Get the intersection of two lists of strings in Perl

Question

In Chapter 4, Section 4.8 (Computing Union, Intersection, or Difference of Unique Lists), the Perl Cookbook provides this technique for getting the intersection of two lists

Answer 1

@a = qw(a B c d);
@b = qw(b c D e f);
@c = grep { $f = lc $_; grep lc $_ eq $f, @b } @a;

gives

B c d

1) I found this solution somewhere but I don't remember where. If somebody knows please let me know.

2) I have no idea how efficient this is compared to other solutions.

3) Sincerly, I do not know how this works. Why there is a semicolon and not a comma?

Answer 2

Array::Utils is what you're looking for.

use Array::Utils qw(:all);

my @a = qw( a b c d );
my @b = qw( c d e f );

my @isect = intersect(@a, @b);
print join(",",@isect) . "\n";

This produces the expected output of

c,d

Edit: I didn't notice that you wanted this done case-insensitively. In that case, you can replace @a with map{lc}@a (and likewise with @b).

Answer 3

Smallest amount of change required from the original solution. Just lower-case the strings.

@a = qw( a b c d );
@b = qw( C D E F );
...
foreach $e (@a, @b) { 
    $union{lc $e}++ && $isect{lc $e}++ 
}

@union = keys %union;
@isect = keys %isect;

Answer 4

Here's one map/grep approach:

my @a = qw(Perl PHP Ruby Python C JavaScript);
my @b = qw(erlang java perl python c snobol lisp);
my @intersection =
    grep { defined }
        @{ { map { lc ,=> $_ } @a } }
           { map { lc } @b };

# @intersection = qw(Perl Python C)