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         perllol - Manipulating Arrays of Arrays in Perl



    Declaration and Access of Arrays of Arrays

         The simplest thing to build an array of arrays (sometimes
         imprecisely 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
         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 array:
             # assign to our array, an array of array references
             @AoA = (
                    [ "fred", "barney" ],
                    [ "george", "jane", "elroy" ],
                    [ "homer", "marge", "bart" ],
             print $AoA[2][2];
         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", ],
                 [ "homer", "bart", "marge", "maggie", ],
                 [ "george", "jane", "elroy", "judy", ],
             print $ref_to_AoA->[2][2];
         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 these:
         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 that:
             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 very important that you make sure to use the `[]' array
         reference constructor.  That's because this will be very
             $AoA[$i] = @tmp;
         You see, assigning a named array like that to a scalar just
         counts the number of elements in @tmp, which probably isn't
         what you want.
         If you are running under `use strict', 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 ' ', <> ];
         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 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
         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";
         Notice that I couldn't say just:
             push $AoA[0], "wilma", "betty";  # WRONG!
         In fact, that wouldn't even compile.  How come?  Because the
         argument to push() must be a real array, not just a
         reference to such.

    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 $aref ( @AoA ) {
                 print "\t [ @$aref ],\n";
         If you wanted to keep track of subscripts, you might do
             for $i ( 0 .. $#AoA ) {
                 print "\t elt $i is [ @{$AoA[$i]} ],\n";
         or maybe even this.  Notice the inner loop.
             for $i ( 0 .. $#AoA ) {
                 for $j ( 0 .. $#{$AoA[$i]} ) {
                     print "elt $i $j is $AoA[$i][$j]\n";
         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} ) {
                     print "elt $i $j is $AoA[$i][$j]\n";
         Hmm... that's still a bit ugly.  How about this:
             for $i ( 0 .. $#AoA ) {
                 $aref = $AoA[$i];
                 $n = @$aref - 1;
                 for $j ( 0 .. $n ) {
                     print "elt $i $j is $AoA[$i][$j]\n";


         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.
         (Remember, of course, that you can always write a loop to do
         a slice operation.)
         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 ];
         but as you might well imagine, this is pretty rough on the
         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 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: Thu Jun  4 16:16:23 MDT 1998

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