=head1 DESCRIPTION
-=head1 Declaration and Access of Arrays of Arrays
+=head2 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.
+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 C<$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" ],
- [ "george", "jane", "elroy" ],
- [ "homer", "marge", "bart" ],
+ [ "fred", "barney", "pebbles", "bambam", "dino", ],
+ [ "george", "jane", "elroy", "judy", ],
+ [ "homer", "bart", "marge", "maggie", ],
);
-
- print $AoA[2][2];
+ say $AoA[2][1];
bart
Now you should be very careful that the outer bracket type
# assign a reference to array of array references
$ref_to_AoA = [
[ "fred", "barney", "pebbles", "bambam", "dino", ],
- [ "homer", "bart", "marge", "maggie", ],
[ "george", "jane", "elroy", "judy", ],
+ [ "homer", "bart", "marge", "maggie", ],
];
-
- print $ref_to_AoA->[2][2];
+ 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
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.
-=head1 Growing Your Own
+=head2 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
$AoA[$i] = [ @tmp ];
}
-It's very important that you make sure to use the C<[]> array reference
-constructor. That's because this will be very wrong:
+It's important you make sure to use the C<[ ]> array reference
+constructor. That's because this wouldn't work:
- $AoA[$i] = @tmp;
+ $AoA[$i] = @tmp; # WRONG!
-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.
+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 C<use strict>, you'll have to add some
-declarations to make it happy:
+If you are running under C<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);
my (@AoA, $i, $line);
for $i ( 0 .. 10 ) {
$line = <>;
- $AoA[$i] = [ split ' ', $line ];
+ $AoA[$i] = [ split " ", $line ];
}
or even just
my (@AoA, $i);
for $i ( 0 .. 10 ) {
- $AoA[$i] = [ split ' ', <> ];
+ $AoA[$i] = [ split " ", <> ];
}
You should in general be leery of using functions that could
my (@AoA, $i);
for $i ( 0 .. 10 ) {
- $AoA[$i] = [ split ' ', scalar(<>) ];
+ $AoA[$i] = [ split " ", scalar(<>) ];
}
If you wanted to have a $ref_to_AoA variable as a reference to an array,
to do something a bit funnier looking:
# add new columns to an existing row
- push @{ $AoA[0] }, "wilma", "betty";
+ push @{ $AoA[0] }, "wilma", "betty"; # explicit deref
+
+Prior to Perl 5.14, this wouldn't even compile:
+
+ push $AoA[0], "wilma", "betty"; # implicit deref
+
+How come? Because once upon a time, the argument to push() had to be a
+real array, not just a reference to one. That's no longer true. In fact,
+the line marked "implicit deref" above works just fine--in this
+instance--to do what the one that says explicit deref did.
+
+The reason I said "in this instance" is because that I<only> works
+because C<$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 C<@{ }>
+always will:
-Notice that I I<couldn't> say just:
+ my $aref = undef;
+ push $aref, qw(some more values); # WRONG!
+ push @$aref, qw(a few more); # ok
- push $AoA[0], "wilma", "betty"; # WRONG!
+If you want to take advantage of this new implicit dereferencing behavior,
+go right ahead: it makes code easier on the eye and wrist. Just understand
+that older releases will choke on it during compilation. Whenever you make
+use of something that works only in some given release of Perl and later,
+but not earlier, you should place a prominent
-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.
+ use v5.14; # needed for implicit deref of array refs by array ops
-=head1 Access and Printing
+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.
+
+=head2 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
set of subscripts.
for $aref ( @AoA ) {
- print "\t [ @$aref ],\n";
+ say "\t [ @$aref ],";
}
If you wanted to keep track of subscripts, you might do this:
for $i ( 0 .. $#AoA ) {
- print "\t elt $i is [ @{$AoA[$i]} ],\n";
+ say "\t elt $i is [ @{$AoA[$i]} ],";
}
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";
+ say "elt $i $j is $AoA[$i][$j]";
}
}
for $i ( 0 .. $#AoA ) {
$aref = $AoA[$i];
for $j ( 0 .. $#{$aref} ) {
- print "elt $i $j is $AoA[$i][$j]\n";
+ say "elt $i $j is $AoA[$i][$j]";
}
}
$aref = $AoA[$i];
$n = @$aref - 1;
for $j ( 0 .. $n ) {
- print "elt $i $j is $AoA[$i][$j]\n";
+ say "elt $i $j is $AoA[$i][$j]";
}
}
-=head1 Slices
+When you get tired of writing a custom print for your data structures,
+you might look at the standard L<Dumpvalue> or L<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 $AoA[0], "wilma", "betty";
+ show @AoA;
+
+will print out:
+
+ 0 0..3 "fred" "barney" "wilma" "betty"
+ 1 0..2 "george" "jane" "elroy"
+ 2 0..2 "homer" "marge" "bart"
+
+Whereas if you comment out the two lines I said you might wish to,
+then it shows it to you this way instead:
+
+ 0 ARRAY(0x8031d0)
+ 0 "fred"
+ 1 "barney"
+ 2 "wilma"
+ 3 "betty"
+ 1 ARRAY(0x803d40)
+ 0 "george"
+ 1 "jane"
+ 2 "elroy"
+ 2 ARRAY(0x803e10)
+ 0 "homer"
+ 1 "marge"
+ 2 "bart"
+
+=head2 Slices
If you want to get at a slice (part of a row) in a multidimensional
array, you're going to have to do some fancy subscripting. That's
because while we have a nice synonym for single elements via the
pointer arrow for dereferencing, no such convenience exists for slices.
-(Remember, of course, that you can always write a loop to do a slice
-operation.)
Here's how to do one operation using a loop. We'll assume an @AoA
variable as before.
That same loop could be replaced with a slice operation:
+ @part = @{$AoA[4]}[7..12];
+
+or spaced out a bit:
+
@part = @{ $AoA[4] } [ 7..12 ];
-but as you might well imagine, this is pretty rough on the reader.
+But as you might well imagine, this can get pretty rough on the reader.
Ah, but what if you wanted a I<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 = map { [ @{ $AoA[$_] } [ 7..12 ] ] } 4 .. 8;
-Although if your manager accused of seeking job security (or rapid
+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:
=head1 SEE ALSO
-perldata(1), perlref(1), perldsc(1)
+L<perldata>, L<perlref>, L<perldsc>
=head1 AUTHOR
Tom Christiansen <F<tchrist@perl.com>>
-Last update: Thu Jun 4 16:16:23 MDT 1998
+Last update: Tue Apr 26 18:30:55 MDT 2011
https://perl5.git.perl.org / perl5.git / blobdiff