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maurelio1234's solution

to Sieve in the PHP Track

Published at Oct 05 2019 · 0 comments
Instructions
Test suite
Solution

Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.

The Sieve of Eratosthenes is a simple, ancient algorithm for finding all prime numbers up to any given limit. It does so by iteratively marking as composite (i.e. not prime) the multiples of each prime, starting with the multiples of 2.

Create your range, starting at two and continuing up to and including the given limit. (i.e. [2, limit])

The algorithm consists of repeating the following over and over:

  • take the next available unmarked number in your list (it is prime)
  • mark all the multiples of that number (they are not prime)

Repeat until you have processed each number in your range.

When the algorithm terminates, all the numbers in the list that have not been marked are prime.

The wikipedia article has a useful graphic that explains the algorithm: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

Notice that this is a very specific algorithm, and the tests don't check that you've implemented the algorithm, only that you've come up with the correct list of primes.

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 sieve/sieve_test.php
    

Source

Sieve of Eratosthenes at Wikipedia http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

Submitting Incomplete Solutions

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

sieve_test.php

<?php

class SieveTest extends PHPUnit\Framework\TestCase
{
    public static function setUpBeforeClass() : void
    {
        require_once 'sieve.php';
    }

    public function testNoPrimesUnderTwo() : void
    {
        $this->assertEquals([], sieve(1));
    }
    public function testFindFirstPrime() : void
    {
        $this->assertEquals([2], sieve(2));
    }
    public function testFindPrimesUpTo10() : void
    {
        $this->assertEquals([2, 3, 5, 7], sieve(10));
    }
    public function testLimitIsPrime() : void
    {
        $this->assertEquals([2, 3, 5, 7, 11, 13], sieve(13));
    }
    public function testFindPrimesUpTo1000() : void
    {
        $this->assertEquals(
            [
                2,
                3,
                5,
                7,
                11,
                13,
                17,
                19,
                23,
                29,
                31,
                37,
                41,
                43,
                47,
                53,
                59,
                61,
                67,
                71,
                73,
                79,
                83,
                89,
                97,
                101,
                103,
                107,
                109,
                113,
                127,
                131,
                137,
                139,
                149,
                151,
                157,
                163,
                167,
                173,
                179,
                181,
                191,
                193,
                197,
                199,
                211,
                223,
                227,
                229,
                233,
                239,
                241,
                251,
                257,
                263,
                269,
                271,
                277,
                281,
                283,
                293,
                307,
                311,
                313,
                317,
                331,
                337,
                347,
                349,
                353,
                359,
                367,
                373,
                379,
                383,
                389,
                397,
                401,
                409,
                419,
                421,
                431,
                433,
                439,
                443,
                449,
                457,
                461,
                463,
                467,
                479,
                487,
                491,
                499,
                503,
                509,
                521,
                523,
                541,
                547,
                557,
                563,
                569,
                571,
                577,
                587,
                593,
                599,
                601,
                607,
                613,
                617,
                619,
                631,
                641,
                643,
                647,
                653,
                659,
                661,
                673,
                677,
                683,
                691,
                701,
                709,
                719,
                727,
                733,
                739,
                743,
                751,
                757,
                761,
                769,
                773,
                787,
                797,
                809,
                811,
                821,
                823,
                827,
                829,
                839,
                853,
                857,
                859,
                863,
                877,
                881,
                883,
                887,
                907,
                911,
                919,
                929,
                937,
                941,
                947,
                953,
                967,
                971,
                977,
                983,
                991,
                997
            ],
            sieve(1000)
        );
    }
}
<?php

function sieve($n) {
    if ($n == 1) return array();

    $ret = range(2,$n);
    $sz = count($ret);

    for($i=0;$i<$sz;$i++) {
        if (!array_key_exists($i, $ret))
            continue;
        for($j=$i+1;$j<$sz;++$j) {
            if (array_key_exists($j, $ret) && $ret[$j] % $ret[$i] == 0) {
                unset($ret[$j]);
            }
        }
    }
    return array_values($ret);
}

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