As JavaFan said, the simplest solution is to just start your loop at 2:

sub isPrime($) {
     $num = shift;
     my ($i,$c);
     for ($i = 2; $i < $num / 2; $i++)
     {
         if ($num % $i == 0){$c += 1;}
     }
     if ($c == 1){return 1;}
     return 0;
}

[download]

The above does however produce a false negative for isPrime(2). It produces a negative for isPrime(1) which is probably correct, but could be considered to be a false depending on your stance on the much debated primality of 1.

Better though is:

use 5.010;
use strict;

sub isPrime (_)
{
    my $num = shift;
    for my $div (2 .. sqrt $num)     # start at 2, end at sqrt($n)
    {
        return if $num % $div == 0;  # return false if $n evenly divid
+es
    }
    return 1;  # return true if we got through the entire loop
}

for (1 .. 30)
{
    if (isPrime)
    {
        say "$_ is prime";
    }
    else
    {
        say;
    }
}

[download]

I've folded in JavaFan's other suggestions, plus changed your prototype from ($) to (_). Prototypes are rarely helpful - you're best off leaving them out usually. In this case though, the (_) prototype is quite cool, because what it does is, if isPrime is called with no argument at all, then it checks $_.

Other suggestions: consider what should happen for isPrime(1), isPrime(0), isPrime(-3) and isPrime("chimpanzee"). Add checks for those special cases before the for my $div loop. The above implementation claims all the above are prime, except -3 which dies.

In mathematical circles, 1 is (these days) not usually regarded as prime, and asking the question of non-natural numbers (i.e. non-integers, and integers lower than 1) does not make sense. Bearing that in mind...

use Carp qw/croak/;
sub isPrime (_)
{
    my $num = shift;
    
    return if $num == 1;  # 1 is not prime
    
    croak "usage: isPrime(NATURAL NUMBER)"
        unless $num =~ /^[1-9][0-9]*$/;
    
    for my $div (2 .. sqrt $num)
    {
        return if $num % $div == 0;
    }
    return 1;
}

[download]
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re: is it prime? by tobyink
in thread is it prime? by Anonymous Monk

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