PERLLOL(1) Perl Programmers Reference Guide PERLLOL(1)
NAME
perllol - Manipulating Arrays of Arrays in Perl
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 (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?
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 ) {
$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 $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 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:
my $aref = undef;
push $aref, qw(some more values); # WRONG!
push @$aref, qw(a few more); # ok
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
use v5.14; # needed for implicit deref of array refs by array ops
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
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]";
}
}
Hmm... that's still a bit ugly. How about this:
for $i ( 0 .. $#AoA ) {
$aref = $AoA[$i];
$n = @$aref - 1;
for $j ( 0 .. $n ) {
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 @_;
}
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.
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];
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
@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 <[email protected]>
Last update: Tue Apr 26 18:30:55 MDT 2011
perl v5.16.3 2013-03-04 PERLLOL(1)
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