Given as input a positive integer, of not too high a value, say below 1000, calculate the N'th prime number.
Finding the 1000th prime takes about 0.35 seconds on a P4-2.2GHz. I've limited the maximum number to 250 to reduce the amount of processing the server has to do.
// generate a random number
$n = mt_rand(1, 250);
// start off the primes array
$primes = array(2);
// set test to 1 to begin with
$test = 1;
while (count($primes) < $n)
// only test odd numbers
// because all even numbers aren't primes
// assume it's prime unless we can prove otherwise
$is_prime = true;
// loop through all the current primes
foreach($primes as $prime)
// if the prime is more than half it's value,
// none of the following ones will work, so break now
if ($test/2 < $prime)
// test if it divides exactly
if (!($test % $prime))
// if so, it's not a prime number
$is_prime = false;
// break now
// if we an't prove otherwise, it's a prime number
// add it to the list
$primes = $test;
// show the n'th prime number
echo 'Prime ' . $n . ' is ' . array_pop($primes);
Prime 25 is 97
J can be used to show prime numbers easily.
Prime numbers start at 0, not 1. So prime number 47 in theory is actually 48.