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FAQ 4.42 How can I tell whether a certain element is contained in a list or array?

 
 
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      02-08-2011
This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

--------------------------------------------------------------------

4.42: How can I tell whether a certain element is contained in a list or array?

(portions of this answer contributed by Anno Siegel and brian d foy)

Hearing the word "in" is an *in*dication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
designed to answer this question quickly and efficiently. Arrays aren't.

That being said, there are several ways to approach this. In Perl 5.10
and later, you can use the smart match operator to check that an item is
contained in an array or a hash:

use 5.010;

if( $item ~~ @array )
{
say "The array contains $item"
}

if( $item ~~ %hash )
{
say "The hash contains $item"
}

With earlier versions of Perl, you have to do a bit more work. If you
are going to make this query many times over arbitrary string values,
the fastest way is probably to invert the original array and maintain a
hash whose keys are the first array's values:

@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
%is_blue = ();
for (@blues) { $is_blue{$_} = 1 }

Now you can check whether $is_blue{$some_color}. It might have been a
good idea to keep the blues all in a hash in the first place.

If the values are all small integers, you could use a simple indexed
array. This kind of an array will take up less space:

@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
@is_tiny_prime = ();
for (@primes) { $is_tiny_prime[$_] = 1 }
# or simply @istiny_prime[@primes] = (1) x @primes;

Now you check whether $is_tiny_prime[$some_number].

If the values in question are integers instead of strings, you can save
quite a lot of space by using bit strings instead:

@articles = ( 1..10, 150..2000, 2017 );
undef $read;
for (@articles) { vec($read,$_,1) = 1 }

Now check whether "vec($read,$n,1)" is true for some $n.

These methods guarantee fast individual tests but require a
re-organization of the original list or array. They only pay off if you
have to test multiple values against the same array.

If you are testing only once, the standard module "List::Util" exports
the function "first" for this purpose. It works by stopping once it
finds the element. It's written in C for speed, and its Perl equivalent
looks like this subroutine:

sub first (&@) {
my $code = shift;
foreach (@_) {
return $_ if &{$code}();
}
undef;
}

If speed is of little concern, the common idiom uses grep in scalar
context (which returns the number of items that passed its condition) to
traverse the entire list. This does have the benefit of telling you how
many matches it found, though.

my $is_there = grep $_ eq $whatever, @array;

If you want to actually extract the matching elements, simply use grep
in list context.

my @matches = grep $_ eq $whatever, @array;



--------------------------------------------------------------------

The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
 
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