Create a hash from an array using map

• map

• We have a list of values. We would like to build a fast look-up table to check for existence.

The time it takes to check if a value can be found in an array is proportional to the length of the array. The complexity is O(n).

If you need to do it a lot of times you might be better off building a hash where the keys are the items coming from the array. The values don't matter as we will check the existance of a key. (Alternatively you can set the values of the hash to be 1 and then you can check if the the value is there.) The time it takes to look up a key in a hash does not depend on the size of the hash. It is O(1). So once we have the hash the look-up will be much faster. Building the hash is proportional to the number of items in the array.

So if we need to look up a very small number of elements or if the original array is small then probably it is better to just use the array.

If we need a lot of look-ups and there are many elements in the original array then building a temporary look-up hash might be a good idea.

We use more memory but we can gain speed.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper qw(Dumper);

my @words = qw(Foo Bar Baz);

my %copy_paste = (Foo => 1, Bar => 1, Baz => 1);

my %lookup = map {$_ => 1} @words;

print Dumper \%copy_paste;
print Dumper \%lookup;

# $VAR1 = {
#           'Bar' => 1,
#           'Baz' => 1,
#           'Foo' => 1
#         };