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Home: Perl Programming Help: Frequently Asked Questions:
How can I tell whether a list or array contains a

 



Jasmine
Administrator

Jan 19, 2001, 3:11 PM

Post #1 of 1 (1233 views)
How can I tell whether a list or array contains a Can't Post

(From the Perl FAQ)

How can I tell whether a list or array contains a certain element?

Hearing the word ``in'' is an indication 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. 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 keep an associative array lying about whose keys are the first array's values.

Code
    @blues = qw/azure cerulean teal turquoise lapis-lazuli/; 
undef %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:

Code
    @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); 
undef @is_tiny_prime;
for (@primes) { $is_tiny_prime[$_] = 1; }

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:

Code
    @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.

Please do not use

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

or worse yet

Code
    $is_there = grep /$whatever/, @array;

These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are regexp characters in $whatever?).



 
 


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