if ($num%2==0){
        $num = $num/2;
        $num = 3*$num+1;
    return steps($num)+1;

Community comments

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Avatar of chaseisabelle

Nice use of recursion. Cool!

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