Re: Prime Number Finder

[elusion]

in reply to Prime Number Finder

Here's the code I've used for this in the past. I believe it's fairly efficent. If you're going to use it like in the root node in this thread, it's best to check only odd numbers, as suggested by ichimunki.

#!usr/bin/perl -w
use strict;

sub prime {
    my $number = shift;
    my $d = 2;
    my $sqrt = sqrt $number;
    while(1) {
        if ($number%$d == 0) {
            return 0;
        }
        if ($d < $sqrt) {
            $d++;
        } else {
            return 1;
        }
    }
}

my $number = $ARGV[0];
print prime($number);

elusion : http://matt.diephouse.com

[Anonymous Monk]

Isn't the math wrong here? I get false primes with this.

[LanceDeeply]

nope! they all look prime to me!
i like it, i'm gonna use it to explain programming loops to a mathmatician friend. i took the liberty to re-arrange a touch...


sub isPrime
{
    my $number = shift;
    my $sqrt = sqrt $number;
    my $d = 2;
    while (1)
    {
        return 0 if ( $number % $d == 0 );
        return 1 if ( $d > $sqrt );
        $d++;
    }
}

[Anonymous Monk]

In fact, you're right, because numbers which only can be devided in 1, themselves and their square root will be marked as prime in this way. make the < into a <= and it is solved.