perllol - Manipulating Arrays of Arrays in Perl


   Declaration and Access of Arrays of Arrays
   The simplest two-level data structure to build in Perl is an array of
   arrays, sometimes casually called a list of lists.  It's reasonably
   easy to understand, and almost everything that applies here will also
   be applicable later on with the fancier data structures.

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

       use 5.010;  # 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];

   Now you should be very careful that the outer bracket type is a round
   one, that is, a parenthesis.  That's because you're assigning to an
   @array, so you need parentheses.  If you wanted there not to be an
   @AoA, but rather just a reference to it, you could 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];

   Notice that the outer bracket type has changed, and so our access
   syntax has also changed.  That's because unlike 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, but an array ref.  So how come you can write


   instead of having to write these:


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

   Growing Your Own
   That's all well and good for declaration of 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 that there's a flat file in which
   each line is a row and each word an element.  If you're trying to
   develop an @AoA array containing all these, here's the right way to do

       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 sitting around with the
   array in it.

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

   It's important you make sure to use the "[ ]" array reference
   constructor.  That's because this wouldn't work:

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

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

   If you are 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

       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 " ", <> ];

   You should in general be leery of using functions that could
   potentially return lists in scalar context without explicitly stating
   such.  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.  What about adding new columns?  If you're
   dealing with just matrices, it's often easiest to use simple

       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 intervening elements to
   "undef" as need be.

   If you wanted just to 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

   Access and Printing
   Now it's time to print your data structure out.  How are you going to
   do that?  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 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
   is 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 $prettily = new Dumpvalue::
                               tick        => q("),
                               compactDump => 1,  # comment these two lines
                                                  # out
                               veryCompact => 1,  # if you want a bigger
                                                  # dump
           dumpValue $prettily @_;

    # 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"

   If you want to get at a slice (part of a row) in a multidimensional
   array, you're going to 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.

   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];

   or spaced out a bit:

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

   But as you might well imagine, this can get pretty rough on the reader.

   Ah, but what if you wanted a two-dimensional slice, such as 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;


   perldata, perlref, perldsc


   Tom Christiansen <>

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


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