304 lines
8.1 KiB
Plaintext
304 lines
8.1 KiB
Plaintext
=head1 NAME
|
|
|
|
perllol - Manipulating Arrays of Arrays in Perl
|
|
|
|
=head1 DESCRIPTION
|
|
|
|
=head1 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.
|
|
|
|
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:
|
|
|
|
# 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 I<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", ],
|
|
[ "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. $ref_to_AoA is a reference to an
|
|
array, whereas @AoA is an array proper. Likewise, C<$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.
|
|
|
|
=head1 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 C<[]> 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.
|
|
|
|
If you are running under C<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 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 C<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 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 reference to such.
|
|
|
|
=head1 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 $aref ( @AoA ) {
|
|
print "\t [ @$aref ],\n";
|
|
}
|
|
|
|
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";
|
|
}
|
|
}
|
|
|
|
=head1 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.
|
|
|
|
@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 ];
|
|
|
|
but as you might well imagine, this is 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 $y run from 7 to 12? Hmm... here's the simple way:
|
|
|
|
@newAoA = ();
|
|
for ($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;
|
|
}
|
|
|
|
|
|
=head1 SEE ALSO
|
|
|
|
perldata(1), perlref(1), perldsc(1)
|
|
|
|
=head1 AUTHOR
|
|
|
|
Tom Christiansen <F<tchrist@perl.com>>
|
|
|
|
Last update: Thu Jun 4 16:16:23 MDT 1998
|