PERLLOL(1) Perl Programmers Reference Guide PERLLOL(1)NAMEperllol - Manipulating Arrays of Arrays in Perl
DESCRIPTION
Declaration and Access of Arrays of Arrays
The simplest thing to build is 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 struc-
tures.
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 refer-
ence 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]
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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 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.
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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 ];
}
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 expli-
citly 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
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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 interven-
ing 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 the
argument to push() must be a real array, not just a refer-
ence to such.
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 ele-
ments, 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 $aref ( @AoA ) {
print "\t [ @$aref ],\n";
}
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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 multidi-
mensional array, you're going to have to do some fancy sub-
scripting. 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.
@part = ();
$x = 4;
for ($y = 7; $y < 13; $y++) {
push @part, $AoA[$x][$y];
}
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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 probably
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 ALSOperldata(1), perlref(1), perldsc(1)AUTHOR
Tom Christiansen <tchrist@perl.com>
Last update: Thu Jun 4 16:16:23 MDT 1998
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