{"id":512,"date":"2011-11-04T13:06:04","date_gmt":"2011-11-04T13:06:04","guid":{"rendered":"http:\/\/psyphi.net\/blog\/?p=512"},"modified":"2011-11-04T13:06:04","modified_gmt":"2011-11-04T13:06:04","slug":"neat-perl-gotcha","status":"publish","type":"post","link":"https:\/\/psyphi.net\/blog\/2011\/11\/neat-perl-gotcha\/","title":{"rendered":"Neat Perl Gotcha"},"content":{"rendered":"

For a while now I’ve been using Test::Perl::Critic as an integral part of my daily coding. Test::Perl::Critic<\/a> wraps Perl::Critic<\/a> 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<\/a>.<\/p>\n

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()<\/code>, int()<\/code> or rand()<\/code>, so most of the time I don’t, but today I wanted to do this:<\/p>\n

my @array = map { int rand * 45 } (1..10);<\/code><\/pre>\n
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<\/code> isn’t what you’d expect, *45<\/code> 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<\/code> seems to exhibit the same behaviour as rand 1<\/code>, yielding a random float between zero and 1. int rand 0<\/code> will always be zero.<\/p>\n
So? Well to stop the parser from taking *45<\/code> as an argument to rand you need to upset PerlCritic and add those parentheses back in:<\/p>\n
my @array = map { int rand() *45 } (1..10);<\/code><\/pre>\n
and it works now so back to work!<\/p>\n
[ The astute amongst you will realise you can just say int rand(45)<\/code>. I know, but that doesn’t work nearly so well as a mildly interesting example. I blame PerlCritic ;) ]<\/p>\n","protected":false},"excerpt":{"rendered":"
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 … Continue reading “Neat Perl Gotcha”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_uf_show_specific_survey":0,"_uf_disable_surveys":false,"footnotes":""},"categories":[11],"tags":[21,714],"class_list":["post-512","post","type-post","status-publish","format-standard","hentry","category-programming","tag-perl","tag-perlcritic"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t