Comparison to an array of a value [duplicate] - perl

Question

This question already has answers here:
How can I verify that a value is present in an array (list) in Perl?
(8 answers)
Closed 9 years ago.
I'm still feeling my way though perl and so there's probably a simple way of doing this but I can find it. I want to compare a single value say A or E to an array that may or may not contain that value, eg A B C D and then perform an action if they match. How should I set this up?
Thanks.

Answer 1

You filter each element of the array to see if it is the element you are looking for and then use the resulting array as a boolean value (not empty = true, empty = false):
#filtered_array = grep { $_ eq 'A' } #array;
if (#filtered_array) {
print "found it!\n";
}

Answer 2

If you store the list in an array then the only way is to examine each element individually in a loop, using grep, or for or any from List::MoreUtils. (grep is the worst of these, as it searches the entire array, even if a match has been found early on.) This is fine if the array is small, but you will hit performance probelms if the array has a significant size and you have to check it frequently.
You can speed things up by representing the same list in a hash, when a check for membership is just a single key lookup.
Alternatively, if the list is enormous, then it is best kept in a database, using SQLite.

Answer 3

Are you stuck on arrays?
Whenever in Perl you're talk about quickly looking up data, you should think in terms of hashes. A hash is a collection of data like an array, but it is keyed, and looking up the key is a very fast operation in Perl.
There's nothing that says the keys to your hash can't be your data, and it is very common in Perl to index an array with a hash in order to quickly search for values.
This turns your array #array into a hash called %arrays_hash.
use strict;
use warnings;
use feature qw(say);
use autodie;
my #array = qw(Alpha Beta Delta Gamma Ohm);
my %array_index;
for my $entry ( #array ) {
$array_index{$entry} = 1; # Doesn't matter. As long as it isn't blank or zero
}
Now, looking up whether or not your data is in your array is very quick. Just simply see if it's a key in your %array_index:
my $item = "Delta"; # Is this in my initial array?
if ( $array_index{$item} ) {
say "Yes! Item '$item' is in my array.";
}
else {
say "No. Item '$item' isn't in my array. David sad.";
}
This is so common, that you'll see a lot of programs that use the map command to index the array. Instead of that for loop, I could have done this:
my %array_index = ( map { $_ => 1 } #array );
or
my %array_index;
map { $array_index{$_} = 1 } #array;
You'll see both. The first one is a one liner. The map command takes each entry in the array, and puts it in $_. Then, it returns the results into an array. Thus, the map will return an array with your data in the even positions (0, 2, 4 8...) and a 1 in the odd positions (1, 3, 5...).
The second one is more literal and easier to understand (or about as easy to understand in a map command). Again, each item in your #array is being assigned to $_, and that is being used as the key in my %array_index hash.
Whether or not you want to use hashes depend upon the length of your array, and how many items of input you'll be searching for. If you're simply searching whether a single item is in your array, I'd probably use List::Utils or List::MoreUtils, or use a for loop to search each value of my array. If I am doing this for multiple values, I am better off with a hash.