PERLLOL(1perl)

💖 PERLLOL 💖 (aka perllol — Manipulating Arrays of Arrays in Perl)

okay bestie, this is the page about arrays of arrays in Perl, straight out of the Perl Programmers Reference Guide. it's giving nested-data-structure bootcamp and honestly? you're gonna eat. let's go 🦋

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📛 NAME 📛

perllol — Manipulating Arrays of Arrays in Perl

🌸 DESCRIPTION 🌸

✨ Declaration and Access of Arrays of Arrays ✨

So the simplest two-level data structure you can build in Perl is an array of arrays — people casually call it a list of lists too, iykyk. It's reasonably easy to understand, and literally almost everything that applies here is gonna apply later on when you level up to the fancier data structures. no cap. 🔥

An array of an array is just a regular old array @AoA that you get at with two subscripts, like $AoA[3][2]. Here's a declaration of the array bestie:

use v5.10;  # so we can use say()

# assign to our array, an array of array references
@AoA = (
       [ "fred", "barney", "pebbles", "bambam", "dino", ],
       [ "george", "jane", "elroy", "judy", ],
       [ "homer", "bart", "marge", "maggie", ],
);
say $AoA[2][1];
  bart

⚠️ okay so be SO careful here: the outer bracket type is a ROUND one, a parenthesis. That's because you're assigning to an @array, so you need parens. periodt.

If you didn't want an @AoA at all, but rather just a reference to it, you'd do something more like this:

# assign a reference to array of array references
$ref_to_AoA = [
    [ "fred", "barney", "pebbles", "bambam", "dino", ],
    [ "george", "jane", "elroy", "judy", ],
    [ "homer", "bart", "marge", "maggie", ],
];
say $ref_to_AoA->[2][1];
  bart

Notice the outer bracket type changed, and so the access syntax changed too. That's because — and this is not like C — in perl you can't freely interchange arrays and references thereto. $ref_to_AoA is a reference to an array, whereas @AoA is an array proper. Likewise $AoA[2] is not an array, it's an array ref. 🫠

So how come you can write these:

$AoA[2][2]
$ref_to_AoA->[2][2]

instead of having to write these:

$AoA[2]->[2]
$ref_to_AoA->[2]->[2]

Well bestie, the rule is: on adjacent brackets only (whether square or curly), you're free to omit the pointer dereferencing arrow. 💅 But you cannot do that for the very first one if it's a scalar containing a reference, which means $ref_to_AoA always needs it. sorry, that's the law.

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🌱 Growing Your Own 🌱

That's all well and good for declaring a fixed data structure, but what if you wanted to add new elements on the fly? Or build it up entirely from scratch? 👀

First, let's look at reading it in from a file. This is something like adding a row at a time. We'll assume there's a flat file where each line is a row and each word is an element. If you're trying to develop an @AoA array containing all of these, here's the right way:

while (<>) {
    @tmp = split;
    push @AoA, [ @tmp ];
}

You might also have loaded that from a function:

for $i ( 1 .. 10 ) {
    $AoA[$i] = [ somefunc($i) ];
}

Or you might have had a temporary variable just sitting around with the array in it:

for $i ( 1 .. 10 ) {
    @tmp = somefunc($i);
    $AoA[$i] = [ @tmp ];
}

it is IMPORTANT that you use the [ ] array reference constructor. Because this next thing? It would NOT work. 💀

$AoA[$i] = @tmp;   # WRONG!

The reason that doesn't do what you want is that assigning a named array like that to a scalar means you're taking an array in scalar context, which means it just counts the number of elements in @tmp. that's it. that's all you get. 😭

If you're running under use strict (and if you aren't, why in the world aren't you?), you'll have to add some declarations to make it happy:

use strict;
my(@AoA, @tmp);
while (<>) {
    @tmp = split;
    push @AoA, [ @tmp ];
}

Of course, you don't need the temporary array to have a name at all:

while (<>) {
    push @AoA, [ split ];
}

You also don't have to use push(). You could just make a direct assignment if you knew where you wanted to put it:

my (@AoA, $i, $line);
for $i ( 0 .. 10 ) {
    $line = <>;
    $AoA[$i] = [ split " ", $line ];
}

or even just

my (@AoA, $i);
for $i ( 0 .. 10 ) {
    $AoA[$i] = [ split " ", <> ];
}

In general you should be leery 👀 of using functions that could potentially return lists in scalar context without explicitly saying so. This would be clearer to the casual reader:

my (@AoA, $i);
for $i ( 0 .. 10 ) {
    $AoA[$i] = [ split " ", scalar(<>) ];
}

If you wanted to have a $ref_to_AoA variable as a reference to an array, you'd have to do something like this:

while (<>) {
    push @$ref_to_AoA, [ split ];
}

Now you can add new rows. But what about adding new columns? 🌈 If you're dealing with just matrices, it's often easiest to use simple assignment:

for $x (1 .. 10) {
    for $y (1 .. 10) {
        $AoA[$x][$y] = func($x, $y);
    }
}

for $x ( 3, 7, 9 ) {
    $AoA[$x][20] += func2($x);
}

It doesn't matter whether those elements are already there or not: it'll gladly create them for you, setting the intervening elements to undef as need be. iconic behaviour tbh. ✨

If you wanted to just append to a row, you'd have to do something a little funnier looking:

# add new columns to an existing row
push $AoA[0]->@*, "wilma", "betty";   # explicit deref

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🖨️ Access and Printing 🖨️

Now it's time to print your data structure out. How are you gonna do that bestie? Well, if you want only one of the elements, it's trivial:

print $AoA[0][0];

If you want to print the whole thing though, you can't say

print @AoA;         # WRONG

because you'll just get references listed, and perl will never automatically dereference things for you. nope. not happening. 🙅

Instead, you have to roll yourself a loop or two. This prints the whole structure, using the shell-style for() construct to loop across the outer set of subscripts:

for $aref ( @AoA ) {
    say "\t [ @$aref ],";
}

If you wanted to keep track of subscripts, you might do this:

for $i ( 0 .. $#AoA ) {
    say "\t elt $i is [ @{$AoA[$i]} ],";
}

or maybe even this. Notice the inner loop 👑

for $i ( 0 .. $#AoA ) {
    for $j ( 0 .. $#{$AoA[$i]} ) {
        say "elt $i $j is $AoA[$i][$j]";
    }
}

As you can see, it's getting a bit complicated. That's why sometimes it's easier to take a temporary on your way through:

for $i ( 0 .. $#AoA ) {
    $aref = $AoA[$i];
    for $j ( 0 .. $#{$aref} ) {
        say "elt $i $j is $AoA[$i][$j]";
    }
}

Hmm... that's still a bit ugly. 🙈 How about this:

for $i ( 0 .. $#AoA ) {
    $aref = $AoA[$i];
    $n = @$aref - 1;
    for $j ( 0 .. $n ) {
        say "elt $i $j is $AoA[$i][$j]";
    }
}

When you get tired of writing a custom print for your data structures, you might look at the standard Dumpvalue or Data::Dumper modules. The former is what the Perl debugger uses, while the latter generates parsable Perl code. For example:

 use v5.14;     # using the + prototype, new to v5.14

 sub show(+) {
    require Dumpvalue;
    state $prettify = Dumpvalue->new(
        tick        => q("),
        compactDump => 1,  # comment these two lines out
        veryCompact => 1,  # if you want a bigger dump
    );
    $prettify->dumpValue(@_);
 }

 # Assign a list of array references to an array.
 my @AoA = (
           [ "fred", "barney" ],
           [ "george", "jane", "elroy" ],
           [ "homer", "marge", "bart" ],
 );
 push $AoA[0]->@*, "wilma", "betty";
 show @AoA;

will print out:

    0  0..3  "fred" "barney" "wilma" "betty"
    1  0..2  "george" "jane" "elroy"
    2  0..2  "homer" "marge" "bart"

Whereas if you comment out the two lines I said you might wish to, then it shows it to you this way instead:

    0  ARRAY(0x8031d0)
       0  "fred"
       1  "barney"
       2  "wilma"
       3  "betty"
    1  ARRAY(0x803d40)
       0  "george"
       1  "jane"
       2  "elroy"
    2  ARRAY(0x803e10)
       0  "homer"
       1  "marge"
       2  "bart"

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🍰 Slices 🍰

If you want to get at a slice (part of a row) in a multidimensional array, you're gonna have to do some fancy subscripting. That's because while we have a nice synonym for single elements via the pointer arrow for dereferencing, no such convenience exists for slices. sorry bestie. 😔

Here's how to do one operation using a loop. We'll assume an @AoA variable as before.

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

That same loop could be replaced with a slice operation:

@part = $AoA[4]->@[ 7..12 ];

Now, what if you wanted a two-dimensional slice, like having $x run from 4..8 and $y run from 7 to 12? Hmm... here's the simple way:

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

We can reduce some of the looping through slices

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

If you were into Schwartzian Transforms, you would probably have selected map for that

@newAoA = map { [ $AoA[$_]->@[ 7..12 ] ] } 4 .. 8;

Although if your manager accused you of seeking job security (or rapid insecurity 💀) through inscrutable code, it would be hard to argue. :-) If I were you, I'd put that in a function:

@newAoA = splice_2D( \@AoA, 4 => 8, 7 => 12 );
sub splice_2D {
    my $lrr = shift;        # ref to array of array refs!
    my ($x_lo, $x_hi,
        $y_lo, $y_hi) = @_;

    return map {
        [ $lrr->[$_]->@[ $y_lo .. $y_hi ] ]
    } $x_lo .. $x_hi;
}

👀 SEE ALSO 👀

perldata, perlref, perldsc

💜 AUTHOR 💜

Tom Christiansen <tchrist@perl.com>

Last update: Tue Apr 26 18:30:55 MDT 2011

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and that's the whole vibe bestie — you are now officially a nested-array main character 👑

this was perl v5.44.0, dated 2026-10-17, from the PERLLOL(1perl) page. slay, go write some @AoAs, and remember: round brackets on the outside, arrows where the law says. mwah 💅🎀