Guides

Introduction

Here’s a micro guide on how to generate prime numbers in a given interval with PHP.

A prime number is a natural number (a positive integer) which has exactly two distinct natural number divisors (1 and the number itself).

The Code

What we want to do is to create a function that takes a minimum and a maximum value of primes values we want to find. Then we try to divide the number by every number bigger than 1 and smaller than itself and determine if it has a remainder on any of the calculation. If it has on every caltulation we’ve got a prime, if not, we don’t.
I’ve made the script to only use odd numbers, because 2 is the only non-odd prime.

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<?php
function prime_generator($from,$to,$return=false){
	if(!($from%2))
		$from++;
	if($from <= 2){
		$from = 3;
		if($return)
			$primes .= "2, ";
		else
			echo "2, ";
	}
 
	for($x=$from; $x <= $to; $x+=2){
		$prime=true;
		for($y=$x-1;$y>1;$y--){
			if(!($x%$y)){
				$prime=false;
				}
		}
		if($prime){
			if($return)
				$primes .= $x . ", ";
			else
				echo $x . ", ";
		}
	}
	if($return)
		return $primes;
}
$primes = prime_generator(0,100, true);
echo $primes;
?>

You can use the function either with or without the $return parameter set. If it’s set to true it will return all primes in the given interval, if not, it gives the output right away.

Ricehigh Micro guides casual scripts, Micro guide, PHP, Prime numbers