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perllol



DESCRIPTION

       Declaration and Access of Arrays of Arrays

       The simplest thing to build is an array of arrays (some­
       times imprecisely called a list of lists).  It's reason­
       ably 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]

       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 ];
           }

           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 refer­
       ence 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 inter­
       vening 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
           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 multi­
       dimensional array, you're going to have to do some fancy

       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 proba­
       bly 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>

  
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