Avatar of Tyresius92

Tyresius92's solution

to Collatz Conjecture in the PHP Track

Published at Jul 26 2019 · 0 comments
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.



require "collatz-conjecture.php";

class CollatzConjecture extends PHPUnit\Framework\TestCase

    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) {
		throw new InvalidArgumentException('Only positive numbers are allowed');

	$result = 0;

	while ($num !== 1) {
		$num = $num % 2 ? $num * 3 + 1 : $num / 2;


	return $result;

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?