[p5sagit/p5-mst-13.2.git] / pod / perldsc.pod

=head1 TITLE

perldsc - Manipulating Complex Data Structures in Perl

=head1 INTRODUCTION

The single feature most sorely lacking in the Perl programming language
prior to its 5.0 release was complex data structures.  Even without direct
language support, some valiant programmers did manage to emulate them, but
it was hard work and not for the faint of heart.  You could occasionally
get away with the C<$m{$LoL,$b}> notation borrowed from I<awk> in which the
keys are actually more like a single concatenated string C<"$LoL$b">, but
traversal and sorting were difficult.  More desperate programmers even
hacked Perl's internal symbol table directly, a strategy that proved hard
to develop and maintain--to put it mildly.

The 5.0 release of Perl let us have complex data structures.  You
may now write something like this and all of a sudden, you'd have a array
with three dimensions!

    for $x (1 .. 10) {
        for $y (1 .. 10) {
            for $z (1 .. 10) {
                $LoL[$x][$y][$z] = 
                    $x ** $y + $z;
            }
        }
    }

Alas, however simple this may appear, underneath it's a much more
elaborate construct than meets the eye!

How do you print it out?  Why can't you just say C<print @LoL>?  How do
you sort it?  How can you pass it to a function or get one of these back
from a function?  Is is an object?  Can you save it to disk to read
back later?  How do you access whole rows or columns of that matrix?  Do
all the values have to be numeric?  

As you see, it's quite easy to become confused.  While some small portion
of the blame for this can be attributed to the reference-based
implementation, it's really more due to a lack of existing documentation with
examples designed for the beginner.

This document is meant to be a detailed but understandable treatment of
the many different sorts of data structures you might want to develop.  It should
also serve as a cookbook of examples.  That way, when you need to create one of these
complex data structures, you can just pinch, pilfer, or purloin
a drop-in example from here.

Let's look at each of these possible constructs in detail.  There are separate
documents on each of the following:

=over 5

=item * arrays of arrays

=item * hashes of arrays

=item * arrays of hashes

=item * hashes of hashes

=item * more elaborate constructs

=item * recursive and self-referential data structures

=item * objects

=back

But for now, let's look at some of the general issues common to all
of these types of data structures. 

=head1 REFERENCES

The most important thing to understand about all data structures in Perl
-- including multidimensional arrays--is that even though they might
appear otherwise, Perl C<@ARRAY>s and C<%HASH>es are all internally
one-dimensional.  They can only hold scalar values (meaning a string,
number, or a reference).  They cannot directly contain other arrays or
hashes, but instead contain I<references> to other arrays or hashes.

You can't use a reference to a array or hash in quite the same way that
you would a real array or hash.  For C or C++ programmers unused to distinguishing
between arrays and pointers to the same, this can be confusing.  If so,
just think of it as the difference between a structure and a pointer to a
structure.  

You can (and should) read more about references in the perlref(1) man
page.  Briefly, references are rather like pointers that know what they
point to.  (Objects are also a kind of reference, but we won't be needing
them right away--if ever.)  That means that when you have something that
looks to you like an access to two-or-more-dimensional array and/or hash,
that what's really going on is that in all these cases, the base type is
merely a one-dimensional entity that contains references to the next
level.  It's just that you can I<use> it as though it were a
two-dimensional one.  This is actually the way almost all C
multidimensional arrays work as well.

    $list[7][12]                        # array of arrays
    $list[7]{string}                    # array of hashes
    $hash{string}[7]                    # hash of arrays
    $hash{string}{'another string'}     # hash of hashes

Now, because the top level only contains references, if you try to print
out your array in with a simple print() function, you'll get something
that doesn't look very nice, like this:

    @LoL = ( [2, 3], [4, 5, 7], [0] );
    print $LoL[1][2];
  7
    print @LoL;
  ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)


That's because Perl doesn't (ever) implicitly dereference your variables.
If you want to get at the thing a reference is referring to, then you have
to do this yourself using either prefix typing indicators, like
C<${$blah}>, C<@{$blah}>, C<@{$blah[$i]}>, or else postfix pointer arrows,
like C<$a-E<gt>[3]>, C<$h-E<gt>{fred}>, or even C<$ob-E<gt>method()-E<gt>[3]>.

=head1 COMMON MISTAKES

The two most common mistakes made in constructing something like
an array of arrays is either accidentally counting the number of
elements or else taking a reference to the same memory location
repeatedly.  Here's the case where you just get the count instead
of a nested array:

    for $i (1..10) {
        @list = somefunc($i);
        $LoL[$i] = @list;       # WRONG!
    } 

That's just the simple case of assigning a list to a scalar and getting
its element count.  If that's what you really and truly want, then you
might do well to consider being a tad more explicit about it, like this:

    for $i (1..10) {
        @list = somefunc($i);
        $counts[$i] = scalar @list;     
    } 

Here's the case of taking a reference to the same memory location
again and again:

    for $i (1..10) {
        @list = somefunc($i);
        $LoL[$i] = \@list;      # WRONG!
    } 

So, just what's the big problem with that?  It looks right, doesn't it?
After all, I just told you that you need an array of references, so by
golly, you've made me one!

Unfortunately, while this is true, it's still broken.  All the references
in @LoL refer to the I<very same place>, and they will therefore all hold
whatever was last in @list!  It's similar to the problem demonstrated in
the following C program:

    #include <pwd.h>
    main() {
        struct passwd *getpwnam(), *rp, *dp;
        rp = getpwnam("root");
        dp = getpwnam("daemon");

        printf("daemon name is %s\nroot name is %s\n", 
                dp->pw_name, rp->pw_name);
    }

Which will print

    daemon name is daemon
    root name is daemon 

The problem is that both C<rp> and C<dp> are pointers to the same location
in memory!  In C, you'd have to remember to malloc() yourself some new
memory.  In Perl, you'll want to use the array constructor C<[]> or the
hash constructor C<{}> instead.   Here's the right way to do the preceding
broken code fragments

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

The square brackets make a reference to a new array with a I<copy>
of what's in @list at the time of the assignment.  This is what
you want.  

Note that this will produce something similar, but it's
much harder to read:

    for $i (1..10) {
        @list = 0 .. $i;
        @{$LoL[$i]} = @list;
    } 

Is it the same?  Well, maybe so--and maybe not.  The subtle difference
is that when you assign something in square brackets, you know for sure
it's always a brand new reference with a new I<copy> of the data.
Something else could be going on in this new case with the C<@{$LoL[$i]}}>
dereference on the left-hand-side of the assignment.  It all depends on
whether C<$LoL[$i]> had been undefined to start with, or whether it
already contained a reference.  If you had already populated @LoL with
references, as in

    $LoL[3] = \@another_list;

Then the assignment with the indirection on the left-hand-side would
use the existing reference that was already there:

    @{$LoL[3]} = @list;

Of course, this I<would> have the "interesting" effect of clobbering
@another_list.  (Have you ever noticed how when a programmer says
something is "interesting", that rather than meaning "intriguing",
they're disturbingly more apt to mean that it's "annoying",
"difficult", or both?  :-)

So just remember to always use the array or hash constructors with C<[]>
or C<{}>, and you'll be fine, although it's not always optimally
efficient.  

Surprisingly, the following dangerous-looking construct will
actually work out fine:

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

That's because my() is more of a run-time statement than it is a
compile-time declaration I<per se>.  This means that the my() variable is
remade afresh each time through the loop.  So even though it I<looks> as
though you stored the same variable reference each time, you actually did
not!  This is a subtle distinction that can produce more efficient code at
the risk of misleading all but the most experienced of programmers.  So I
usually advise against teaching it to beginners.  In fact, except for
passing arguments to functions, I seldom like to see the gimme-a-reference
operator (backslash) used much at all in code.  Instead, I advise
beginners that they (and most of the rest of us) should try to use the
much more easily understood constructors C<[]> and C<{}> instead of
relying upon lexical (or dynamic) scoping and hidden reference-counting to
do the right thing behind the scenes.

In summary:

    $LoL[$i] = [ @list ];       # usually best
    $LoL[$i] = \@list;          # perilous; just how my() was that list?
    @{ $LoL[$i] } = @list;      # way too tricky for most programmers


=head1 CAVEAT ON PRECEDENCE 

Speaking of things like C<@{$LoL[$i]}>, the following are actually the
same thing:

    $listref->[2][2]    # clear
    $$listref[2][2]     # confusing

That's because Perl's precedence rules on its five prefix dereferencers
(which look like someone swearing: C<$ @ * % &>) make them bind more
tightly than the postfix subscripting brackets or braces!  This will no
doubt come as a great shock to the C or C++ programmer, who is quite
accustomed to using C<*a[i]> to mean what's pointed to by the I<i'th>
element of C<a>.  That is, they first take the subscript, and only then
dereference the thing at that subscript.  That's fine in C, but this isn't C.

The seemingly equivalent construct in Perl, C<$$listref[$i]> first does
the deref of C<$listref>, making it take $listref as a reference to an
array, and then dereference that, and finally tell you the I<i'th> value
of the array pointed to by $LoL. If you wanted the C notion, you'd have to
write C<${$LoL[$i]}> to force the C<$LoL[$i]> to get evaluated first
before the leading C<$> dereferencer.

=head1 WHY YOU SHOULD ALWAYS C<use strict>

If this is starting to sound scarier than it's worth, relax.  Perl has
some features to help you avoid its most common pitfalls.  The best
way to avoid getting confused is to start every program like this:

    #!/usr/bin/perl -w
    use strict;

This way, you'll be forced to declare all your variables with my() and
also disallow accidental "symbolic dereferencing".  Therefore if you'd done
this:

    my $listref = [
        [ "fred", "barney", "pebbles", "bambam", "dino", ],
        [ "homer", "bart", "marge", "maggie", ],
        [ "george", "jane", "alroy", "judy", ],
    ];

    print $listref[2][2];

The compiler would immediately flag that as an error I<at compile time>,
because you were accidentally accessing C<@listref>, an undeclared
variable, and it would thereby remind you to instead write:

    print $listref->[2][2]

=head1 DEBUGGING

The standard Perl debugger in 5.001 doesn't do a very nice job of 
printing out complex data structures.  However, the perl5db that
Ilya Zakharevich E<lt>F<ilya@math.ohio-state.edu>E<gt>
wrote, which is accessible at

    ftp://ftp.perl.com/pub/perl/ext/perl5db-kit-0.9.tar.gz

has several new features, including command line editing as well
as the C<x> command to dump out complex data structures.  For example, 
given the assignment to $LoL above, here's the debugger output:

    DB<1> X $LoL
    $LoL = ARRAY(0x13b5a0)
       0  ARRAY(0x1f0a24)
          0  'fred'
          1  'barney'
          2  'pebbles'
          3  'bambam'
          4  'dino'
       1  ARRAY(0x13b558)
          0  'homer'
          1  'bart'
          2  'marge'
          3  'maggie'
       2  ARRAY(0x13b540)
          0  'george'
          1  'jane'
          2  'alroy'
          3  'judy'

There's also a lower-case B<x> command which is nearly the same.

=head1 SEE ALSO

perlref(1), perldata(1)

=head1 AUTHOR

Tom Christiansen E<lt>F<tchrist@perl.com>E<gt>

Last update: 
Sat Oct  7 22:41:09 MDT 1995