Question

I have a XAMPP install, with pretty much the default config.

Performance isn’t much of a problem in general as I use PHP mostly to run web pages and small web apps. Waiting a couple seconds for a page is not unusual.

However, I have recently taken up the problems from Project Euler and decided to do them in PHP.

Try as I may, I couldn’t get my code to run in less than 1 minute 1 second (optimized down from almost 3 min) and I was getting pretty embarrassed, especially considering most posters on Pjt Euler reported times of 1-3 seconds. (#7, find the 10001th prime)

I ported my code to C#, and the same task completed in a blink. 0.4 seconds. Same algorithm, the only notable difference in the code is that I used a List in C# to replace the array I was using in PHP.

While I did expect C# to outperform php, this difference leads me to suspect a gross configuration problem, but I have no idea where to look.

What could be the cause of this poor performance?

---

Edit: Here is the code:

In PHP:

/*
  * Project Euler #7:
  * By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
  * What is the 10001st prime number?
  */

ini_set('max_execution_time', 300);  
echo "start time:" . date("i:s:u") . "<br />";
function isPrime($number, $prevPrimes)
{       
    foreach ($prevPrimes as $key =>$prime)
    {
        if ($prime == 1)
        {
            continue;
        }
        elseif ($number % $prime == 0)
        {
            return 0;
        }
    }
    // If we get to here, $number is prime
    return $number; 
}
$primes = array();
$i = 0;
$nbPrimes = 0;
while ($nbPrimes <10001)
{
    $i++;
    if ($i % 2 != 0)
    {
        $result = isPrime($i, $primes);

        if ($result != 0)
        {
            $primes[] = $i;
            $nbPrimes++;
        }
    }
}
echo "#$nbPrimes: $result<br>";

echo "End time:" . date("i:s:u") . "<br />";

In C#:

public static void RunSnippet()
{
    Stopwatch stopwatch = new Stopwatch();
    stopwatch.Start();

    List<int> primes = new List<int>();
    int i = 0;
    int nbPrimes = 0;
    int result =0;
    while (nbPrimes <10001)
    {
        i++;
        if (i % 2 != 0)
        {
            result = isPrime(i, primes);

            if (result != 0)
            {
                primes.Add(i);
                nbPrimes++;
            }
        }
    }
    stopwatch.Stop();
    Console.WriteLine("Time elapsed: {0}",
    stopwatch.Elapsed);
    Console.WriteLine ("#" + nbPrimes + ": " + result.ToString());
}
public static int isPrime(int number, List<int> prevPrimes)
{
    foreach (int prime in prevPrimes)
    {
        if (prime == 1)
        {
            continue;
        }
        else if (number % prime == 0)
        {
            return 0;
        }
    }
    // If we get to here, number is prime
    return number;  
}   

Answer 1

FINAL EDIT

Here is the PHP code from Bakudan’s logic, which returns this result:

start time:44:25:000000
#10001: 104759
End time:44:26:000000

The Code:

<?php
echo "start time:" . date("i:s:u") . "\n";

function isPrime($number, &$primes)
{
    if ($number === 1) return false;
    elseif ($number %2 === 0) return false;
    elseif ($number < 4) return true;
    elseif ($number < 9) return true;
    elseif ($number %3 === 0) return false;
    else $r = floor(sqrt($number));

    $f = 5;
    while ($f <= $r) {
        if ($number % $f ===0) return false;
        if ($number % ($f+2) === 0) return false;
        $f = $f + 6;
    }

    return true;
}

$primes = array();
$nbPrimes = $i = 0;
while ($nbPrimes < 10001)
{
    $i++;
    if (isPrime($i, $primes) !== false)
    {
        $primes[] = $i;
        $nbPrimes++;
    }
}
echo "#$nbPrimes: " . end($primes) . "\n";
echo "End time:" . date("i:s:u") . "\n";

Bakudan gave me the pseudo code, I Just translated and wrote it out for the OP’s script above.

---

EDIT 2

I cleaned up the code a bit, didn’t improve anything, may enhance “readability”. But yea, I think this is the best you will get with PHP, which on an i7 without apache yields 5 seconds.

    <?php
    echo "start time:" . date("i:s:u") . "\n";

    function isPrime($number, &$primes)
    {
        foreach($primes as $prime) {
            if ($number % $prime === 0 && $prime > 1)
                    return false;
        }
    }

    $primes = array();
    $nbPrimes = $i = 1;
    while ($nbPrimes <= 10001)
    {
        if ($i % 2 !== 0 && isPrime($i, $primes) !== false)
        {
            $primes[] = $i;
            $nbPrimes++;
        }
        $i++;
    }
    echo "#$nbPrimes: " . end($primes) . "\n";
    echo "End time:" . date("i:s:u") . "\n";

---

EDIT

Knocked another second off by moving the $prime === 1 to be after the $number % $prime check in the same if statement.

start time:29:40:000000
#10001: 104743
End time:29:45:000000

---

Taking Hannes suggestion of strict checking and passing the array as reference plus adding a few tweaks of my own (modifying the array inside the function):

ini_set('max_execution_time', 300);
echo "start time:" . date("i:s:u") . "\n";

function isPrime($number, &$prevPrimes)
{
   foreach ($prevPrimes as $prime) {
        if ($number % $prime === 0 && $prime !== 1)
        {
            return false;
        }
    }

    // If we get to here, $number is prime
    $prevPrimes[] = $number;
    return $number;
}

$primes = array();
$i = 0;
$nbPrimes = 0;
while ($nbPrimes < 10001)
{
    $i++;
    if ($i % 2 !== 0)
    {
        $result = isPrime($i, $primes);

        if ($result !== 0)
        {
            $nbPrimes++;
        }
    }
}
echo "#$nbPrimes: $result\n";

echo "End time:" . date("i:s:u") . "\n";

Which ended up being:

start time:52:08:000000
#10001: 104743
End time:52:15:000000

VS your code:

start time:50:44:000000
#10001: 104743
End time:51:17:000000

A good improvement there, but nothing like C#, just goes to show the power of a compiled language 🙂