🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of HadiModarres

HadiModarres's solution

to Nth Prime in the PHP Track

Published at Jul 30 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Given a number n, determine what the nth prime is.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

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 nth-prime/nth-prime_test.php


A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7

Submitting Incomplete Solutions

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



class NthPrimeTest extends PHPUnit\Framework\TestCase
    public static function setUpBeforeClass() : void
        require 'nth-prime.php';

    public function testFirstPrime()
        $this->assertEquals(2, prime(1));
    public function testSecondPrime()
        $this->assertEquals(3, prime(2));
    public function testSixthPrime()
        $this->assertEquals(13, prime(6));
    public function testBigPrime()
        $this->assertEquals(104743, prime(10001));
    public function testZeroPrime()
        $this->assertEquals(false, prime(0));

function prime(int $nth){
   if ($nth === 0){
       return false;
    if ($nth === 1){
        return 2;
    $previousPrime = prime($nth - 1);
    $currentNum = $previousPrime+1;
    while (!isPrime($currentNum)) {
    return $currentNum;

function isPrime(int $num){
    for ($i = 2;$i<$num/2+1;$i++){
        if ($num % $i ===0){
            return false;
    return true;

echo prime(99);

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?