ajguitarmaniac has asked for the wisdom of the Perl Monks concerning the following question:

Greetings Monks! This one sounds real simple but for some reason I'm having a hell of a time figuring it out! Here goes.. I have 2 arrays, say 

my @arr1 = ('1','2','3','4','5','6','7','8','9');
my @arr2 = ('1','4','5','9','7');

[download]

I need to pick out those elements that are found in @arr1 but not in @arr2 and store them into another array(say @arr3) and print the values consequently.It must be noted that elements in @arr2 will for sure exist in @arr1. Help me out Monks!

Replies are listed 'Best First'.
Re: Array comparison using perl
by Khen1950fx (Canon) on Dec 21, 2010 at 08:37 UTC
    That's easy enough. Using List::Compare: 
    #!/usr/bin/perl

use strict;
use warnings;
use List::Compare;

my @arr1 = (1, 2, 3, 4, 5, 6, 7, 8, 9);
my @arr2 = (1, 4, 5, 9 ,7);

my $lc = List::Compare->new(\@arr1, \@arr2);
my @arr3 = $lc->get_unique;
print "@arr3\n";

    [download]
[reply]
[d/l]

      Thanks Khen1950fx!

[reply]
Re: Array comparison using perl
by fisher (Priest) on Dec 21, 2010 at 11:34 UTC
    ...and not using any modules: 
    #!/usr/bin/env perl
#

use strict;
use warnings;


my @a1 = (1, 2, 3, 4, 5, 6, 7, 8, 9);
my @a2 = (1, 4, 5, 9 ,7);

print  "\@a1 is (".(join ",", @a1).")\n";
print  "\@a2 is (".(join ",", @a2).")\n";

my ($e1, $f);
my @a3;

for $e1 (@a1) {
    $f = 0;
    for (@a2) {
        if ($e1 eq $_) { $f++; }
    }
    unless ($f) { push @a3, $e1 }
}

print  "\@a3 is (".(join ",", @a3).")\n";

    [download]
[reply]
[d/l]

      and what happens when the number of elements increases? This code is O(n2) which doesn't scale at all well. Better is something like:

          my %hits;
    
    ++$hits{$_} for @a1;

    my @matched = grep {exists $hits{$_}} @a2;

      [download]
      True laziness is hard work
[reply]
[d/l]

      Right on fisher! Thanks!

[reply]
Re: Array comparison using perl
by samarzone (Pilgrim) on Dec 21, 2010 at 08:20 UTC

    You will need to build some logic based on hashes, loops or may be grep. Submit your latest effort.

    -- Regards - Samar
[reply]

Back to Seekers of Perl Wisdom