NAME
perllol - 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). Its 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
@AoAthat you can get at with two subscripts, like
$AoA[3][2]. Heres 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. Thats because youre 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", ],
[ "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. Thats because unlike C, in perl you cant freely interchange arrays and references thereto.
$ref_to_AoAis a reference to an array, whereas
@AoAis 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, thats 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 its a scalar containing a reference, which means that
$ref_to_AoAalways needs it.
Growing Your Own
Thats 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, lets look at reading it in from a file. This is something like adding a row at a time. Well assume that theres a flat file in which each line is a row and each word an element. If youre trying to develop an
@AoAarray containing all these, heres 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 ];
}
Its very important that you make sure to use the
[]array reference constructor. Thats 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 isnt what you want.
If you are running under
use strict, youll have to add some declarations to make it happy:
use strict;
my(@AoA, @tmp);
while (<>) {
@tmp = split;
push @AoA, [ @tmp ];
}
Of course, you dont need the temporary array to have a name at all:
while (<>) {
push @AoA, [ split ];
}
You also dont 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
$ref_to_AoAvariable as a reference to an array, youd have to do something like this:
while (<>) {
push @$ref_to_AoA, [ split ];
}
Now you can add new rows. What about adding new columns? If youre dealing with just matrices, its 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 doesnt matter whether those elements are already there or not: itll gladly create them for you, setting intervening elements to
undefas need be.
If you wanted just to append to a row, youd have to do something a bit funnier looking:
# add new columns to an existing row
push @{ $AoA[0] }, "wilma", "betty";
Notice that I couldnt say just:
push $AoA[0], "wilma", "betty"; # WRONG!
In fact, that wouldnt even compile. How come? Because the argument to push() must be a real array, not just a reference to such.
Access and Printing
Now its time to print your data structure out. How are you going to do that? Well, if you want only one of the elements, its trivial:
print $AoA[0][0];
If you want to print the whole thing, though, you cant say
print @AoA; # WRONG
because youll 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, its getting a bit complicated. Thats 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... thats 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 multidimensional array, youre going to have to do some fancy subscripting. Thats 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.)
Heres how to do one operation using a loop. Well assume an
@AoAvariable 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 two-dimensional slice, such as having
$xrun from 4..8 and
$yrun from 7 to 12? Hmm... heres the simple way:
@newAoA = ();
for ($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, Id 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(1), perlref(1), perldsc(1)
AUTHOR
Tom Christiansen <tchrist@perl.com>
Last update: Thu Jun 4 16:16:23 MDT 1998