=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 $LoL[3][2]. Here's
-a declaration of the array:
+An array of an array is just a regular old array @AoA that you can
+get at with two subscripts, like C<$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 parens. 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", ],
];
+ say $ref_to_AoA->[2][1];
+ bart
- print $ref_to_LoL->[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, $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 array.
+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);
- for $i ( 0 .. 10 )
+ my (@AoA, $i, $line);
+ for $i ( 0 .. 10 ) {
$line = <>;
- $LoL[$i] = [ split ' ', $line ];
- }
+ $AoA[$i] = [ split " ", $line ];
+ }
or even just
- my (@LoL, $i);
- for $i ( 0 .. 10 )
- $LoL[$i] = [ split ' ', <> ];
- }
+ my (@AoA, $i);
+ for $i ( 0 .. 10 ) {
+ $AoA[$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);
- for $i ( 0 .. 10 )
- $LoL[$i] = [ split ' ', scalar(<>) ];
- }
+ my (@AoA, $i);
+ for $i ( 0 .. 10 ) {
+ $AoA[$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 not only have to declare $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
-just dealing with matrices, it's often easiest to use simple assignment:
+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.
-If you just wanted to append to a row, you'd have
+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 @{ $LoL[0] }, "wilma", "betty";
+ push @{ $AoA[0] }, "wilma", "betty"; # explicit deref
+
+Prior to Perl 5.14, this wouldn't even compile:
-Notice that I I<couldn't> just say:
+ push $AoA[0], "wilma", "betty"; # implicit deref
- push $LoL[0], "wilma", "betty"; # WRONG!
+How come? Because once upon a time, the argument to push() had to be a
+real array, not just a reference to one. That's no longer true. In fact,
+the line marked "implicit deref" above works just fine--in this
+instance--to do what the one that says explicit deref did.
-In fact, that wouldn't even compile. How come? Because the argument
-to push() must be a real array, not just a reference to such.
+The reason I said "in this instance" is because that I<only> works
+because C<$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:
-=head1 Access and Printing
+ my $aref = undef;
+ push $aref, qw(some more values); # WRONG!
+ push @$aref, qw(a few more); # ok
-Now it's time to print your data structure out. How
-are you going to do that? Well, if you only want one
+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
+
+ 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
-just say
+say
- print @LoL; # WRONG
+ print @AoA; # WRONG
-because you'll just get references listed, and perl will never
-automatically dereference things for you. Instead, you have to
+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.
+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
+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]";
}
}
-Hm... that's still a bit ugly. How about this:
+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;
+ $x = 4;
for ($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.
+or spaced out a bit:
+
+ @part = @{ $AoA[4] } [ 7..12 ];
+
+But as you might well imagine, this can get pretty rough on the reader.
Ah, but what if you wanted a I<two-dimensional slice>, such as having
-$x run from 4..8 and $y run from 7 to 12? Hm... here's the simple way:
+$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
-perldata(1), perlref(1), perldsc(1)
+L<perldata>, L<perlref>, L<perldsc>
=head1 AUTHOR
-Tom Christiansen <tchrist@perl.com>
+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