There are a couple options. You need to know the index, so foreach is out. You could do something like this: 
for(my $x = 0; $x <= $#array; $x++) {
    next unless $array[$x];
    # do stuff with $x and $array[$x]
}

[download]

But that is obviously inefficient, especially if your list is VERY sparse. Like with elements numbered 100, 435, 1040, etc.

Why not use a hash? You can convert a sparse list to a hash like so:

my %hash = 
    map { $x => $y } 
    grep { my $y = $list[$x]; $y ne $undef; [$x, $y] }
    for(my $x = 0; $x <= $#list; $x++);

[download]

And use it like this:

$list{100} = 'dog';
$list{435} = 'cat';
$list{1040} = 'tax form';

while(($index, $element) = each %list) {
    # do stuff with $index and $element
}

[download]

Other alternatives include using a pseudohash, or writing a tied array implementation that acts like a list in your code but works like a hash behind the scenes.

Good luck!

In reply to Re: Efficient access to sparse lists? by mwp
in thread Efficient access to sparse lists? by Improv

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