Perl equivalent of (Python-) list comprehension

Question

I'm looking for ways to express this Python snippet in Perl:

data = {"A": None, "B": "yes", "C": None}
key_list = [k for k in data if data[k]]  
# in this case the same as filter(lambda k: data[k], data) but let's ignore that

So looking at it one way, I just want the keys where the values are None or undef. Looking at it another way, what I want is the concise perl equivalent of a list comprehension with conditional.

Answer 1

I think you want grep:

#!/usr/bin/env perl
use strict;
use warnings;

my %data = ( A => undef, B => 'yes', C => undef );

my @keys = grep { defined $data{$_} } keys %data;

print "Key: $_\n" for @keys;

I also think that I type too slowly, and that I should reload the page before posting answers. By the way, either a value of 0 or undef can be a good way to handle null values, but make sure you remember which you're using. A false value and and undefined value aren't the same thing in Perl. To clarify: undef returns false in a boolean test, but so does 0. If 0 is a valid value, then you want to explicitly test for definedness, not simply truth. (I mention it because James went for 0 and I went the other way, and you may or may not know if it matters.)