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PERLLOL

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlLoL - Manipulating Lists of Lists in Perl

DESCRIPTION

Declaration and Access of Lists of Lists

The simplest thing to build is a list of lists (sometimes called an arrayof arrays). It's reasonably easy to understand, and almost everythingthat applies here will also be applicable later on with the fancier datastructures.

A list of lists, or an array of an array if you would, is just a regularold array @LoL that you can get at with two subscripts, like $LoL[3][2]. Here'sa declaration of the array:

    # assign to our array a list of list references    @LoL = (           [ "fred", "barney" ],           [ "george", "jane", "elroy" ],           [ "homer", "marge", "bart" ],    );

    print $LoL[2][2];  bart

Now you should be very careful that the outer bracket typeis a round one, that is, parentheses. That's because you're assigning toan @list, so you need parentheses. If you wanted there not to be an @LoL,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 = [        [ "fred", "barney", "pebbles", "bambam", "dino", ],        [ "homer", "bart", "marge", "maggie", ],        [ "george", "jane", "alroy", "judy", ],    ];

    print $ref_to_LoL->[2][2];

Notice that the outer bracket type has changed, and so our access syntaxhas also changed. That's because unlike C, in perl you can't freelyinterchange arrays and references thereto. $ref_to_LoL is a reference to anarray, whereas @LoL is an array proper. Likewise, $LoL[2] is not anarray, but an array ref. So how come you can write these:

    $LoL[2][2]    $ref_to_LoL->[2][2]

instead of having to write these:

    $LoL[2]->[2]    $ref_to_LoL->[2]->[2]

Well, that's because the rule is that on adjacent brackets only (whethersquare 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 containinga reference, which means that $ref_to_LoL always needs it.

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 buildit up entirely from scratch?

First, let's look at reading it in from a file. This is something likeadding a row at a time. We'll assume that there's a flat file in whicheach 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:

    while (<>) {        @tmp = split;        push @LoL, [ @tmp ];    }

You might also have loaded that from a function:

    for $i ( 1 .. 10 ) {        $LoL[$i] = [ somefunc($i) ];    }

Or you might have had a temporary variable sitting around with thelist in it.

    for $i ( 1 .. 10 ) {        @tmp = somefunc($i);        $LoL[$i] = [ @tmp ];    }

It's very important that you make sure to use the [] list referenceconstructor. That's because this will be very wrong:

    $LoL[$i] = @tmp;

You see, assigning a named list like that to a scalar just counts thenumber of elements in @tmp, which probably isn't what you want.

If you are running under use strict, you'll have to add somedeclarations to make it happy:

    use strict;    my(@LoL, @tmp);    while (<>) {        @tmp = split;        push @LoL, [ @tmp ];    }

Of course, you don't need the temporary array to have a name at all:

    while (<>) {        push @LoL, [ split ];    }

You also don't have to use push(). You could just make a direct assignmentif you knew where you wanted to put it:

    my (@LoL, $i, $line);    for $i ( 0 .. 10 ) {        $line = <>;        $LoL[$i] = [ split ' ', $line ];    }

or even just

    my (@LoL, $i);    for $i ( 0 .. 10 ) {        $LoL[$i] = [ split ' ', <> ];    }

You should in general be leery of using potential list functionsin a 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(<>) ];    }

If you wanted to have a $ref_to_LoL 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 also having toinitialize it to a reference to an empty list. (This was a bug inperl 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'redealing 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);        }    }

    for $x ( 3, 7, 9 ) {        $LoL[$x][20] += func2($x);    }

It doesn't matter whether those elements are alreadythere or not: it'll gladly create them for you, settingintervening elements to undef as need be.

If you wanted just to append to a row, you'd haveto do something a bit funnier looking:

    # add new columns to an existing row    push @{ $LoL[0] }, "wilma", "betty";

Notice that I couldn't say just:

    push $LoL[0], "wilma", "betty";  # WRONG!

In fact, that wouldn't even compile. How come? Because the argumentto push() must be a real array, not just a reference to such.

Access and Printing

Now it's time to print your data structure out. Howare you going to do that? Well, if you want only oneof the elements, it's trivial:

    print $LoL[0][0];

If you want to print the whole thing, though, you can'tsay

    print @LoL;         # WRONG

because you'll get just references listed, and perl will neverautomatically dereference things for you. Instead, you have toroll yourself a loop or two. This prints the whole structure,using the shell-style for() construct to loop across the outerset of subscripts.

    for $aref ( @LoL ) {        print "\t [ @$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";    }

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";        }    }

As you can see, it's getting a bit complicated. That's whysometimes is easier to take a temporary on your way through:

    for $i ( 0 .. $#LoL ) {        $aref = $LoL[$i];        for $j ( 0 .. $#{$aref} ) {            print "elt $i $j is $LoL[$i][$j]\n";        }    }

Hmm... that's still a bit ugly. How about this:

    for $i ( 0 .. $#LoL ) {        $aref = $LoL[$i];        $n = @$aref - 1;        for $j ( 0 .. $n ) {            print "elt $i $j is $LoL[$i][$j]\n";        }    }

Slices

If you want to get at a slice (part of a row) in a multidimensionalarray, you're going to have to do some fancy subscripting. That'sbecause while we have a nice synonym for single elements via thepointer arrow for dereferencing, no such convenience exists for slices.(Remember, of course, that you can always write a loop to do a sliceoperation.)

Here's how to do one operation using a loop. We'll assume an @LoLvariable as before.

    @part = ();    $x = 4;    for ($y = 7; $y < 13; $y++) {        push @part, $LoL[$x][$y];    }

That same loop could be replaced with a slice operation:

    @part = @{ $LoL[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$x run from 4..8 and $y run from 7 to 12? Hmm... here's the simple way:

    @newLoL = ();    for ($startx = $x = 4; $x <= 8; $x++) {        for ($starty = $y = 7; $y <= 12; $y++) {            $newLoL[$x - $startx][$y - $starty] = $LoL[$x][$y];        }    }

We can reduce some of the looping through slices

    for ($x = 4; $x <= 8; $x++) {        push @newLoL, [ @{ $LoL[$x] } [ 7..12 ] ];    }

If you were into Schwartzian Transforms, you would probablyhave selected map for that

    @newLoL = map { [ @{ $LoL[$_] } [ 7..12 ] ] } 4 .. 8;

Although if your manager accused of seeking job security (or rapidinsecurity) 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 );    sub splice_2D {        my $lrr = shift;        # ref to list of list refs!        my ($x_lo, $x_hi,            $y_lo, $y_hi) = @_;

        return map {            [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ]        } $x_lo .. $x_hi;    }

AUTHOR

Tom Christiansen <tchristAATTperl.com>

Last udpate: Sat Oct 7 19:35:26 MDT 1995

Index

NAME
DESCRIPTION
Declaration and Access of Lists of Lists
Growing Your Own
Access and Printing
Slices
SEE ALSO
AUTHOR

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