FM / pod

/usr/share/perl5/pod

Name	Size	Mode	Actions
a2p.pod	6107	0644	edit dl rm
perl.pod	15748	0644	edit dl rm
perl56delta.pod	107184	0644	edit dl rm
perl58delta.pod	114908	0644	edit dl rm
perl561delta.pod	124697	0644	edit dl rm
perl570delta.pod	21655	0644	edit dl rm
perl571delta.pod	30382	0644	edit dl rm
perl572delta.pod	25551	0644	edit dl rm
perl573delta.pod	4644	0644	edit dl rm
perl581delta.pod	38060	0644	edit dl rm
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perl585delta.pod	5889	0644	edit dl rm
perl586delta.pod	4651	0644	edit dl rm
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perl589delta.pod	53905	0644	edit dl rm
perl590delta.pod	34951	0644	edit dl rm
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perl593delta.pod	17034	0644	edit dl rm
perl594delta.pod	12661	0644	edit dl rm
perl595delta.pod	18934	0644	edit dl rm
perl5004delta.pod	56240	0644	edit dl rm
perl5005delta.pod	34285	0644	edit dl rm
perl5100delta.pod	53922	0644	edit dl rm
perl5101delta.pod	43877	0644	edit dl rm
perlaix.pod	18128	0644	edit dl rm
perlamiga.pod	7031	0644	edit dl rm
perlapi.pod	169428	0644	edit dl rm
perlapio.pod	19332	0644	edit dl rm
perlapollo.pod	829	0644	edit dl rm
perlartistic.pod	6802	0644	edit dl rm
perlbeos.pod	2939	0644	edit dl rm
perlbook.pod	663	0644	edit dl rm
perlboot.pod	28355	0644	edit dl rm
perlbot.pod	11675	0644	edit dl rm
perlbs2000.pod	7929	0644	edit dl rm
perlcall.pod	55173	0644	edit dl rm
perlce.pod	9257	0644	edit dl rm
perlcheat.pod	4153	0644	edit dl rm
perlclib.pod	7684	0644	edit dl rm
perlcn.pod	4915	0644	edit dl rm
perlcommunity.pod	6398	0644	edit dl rm
perlcompile.pod	9516	0644	edit dl rm
perlcygwin.pod	28183	0644	edit dl rm
perldata.pod	36879	0644	edit dl rm
perldbmfilter.pod	4987	0644	edit dl rm
perldebguts.pod	31887	0644	edit dl rm
perldebtut.pod	21304	0644	edit dl rm
perldebug.pod	37535	0644	edit dl rm
perldelta.pod	43877	0644	edit dl rm
perldgux.pod	2821	0644	edit dl rm
perldiag.pod	183151	0644	edit dl rm
perldoc.pod	7310	0644	edit dl rm
perldos.pod	10853	0644	edit dl rm
perldsc.pod	25447	0644	edit dl rm
perlebcdic.pod	66607	0644	edit dl rm
perlembed.pod	38033	0644	edit dl rm
perlepoc.pod	3779	0644	edit dl rm
perlfaq.pod	24841	0644	edit dl rm
perlfaq1.pod	17762	0644	edit dl rm
perlfaq2.pod	21614	0644	edit dl rm
perlfaq3.pod	38534	0644	edit dl rm
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perlfaq5.pod	49201	0644	edit dl rm
perlfaq6.pod	38538	0644	edit dl rm
perlfaq7.pod	37588	0644	edit dl rm
perlfaq8.pod	46128	0644	edit dl rm
perlfaq9.pod	24191	0644	edit dl rm
perlfilter.pod	21140	0644	edit dl rm
perlfork.pod	11440	0644	edit dl rm
perlform.pod	16894	0644	edit dl rm
perlfreebsd.pod	1950	0644	edit dl rm
perlfunc.pod	291909	0644	edit dl rm
perlglossary.pod	112139	0644	edit dl rm
perlgpl.pod	18806	0644	edit dl rm
perlguts.pod	105182	0644	edit dl rm
perlhack.pod	121512	0644	edit dl rm
perlhaiku.pod	1504	0644	edit dl rm
perlhist.pod	38766	0644	edit dl rm
perlhpux.pod	28650	0644	edit dl rm
perlhurd.pod	1990	0644	edit dl rm
perlintern.pod	26408	0644	edit dl rm
perlintro.pod	21491	0644	edit dl rm
perliol.pod	33755	0644	edit dl rm
perlipc.pod	67761	0644	edit dl rm
perlirix.pod	4397	0644	edit dl rm
perljp.pod	8070	0644	edit dl rm
perlko.pod	7892	0644	edit dl rm
perllexwarn.pod	14435	0644	edit dl rm
perllinux.pod	1492	0644	edit dl rm
perllocale.pod	41616	0644	edit dl rm
perllol.pod	8252	0644	edit dl rm
perlmachten.pod	4500	0644	edit dl rm
perlmacos.pod	2111	0644	edit dl rm
perlmacosx.pod	11322	0644	edit dl rm
perlmint.pod	9539	0644	edit dl rm
perlmod.pod	24494	0644	edit dl rm
perlmodinstall.pod	13929	0644	edit dl rm
perlmodlib.pod	77333	0644	edit dl rm
perlmodstyle.pod	21114	0644	edit dl rm
perlmpeix.pod	15087	0644	edit dl rm
perlmroapi.pod	2959	0644	edit dl rm
perlnetware.pod	6488	0644	edit dl rm
perlnewmod.pod	11214	0644	edit dl rm
perlnumber.pod	8352	0644	edit dl rm
perlobj.pod	21813	0644	edit dl rm
perlop.pod	96225	0644	edit dl rm
perlopenbsd.pod	1207	0644	edit dl rm
perlopentut.pod	37997	0644	edit dl rm
perlos2.pod	92783	0644	edit dl rm
perlos390.pod	16074	0644	edit dl rm
perlos400.pod	4618	0644	edit dl rm
perlothrtut.pod	40657	0644	edit dl rm
perlpacktut.pod	51047	0644	edit dl rm
perlperf.pod	51328	0644	edit dl rm
perlplan9.pod	5125	0644	edit dl rm
perlpod.pod	21626	0644	edit dl rm
perlpodspec.pod	67792	0644	edit dl rm
perlport.pod	86302	0644	edit dl rm
perlpragma.pod	4320	0644	edit dl rm
perlqnx.pod	4246	0644	edit dl rm
perlre.pod	82798	0644	edit dl rm
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perlrebackslash.pod	20109	0644	edit dl rm
perlrecharclass.pod	21909	0644	edit dl rm
perlref.pod	26465	0644	edit dl rm
perlreftut.pod	18670	0644	edit dl rm
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perlrepository.pod	23453	0644	edit dl rm
perlrequick.pod	17617	0644	edit dl rm
perlreref.pod	11996	0644	edit dl rm
perlretut.pod	115005	0644	edit dl rm
perlriscos.pod	1511	0644	edit dl rm
perlrun.pod	49928	0644	edit dl rm
perlsec.pod	23542	0644	edit dl rm
perlsolaris.pod	29131	0644	edit dl rm
perlstyle.pod	8618	0644	edit dl rm
perlsub.pod	54222	0644	edit dl rm
perlsymbian.pod	16222	0644	edit dl rm
perlsyn.pod	31090	0644	edit dl rm
perlthrtut.pod	46510	0644	edit dl rm
perltie.pod	36665	0644	edit dl rm
perltoc.pod	642987	0644	edit dl rm
perltodo.pod	48360	0644	edit dl rm
perltooc.pod	51425	0644	edit dl rm
perltoot.pod	68625	0644	edit dl rm
perltrap.pod	41151	0644	edit dl rm
perltru64.pod	7736	0644	edit dl rm
perltw.pod	5384	0644	edit dl rm
perlunicode.pod	56169	0644	edit dl rm
perlunifaq.pod	12878	0644	edit dl rm
perluniintro.pod	32973	0644	edit dl rm
perlunitut.pod	7907	0644	edit dl rm
perlutil.pod	9881	0644	edit dl rm
perluts.pod	3180	0644	edit dl rm
perlvar.pod	59133	0644	edit dl rm
perlvmesa.pod	3952	0644	edit dl rm
perlvms.pod	52545	0644	edit dl rm
perlvos.pod	5567	0644	edit dl rm
perlwin32.pod	40809	0644	edit dl rm
perlxs.pod	74194	0644	edit dl rm
perlxstut.pod	49670	0644	edit dl rm

Edit: /usr/share/perl5/pod/perllol.pod (8252B)

=head1 NAME

perllol - Manipulating Arrays of Arrays in Perl

=head1 DESCRIPTION

=head2 Declaration and Access of Arrays of Arrays

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.

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.

=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
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.

=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 $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";
	}
    }

=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 @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 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:

@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

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