use strict;
use warnings;

print nearPrime(100000);

sub nearPrime {
    my $num = shift; $_ = 0;
    return $num if isPrime($num);
    while (1) {
        $_++;
        return ($num - $_) if isPrime($num - $_);
        return ($num + $_) if isPrime($num + $_);
    }
}

BEGIN {
    my @primes = (2); my $max = 2;
    sub isPrime {
        my $num = shift;
        my $root = int sqrt $num;
        if ($root > $max) {
            for (($max+1)..$root) {
                 push(@primes, $_) if isPrime($_);
            }
            $max = $root;
        }
        for (@primes) {
            return 0 if ($num % $_ == 0);
        }
        return 1;
    }
}

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In reply to Re: a close prime number by TedPride
in thread a close prime number by Anonymous Monk

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