Counting array elements in Perl

Edit: Hash versus Array

As cincodenada correctly pointed out in the comment, ysth gave a better answer: I should have answered your question with another question: "Do you really want to use a Perl array? A hash may be more appropriate."

An array allocates memory for all possible indices up to the largest used so-far. In your example, you allocate 24 cells (but use only 3). By contrast, a hash only allocates space for those fields that are actually used.

Array solution: scalar grep

Here are two possible solutions (see below for explanation):

print scalar(grep {defined $_} @a), "\n";  # prints 3print scalar(grep $_, @a), "\n";            # prints 3

Explanation: After adding $a[23], your array really contains 24 elements --- but most of them are undefined (which also evaluates as false). You can count the number of defined elements (as done in the first solution) or the number of true elements (second solution).

What is the difference? If you set $a[10]=0, then the first solution will count it, but the second solution won't (because 0 is false but defined). If you set $a[3]=undef, none of the solutions will count it.

Hash solution (by yst)

As suggested by another solution, you can work with a hash and avoid all the problems:

$a{0}  = 1;$a{5}  = 2;$a{23} = 3;print scalar(keys %a), "\n";  # prints 3

This solution counts zeros and undef values.

It sounds like you want a sparse array. A normal array would have 24 items in it, but a sparse array would have 3. In Perl we emulate sparse arrays with hashes:

#!/usr/bin/perluse strict;use warnings;my %sparse;@sparse{0, 5, 23} = (1 .. 3);print "there are ", scalar keys %sparse, " items in the sparse array\n",    map { "\t$sparse{$_}\n" } sort { $a <=> $b } keys %sparse;

The keys function in scalar context will return the number of items in the sparse array. The only downside to using a hash to emulate a sparse array is that you must sort the keys before iterating over them if their order is important.

You must also remember to use the delete function to remove items from the sparse array (just setting their value to undef is not enough).

Maybe you want a hash instead (or in addition). Arrays are an ordered set of elements; if you create $foo[23], you implicitly create $foo[0] through $foo[22].