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NAME

     perllol - Manipulating Arrays of Arrays in Perl



DESCRIPTION


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
     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 array:

         # assign to our array, an array of array references
         @AoA = (
                [ "fred", "barney" ],
                [ "george", "jane", "elroy" ],
                [ "homer", "marge", "bart" ],
         );

         print $AoA[2][2];
       bart

     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:

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

     instead of having to write these:

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

     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
     wrong:

         $AoA[$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
     this:

         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";
             }
         }




Slices
     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
     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 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(1), perlref(1), perldsc(1)



AUTHOR
     Tom Christiansen <tchrist@perl.com>

     Last update: Thu Jun  4 16:16:23 MDT 1998








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