=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 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.
-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 = (
- [ "fred", "barney" ],
- [ "george", "jane", "elroy" ],
- [ "homer", "marge", "bart" ],
- );
+ use 5.010; # so we can use say()
- print $LoL[2][2];
+ # 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, 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", ],
+ [ "george", "jane", "elroy", "judy", ],
[ "homer", "bart", "marge", "maggie", ],
- [ "george", "jane", "alroy", "judy", ],
];
-
- print $ref_to_LoL->[2][2];
+ 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. $ref_to_LoL is a reference to an
-array, whereas @LoL is an array proper. Likewise, C<$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
-constructor. That's because this will be very wrong:
+It's important you make sure to use the C<[ ]> array reference
+constructor. That's because this wouldn't work:
- $LoL[$i] = @tmp;
+ $AoA[$i] = @tmp; # WRONG!
-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.
+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 C<use strict>, you'll have to add some
-declarations to make it happy:
+If you are running under C<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(@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 ];
+ push @$ref_to_AoA, [ split ];
}
-Actually, if you were using strict, you'd have to declare not only
-$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
-perl version 5.001m that's been fixed for the 5.002 release.)
-
- my $ref_to_LoL = [];
- while (<>) {
- push @$ref_to_LoL, [ 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
to do something a bit funnier looking:
# add new columns to an existing row
- push @{ $LoL[0] }, "wilma", "betty";
+ push @{ $AoA[0] }, "wilma", "betty"; # explicit deref
+
+Prior to Perl 5.14, this wouldn't even compile:
+
+ push $AoA[0], "wilma", "betty"; # implicit deref
+
+How come? Because once upon a time, the argument to push() had to be a
+real array, not just a reference to one. That's no longer true. In fact,
+the line marked "implicit deref" above works just fine--in this
+instance--to do what the one that says explicit deref did.
-Notice that I I<couldn't> say just:
+The reason I said "in this instance" is because that I<only> works
+because C<$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 C<@{ }>
+always will:
- push $LoL[0], "wilma", "betty"; # WRONG!
+ my $aref = undef;
+ push $aref, qw(some more values); # WRONG!
+ push @$aref, qw(a few more); # ok
-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.
+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
-=head1 Access and Printing
+ 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.
+
+=head2 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 $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
using the shell-style for() construct to loop across the outer
set of subscripts.
- for $aref ( @LoL ) {
- print "\t [ @$aref ],\n";
+ for $aref ( @AoA ) {
+ say "\t [ @$aref ],";
}
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 ) {
+ say "\t elt $i is [ @{$AoA[$i]} ],";
}
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]} ) {
+ 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 .. $#LoL ) {
- $aref = $LoL[$i];
+ for $i ( 0 .. $#AoA ) {
+ $aref = $AoA[$i];
for $j ( 0 .. $#{$aref} ) {
- print "elt $i $j is $LoL[$i][$j]\n";
+ say "elt $i $j is $AoA[$i][$j]";
}
}
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";
+ say "elt $i $j is $AoA[$i][$j]";
}
}
-=head1 Slices
+When you get tired of writing a custom print for your data structures,
+you might look at the standard L<Dumpvalue> or L<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"
+
+=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
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 @LoL
+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, $LoL[$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];
+
+or spaced out a bit:
+
+ @part = @{ $AoA[4] } [ 7..12 ];
-but as you might well imagine, this is pretty rough on the reader.
+But as you might well imagine, this can get 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][$y];
}
}
We can reduce some of the looping through slices
for ($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 $lrr = shift; # ref to array of array refs!
my ($x_lo, $x_hi,
$y_lo, $y_hi) = @_;
=head1 SEE ALSO
-perldata(1), perlref(1), perldsc(1)
+L<perldata>, L<perlref>, L<perldsc>
=head1 AUTHOR
Tom Christiansen <F<tchrist@perl.com>>
-Last udpate: Sat Oct 7 19:35:26 MDT 1995
+Last update: Tue Apr 26 18:30:55 MDT 2011
https://perl5.git.perl.org / perl5.git / blobdiff