Programming Perl, 4th Edition

Slices

If you want to access a slice (part of a row) of a multidimensional array, you’re going to have to do some fancy subscripting. The pointer arrows give us a nice way to access a single element, but no such convenience exists for slices. You can always use a loop to extract the elements of your slice one by one:

@part = ();
for ($y = 7; $y < 13; $y++) {
    push @part, $AoA[4][$y];
}

This particular loop could be replaced with an array slice:

@part = @{ $AoA[4] } [ 7..12 ];

If you want a two-dimensional slice, say, with $x running from 4..8 and $y from 7..12, here’s one way to do it:

@newAoA = ();
for ($startx = $x = 4; $x <= 8; $x++) {
    for ($starty = $y = 7; $y <= 12; $y++) {
        $newAoA[$x – $startx][$y – $starty] = $AoA[$x][$y];
    }
}

In this example, the individual values within our destination two-dimensional array, @newAoA, are assigned one by one, taken from a two-dimensional subarray of @AoA. An alternative is to create anonymous arrays, each consisting of a desired slice of an @AoA subarray, and then put references to these anonymous arrays into @newAoA. We would then be writing references into @newAoA (subscripted once, so to speak) instead of subarray values into a twice-subscripted @newAoA. This method eliminates the innermost loop:

for ($x = 4; $x <= 8; $x++) {
    push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ];
}

Of course, if you do this often, you should probably write a subroutine called something like extract_rectangle. And if you do it very often with large collections of multidimensional ...