perllol


DESCRIPTION
   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
       array:

           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];
         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", ],
               [ "george", "jane", "elroy", "judy", ],
               [ "homer", "bart", "marge", "maggie", ],
           ];
           say $ref_to_AoA->[2][1];
         bart

       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
       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 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
       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 ) {

       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";   # explicit deref

       Prior to Perl 5.14, this wouldn't even compile:

           push $AoA[0], "wilma", "betty";        # implicit deref

       How come?  Because once upon a time, the argument to push() had to be
       be a real array, not just a reference to one. That's no longer true.
       In fact, the line marked "implicit deref" above works just fine--in
       this instance--to do what the one that says explicit deref did.

       The reason I said "in this instance" is because that only works because
       $AoA[0] already held an array reference.  If you try that on an
       undefined variable, you'll take an exception.  That's because the
       implicit derefererence will never autovivify an undefined variable the
       way "@{ }" always will:


       directive at the top of the file that needs it.  That way when somebody
       tries to run the new code under an old perl, rather than getting an
       error like

           Type of arg 1 to push must be array (not array element) at /tmp/a line 8, near ""betty";"
           Execution of /tmp/a aborted due to compilation errors.

       they'll be politely informed that

           Perl v5.14.0 required--this is only v5.12.3, stopped at /tmp/a line 1.
           BEGIN failed--compilation aborted at /tmp/a line 1.

   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]";
               }

       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"

   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.
           @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;
           }

SEE ALSO
       perldata, perlref, perldsc

AUTHOR
       Tom Christiansen <tchrist@perl.com>

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