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PERLLOL(1pm)           Perl Programmers Reference Guide           PERLLOL(1pm)


       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 ];

       Now, 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(1), perlref(1), perldsc(1)


       Tom Christiansen <>

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

perl v5.34.0                      2020-10-04                      PERLLOL(1pm)

perl 5.34.0 - Generated Sat Feb 26 07:56:17 CST 2022
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