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?
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 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 $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 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
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]";
}
}
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 @_;
}
# 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.
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][$y];
}
}
We can reduce some of the looping through slices
for ($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 AT perl.com>
Last update: Tue Apr 26 18:30:55 MDT 2011
perl v5.16.3 2013-03-04 PERLLOL(1)
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