=head1 NAME
-perlLoL - Manipulating Lists of Lists in Perl
+perllol - Manipulating Arrays of Arrays in Perl
=head1 DESCRIPTION
-=head1 Declaration and Access of Lists of Lists
+=head2 Declaration and Access of Arrays of Arrays
-The simplest thing to build is a list of lists (sometimes called an array
-of arrays). It's reasonably easy to understand, and almost everything
-that applies here will also be applicable later on with the fancier data
-structures.
+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 structures.
-A list of lists, or an array of an array if you would, is just a regular
-old array @LoL that you can get at with two subscripts, like C<$LoL[3][2]>. Here's
-a declaration of the array:
+An array of an array is just a regular old array @AoA that you can
+get at with two subscripts, like C<$AoA[3][2]>. Here's a declaration
+of the array:
- # assign to our array a list of list references
- @LoL = (
+ # assign to our array, an array of array references
+ @AoA = (
[ "fred", "barney" ],
[ "george", "jane", "elroy" ],
[ "homer", "marge", "bart" ],
);
- print $LoL[2][2];
+ print $AoA[2][2];
bart
Now you should be very careful that the outer bracket type
-is a round one, that is, parentheses. That's because you're assigning to
-an @list, so you need parentheses. If you wanted there I<not> to be an @LoL,
+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 list of list references
- $ref_to_LoL = [
+ # assign a reference to array of array references
+ $ref_to_AoA = [
[ "fred", "barney", "pebbles", "bambam", "dino", ],
[ "homer", "bart", "marge", "maggie", ],
- [ "george", "jane", "alroy", "judy", ],
+ [ "george", "jane", "elroy", "judy", ],
];
- print $ref_to_LoL->[2][2];
+ print $ref_to_AoA->[2][2];
-Notice that the outer bracket type has changed, and so our access syntax
+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. $ref_to_LoL is a reference to an
-array, whereas @LoL is an array proper. Likewise, C<$LoL[2]> is not an
+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:
- $LoL[2][2]
- $ref_to_LoL->[2][2]
+ $AoA[2][2]
+ $ref_to_AoA->[2][2]
instead of having to write these:
- $LoL[2]->[2]
- $ref_to_LoL->[2]->[2]
+ $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_LoL always needs it.
+a reference, which means that $ref_to_AoA always needs it.
-=head1 Growing Your Own
+=head2 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
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
-@LoL list containing all these, here's the right way to do that:
+@AoA array containing all these, here's the right way to do that:
while (<>) {
@tmp = split;
- push @LoL, [ @tmp ];
- }
+ push @AoA, [ @tmp ];
+ }
You might also have loaded that from a function:
for $i ( 1 .. 10 ) {
- $LoL[$i] = [ somefunc($i) ];
+ $AoA[$i] = [ somefunc($i) ];
}
Or you might have had a temporary variable sitting around with the
-list in it.
+array in it.
for $i ( 1 .. 10 ) {
@tmp = somefunc($i);
- $LoL[$i] = [ @tmp ];
+ $AoA[$i] = [ @tmp ];
}
-It's very important that you make sure to use the C<[]> list reference
+It's very important that you make sure to use the C<[]> array reference
constructor. That's because this will be very wrong:
- $LoL[$i] = @tmp;
+ $AoA[$i] = @tmp;
-You see, assigning a named list like that to a scalar just counts the
-number of elements in @tmp, which probably isn't what you want.
+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(@LoL, @tmp);
+ my(@AoA, @tmp);
while (<>) {
@tmp = split;
- push @LoL, [ @tmp ];
- }
+ push @AoA, [ @tmp ];
+ }
Of course, you don't need the temporary array to have a name at all:
while (<>) {
- push @LoL, [ split ];
- }
+ 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 (@LoL, $i, $line);
+ my (@AoA, $i, $line);
for $i ( 0 .. 10 ) {
$line = <>;
- $LoL[$i] = [ split ' ', $line ];
- }
+ $AoA[$i] = [ split ' ', $line ];
+ }
or even just
- my (@LoL, $i);
+ my (@AoA, $i);
for $i ( 0 .. 10 ) {
- $LoL[$i] = [ split ' ', <> ];
- }
+ $AoA[$i] = [ split ' ', <> ];
+ }
-You should in general be leery of using potential list functions
-in a scalar context without explicitly stating such.
-This would be clearer to the casual reader:
+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 (@LoL, $i);
+ my (@AoA, $i);
for $i ( 0 .. 10 ) {
- $LoL[$i] = [ split ' ', scalar(<>) ];
- }
+ $AoA[$i] = [ split ' ', scalar(<>) ];
+ }
-If you wanted to have a $ref_to_LoL variable as a reference to an array,
+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_LoL, [ split ];
- }
-
-Actually, if you were using strict, you'd have to declare not only
-$ref_to_LoL as you had to declare @LoL, but you'd I<also> having to
-initialize it to a reference to an empty list. (This was a bug in 5.001m
-that's been fixed for the 5.002 release.)
-
- my $ref_to_LoL = [];
- while (<>) {
- push @$ref_to_LoL, [ split ];
- }
+ push @$ref_to_AoA, [ split ];
+ }
-Ok, now you can add new rows. What about adding new columns? If you're
+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) {
- $LoL[$x][$y] = func($x, $y);
+ $AoA[$x][$y] = func($x, $y);
}
}
for $x ( 3, 7, 9 ) {
- $LoL[$x][20] += func2($x);
- }
+ $AoA[$x][20] += func2($x);
+ }
-It doesn't matter whether those elements are already
+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.
to do something a bit funnier looking:
# add new columns to an existing row
- push @{ $LoL[0] }, "wilma", "betty";
+ push @{ $AoA[0] }, "wilma", "betty";
Notice that I I<couldn't> say just:
- push $LoL[0], "wilma", "betty"; # WRONG!
+ 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
+=head2 Access and Printing
-Now it's time to print your data structure out. How
+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 $LoL[0][0];
+ print $AoA[0][0];
If you want to print the whole thing, though, you can't
say
- print @LoL; # WRONG
+ print @AoA; # WRONG
because you'll get just references listed, and perl will never
-automatically dereference things for you. Instead, you have to
+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.
+set of subscripts.
- for $aref ( @LoL ) {
+ for $aref ( @AoA ) {
print "\t [ @$aref ],\n";
}
If you wanted to keep track of subscripts, you might do this:
- for $i ( 0 .. $#LoL ) {
- print "\t elt $i is [ @{$LoL[$i]} ],\n";
+ for $i ( 0 .. $#AoA ) {
+ print "\t elt $i is [ @{$AoA[$i]} ],\n";
}
or maybe even this. Notice the inner loop.
- for $i ( 0 .. $#LoL ) {
- for $j ( 0 .. $#{$LoL[$i]} ) {
- print "elt $i $j is $LoL[$i][$j]\n";
+ 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
+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 .. $#LoL ) {
- $aref = $LoL[$i];
+ for $i ( 0 .. $#AoA ) {
+ $aref = $AoA[$i];
for $j ( 0 .. $#{$aref} ) {
- print "elt $i $j is $LoL[$i][$j]\n";
+ print "elt $i $j is $AoA[$i][$j]\n";
}
}
Hmm... that's still a bit ugly. How about this:
- for $i ( 0 .. $#LoL ) {
- $aref = $LoL[$i];
+ for $i ( 0 .. $#AoA ) {
+ $aref = $AoA[$i];
$n = @$aref - 1;
for $j ( 0 .. $n ) {
- print "elt $i $j is $LoL[$i][$j]\n";
+ print "elt $i $j is $AoA[$i][$j]\n";
}
}
-=head1 Slices
+=head2 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
(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 @LoL
+Here's how to do one operation using a loop. We'll assume an @AoA
variable as before.
@part = ();
- $x = 4;
+ $x = 4;
for ($y = 7; $y < 13; $y++) {
- push @part, $LoL[$x][$y];
- }
+ push @part, $AoA[$x][$y];
+ }
That same loop could be replaced with a slice operation:
- @part = @{ $LoL[4] } [ 7..12 ];
+ @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 $y run from 7 to 12? Hmm... here's the simple way:
- @newLoL = ();
+ @newAoA = ();
for ($startx = $x = 4; $x <= 8; $x++) {
- for ($starty = $y = 7; $x <= 12; $y++) {
- $newLoL[$x - $startx][$y - $starty] = $LoL[$x][$y];
+ for ($starty = $y = 7; $y <= 12; $y++) {
+ $newAoA[$x - $startx][$y - $starty] = $AoA[$x][$y];
}
- }
+ }
-We can reduce some of the looping through slices
+We can reduce some of the looping through slices
for ($x = 4; $x <= 8; $x++) {
- push @newLoL, [ @{ $LoL[$x] } [ 7..12 ] ];
+ push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ];
}
If you were into Schwartzian Transforms, you would probably
have selected map for that
- @newLoL = map { [ @{ $LoL[$_] } [ 7..12 ] ] } 4 .. 8;
+ @newAoA = map { [ @{ $AoA[$_] } [ 7..12 ] ] } 4 .. 8;
-Although if your manager accused of seeking job security (or rapid
+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:
- @newLoL = splice_2D( \@LoL, 4 => 8, 7 => 12 );
+ @newAoA = splice_2D( \@AoA, 4 => 8, 7 => 12 );
sub splice_2D {
- my $lrr = shift; # ref to list of list refs!
- my ($x_lo, $x_hi,
+ 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 ] ]
+ return map {
+ [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ]
} $x_lo .. $x_hi;
- }
+ }
=head1 SEE ALSO
=head1 AUTHOR
-Tom Christiansen E<lt>F<tchrist@perl.com>E<gt>
+Tom Christiansen <F<tchrist@perl.com>>
-Last udpate: Sat Oct 7 19:35:26 MDT 1995
+Last update: Thu Jun 4 16:16:23 MDT 1998
https://perl5.git.perl.org / perl5.git / blobdiff