For a while now I’ve been using Test::Perl::Critic as an integral part of my daily coding. Test::Perl::Critic wraps Perl::Critic into a set of tests which can be automatically run against a distribution. Perl Critic implements Damien Conway’s set of standard Perl Best Practices.
I’m not going to go into the arguments of whether they’re good or bad rules right now. Suffice to say I use nearly all of them and my code has improved because of it. Anyway, one of the rules states you shouldn’t use parentheses for core method calls like length(), int() or rand(), so most of the time I don’t, but today I wanted to do this:
my @array = map { int rand * 45 } (1..10);The results come out as an array of zeros. Why? Well it’s not a broken PRNG, that’s for sure. It’s a simple case of misunderstood parsing, operator precedence and Perl internals. Looking closely * 45 isn’t what you’d expect, *45 refers to the typeglob named 45 in the main package. I actually have no idea how this is cast into a number in Perl but however it’s done, it evaluates to zero. rand 0 seems to exhibit the same behaviour as rand 1, yielding a random float between zero and 1. int rand 0 will always be zero.
So? Well to stop the parser from taking *45 as an argument to rand you need to upset PerlCritic and add those parentheses back in:
my @array = map { int rand() *45 } (1..10);and it works now so back to work!
[ The astute amongst you will realise you can just say int rand(45). I know, but that doesn’t work nearly so well as a mildly interesting example. I blame PerlCritic ;) ]