How to subtract an array from an array?

Question

When I try the following

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my @bl = qw(red green blue);
my @a = qw(green yellow purple blue pink);

print Dumper [grep {not @bl} @a];

I get an empty array. I would have expected that @bl was subtracted from @a, so the output was yellow purple pink.

What's wrong here?

Answer 1

You need to turn @bl into a hash to perform the set difference:

my %in_bl = map {$_ => 1} @bl;
my @diff  = grep {not $in_bl{$_}} @a;

Answer 2

See perlfaq4: How do I compute the difference of two arrays?

In your code, not is probably not doing what you think it is doing.

not @bl will always be 1 if @bl is an empty array, and undef if @bl is not empty. It doesn't mean "elements not in @bl" in any sense.