PERLLOL(1)

💖 PERLLOL — the perllol manual page, but make it a group chat 💖

okay bestie, sit down, hydrate, because today we're learning perllol — that's the Perl Programmers Reference Guide entry for Manipulating Arrays of Arrays in Perl. yes the manual is literally called "lol". no I will not be taking questions about that. 🎀

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✨ NAME ✨

perllol — Manipulating Arrays of Arrays in Perl

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🌸 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, which people casually call a list of lists. it's genuinely not that deep, and like, almost everything here also works later on with the fancier data structures, so this is the tutorial that keeps on giving. 💅

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

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

now listen, be very careful that the outer bracket type is a round one, i.e. a parenthesis. that's because you're assigning to an @array, so you need parens, obviously. but if you didn't want an @AoA at all and just wanted 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, so our access syntax changed too. that's because — unlike C — in perl you cannot freely interchange arrays and references to them. $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 why can you 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 that on adjacent brackets only (square or curly, don't care), 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. periodt. 👑

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🌈 Growing Your Own

cool cool, that's all fine for a fixed data structure, but what if you wanna add new elements on the fly, or build the whole thing from scratch? iconic question, let's go.

first, reading it in from a file. this is 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 build an @AoA array with all of that in it, here's the right way:

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

you might also have loaded it from a function:

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

or maybe you had a temporary variable just vibing with the array inside it.

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

it is important that you use the [ ] array reference constructor. because this 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 is taking an array in scalar context, which literally just counts the number of elements in @tmp. so you'd get a number. not the vibe. 💀

if you're running under use strict (and if you aren't — bestie, 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 stating such. this next one is clearer to the casual reader:

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

if you wanted 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 ];
}

so now you can add new rows. what about adding new columns? if you're dealing with just matrices, it's usually 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 does not matter whether those elements already exist or not — it'll gladly create them for you, setting intervening elements to undef as need be. no cap.

if you wanted to just append to a row, you'd have to do something a bit 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? well, if you want only one element, it's trivial:

print $AoA[0][0];

but if you want to print the whole thing, you can't say

print @AoA;         # WRONG

because you'll get just references listed, and perl will never automatically dereference things for you. 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. tragic, I know. 😭

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 $x running from 4..8 and $y running 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;
}

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🔥 SEE ALSO 🔥

perldata, perlref, perldsc

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👑 AUTHOR 👑

Tom Christiansen <tchrist@perl.com>

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

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ok and that's the whole thing, bestie. go forth and nest your arrays responsibly. 🫶

this was perl v5.42.3, dated 2026-08-03, brought to you by the PERLLOL(1) girlies — stay hydrated, deref your refs, and remember: [ ] not ( ) unless it's the outer one. mwah. 💖✨🎀🦋🌸💅