FM / pod

/usr/share/perl5/pod

Name	Size	Mode	Actions
perl.pod	16270	0644	edit dl rm
perl56delta.pod	107201	0644	edit dl rm
perl58delta.pod	115165	0644	edit dl rm
perl561delta.pod	124713	0644	edit dl rm
perl581delta.pod	38061	0644	edit dl rm
perl582delta.pod	4470	0644	edit dl rm
perl583delta.pod	6335	0644	edit dl rm
perl584delta.pod	7363	0644	edit dl rm
perl585delta.pod	5889	0644	edit dl rm
perl586delta.pod	4651	0644	edit dl rm
perl587delta.pod	8357	0644	edit dl rm
perl588delta.pod	25272	0644	edit dl rm
perl589delta.pod	53900	0644	edit dl rm
perl5004delta.pod	56240	0644	edit dl rm
perl5005delta.pod	34283	0644	edit dl rm
perl5100delta.pod	55535	0644	edit dl rm
perl5101delta.pod	43888	0644	edit dl rm
perl5120delta.pod	89272	0644	edit dl rm
perl5121delta.pod	10141	0644	edit dl rm
perl5122delta.pod	9603	0644	edit dl rm
perl5123delta.pod	4100	0644	edit dl rm
perl5124delta.pod	3672	0644	edit dl rm
perl5125delta.pod	7683	0644	edit dl rm
perl5140delta.pod	144324	0644	edit dl rm
perl5141delta.pod	7966	0644	edit dl rm
perl5142delta.pod	6892	0644	edit dl rm
perl5143delta.pod	7760	0644	edit dl rm
perl5144delta.pod	6327	0644	edit dl rm
perl5160delta.pod	133651	0644	edit dl rm
perl5161delta.pod	6142	0644	edit dl rm
perl5162delta.pod	3594	0644	edit dl rm
perl5163delta.pod	4085	0644	edit dl rm
perl5180delta.pod	119431	0644	edit dl rm
perl5181delta.pod	6595	0644	edit dl rm
perl5182delta.pod	5335	0644	edit dl rm
perl5184delta.pod	4642	0644	edit dl rm
perl5200delta.pod	115699	0644	edit dl rm
perl5201delta.pod	10899	0644	edit dl rm
perl5202delta.pod	12509	0644	edit dl rm
perl5203delta.pod	9392	0644	edit dl rm
perl5220delta.pod	130963	0644	edit dl rm
perl5221delta.pod	10767	0644	edit dl rm
perl5222delta.pod	12629	0644	edit dl rm
perl5223delta.pod	8456	0644	edit dl rm
perl5224delta.pod	4460	0644	edit dl rm
perl5240delta.pod	64927	0644	edit dl rm
perl5241delta.pod	8215	0644	edit dl rm
perl5242delta.pod	4113	0644	edit dl rm
perl5243delta.pod	11428	0644	edit dl rm
perl5244delta.pod	4510	0644	edit dl rm
perl5260delta.pod	101836	0644	edit dl rm
perl5261delta.pod	7927	0644	edit dl rm
perl5262delta.pod	7880	0644	edit dl rm
perl5263delta.pod	7063	0644	edit dl rm
perl5280delta.pod	72113	0644	edit dl rm
perlaix.pod	20437	0644	edit dl rm
perlamiga.pod	5749	0644	edit dl rm
perlandroid.pod	7871	0644	edit dl rm
perlapi.pod	443535	0644	edit dl rm
perlapio.pod	19285	0644	edit dl rm
perlartistic.pod	7010	0644	edit dl rm
perlbook.pod	8338	0644	edit dl rm
perlboot.pod	294	0644	edit dl rm
perlbot.pod	304	0644	edit dl rm
perlbs2000.pod	8058	0644	edit dl rm
perlcall.pod	56706	0644	edit dl rm
perlce.pod	14602	0644	edit dl rm
perlcheat.pod	4481	0644	edit dl rm
perlclib.pod	9619	0644	edit dl rm
perlcn.pod	4691	0644	edit dl rm
perlcommunity.pod	7217	0644	edit dl rm
perlcygwin.pod	27199	0644	edit dl rm
perldata.pod	46743	0644	edit dl rm
perldbmfilter.pod	4981	0644	edit dl rm
perldebguts.pod	38535	0644	edit dl rm
perldebtut.pod	22152	0644	edit dl rm
perldebug.pod	39258	0644	edit dl rm
perldelta.pod	7063	0644	edit dl rm
perldeprecation.pod	18169	0644	edit dl rm
perldiag.pod	284572	0644	edit dl rm
perldos.pod	10522	0644	edit dl rm
perldsc.pod	25614	0644	edit dl rm
perldtrace.pod	7958	0644	edit dl rm
perlebcdic.pod	84233	0644	edit dl rm
perlembed.pod	37196	0644	edit dl rm
perlexperiment.pod	7195	0644	edit dl rm
perlfork.pod	13355	0644	edit dl rm
perlform.pod	16608	0644	edit dl rm
perlfreebsd.pod	1610	0644	edit dl rm
perlfunc.pod	392957	0644	edit dl rm
perlgit.pod	33509	0644	edit dl rm
perlgpl.pod	13815	0644	edit dl rm
perlguts.pod	139329	0644	edit dl rm
perlhack.pod	40445	0644	edit dl rm
perlhacktips.pod	55509	0644	edit dl rm
perlhacktut.pod	6153	0644	edit dl rm
perlhaiku.pod	1508	0644	edit dl rm
perlhist.pod	53546	0644	edit dl rm
perlhpux.pod	30509	0644	edit dl rm
perlhurd.pod	1993	0644	edit dl rm
perlintern.pod	54572	0644	edit dl rm
perlinterp.pod	33687	0644	edit dl rm
perlintro.pod	22119	0644	edit dl rm
perliol.pod	34185	0644	edit dl rm
perlipc.pod	70829	0644	edit dl rm
perlirix.pod	4395	0644	edit dl rm
perljp.pod	7521	0644	edit dl rm
perlko.pod	12259	0644	edit dl rm
perllexwarn.pod	355	0644	edit dl rm
perllinux.pod	1488	0644	edit dl rm
perllocale.pod	68678	0644	edit dl rm
perllol.pod	9580	0644	edit dl rm
perlmacos.pod	1001	0644	edit dl rm
perlmacosx.pod	12060	0644	edit dl rm
perlmod.pod	26250	0644	edit dl rm
perlmodinstall.pod	12792	0644	edit dl rm
perlmodlib.pod	76482	0644	edit dl rm
perlmodstyle.pod	22575	0644	edit dl rm
perlmroapi.pod	3212	0644	edit dl rm
perlnetware.pod	6648	0644	edit dl rm
perlnewmod.pod	11036	0644	edit dl rm
perlnumber.pod	8353	0644	edit dl rm
perlobj.pod	35537	0644	edit dl rm
perlootut.pod	26783	0644	edit dl rm
perlop.pod	136252	0644	edit dl rm
perlopenbsd.pod	1204	0644	edit dl rm
perlopentut.pod	9455	0644	edit dl rm
perlos2.pod	93351	0644	edit dl rm
perlos390.pod	15674	0644	edit dl rm
perlos400.pod	4768	0644	edit dl rm
perlpacktut.pod	51282	0644	edit dl rm
perlperf.pod	49881	0644	edit dl rm
perlplan9.pod	5125	0644	edit dl rm
perlpod.pod	22196	0644	edit dl rm
perlpodspec.pod	68476	0644	edit dl rm
perlpolicy.pod	25629	0644	edit dl rm
perlport.pod	87602	0644	edit dl rm
perlpragma.pod	5176	0644	edit dl rm
perlqnx.pod	6673	0644	edit dl rm
perlre.pod	120901	0644	edit dl rm
perlreapi.pod	30334	0644	edit dl rm
perlrebackslash.pod	31817	0644	edit dl rm
perlrecharclass.pod	49029	0644	edit dl rm
perlref.pod	35304	0644	edit dl rm
perlreftut.pod	18790	0644	edit dl rm
perlreguts.pod	38328	0644	edit dl rm
perlrepository.pod	509	0644	edit dl rm
perlrequick.pod	18497	0644	edit dl rm
perlreref.pod	14744	0644	edit dl rm
perlretut.pod	121257	0644	edit dl rm
perlriscos.pod	1529	0644	edit dl rm
perlrun.pod	53550	0644	edit dl rm
perlsec.pod	26184	0644	edit dl rm
perlsolaris.pod	29822	0644	edit dl rm
perlsource.pod	6876	0644	edit dl rm
perlstyle.pod	8630	0644	edit dl rm
perlsub.pod	72967	0644	edit dl rm
perlsymbian.pod	15359	0644	edit dl rm
perlsyn.pod	44512	0644	edit dl rm
perlsynology.pod	7778	0644	edit dl rm
perlthrtut.pod	46459	0644	edit dl rm
perltie.pod	38607	0644	edit dl rm
perltoc.pod	694171	0644	edit dl rm
perltodo.pod	376	0644	edit dl rm
perltooc.pod	294	0644	edit dl rm
perltoot.pod	294	0644	edit dl rm
perltrap.pod	10620	0644	edit dl rm
perltru64.pod	8492	0644	edit dl rm
perltw.pod	4477	0644	edit dl rm
perlunicode.pod	82491	0644	edit dl rm
perlunicook.pod	25488	0644	edit dl rm
perlunifaq.pod	13647	0644	edit dl rm
perluniintro.pod	38340	0644	edit dl rm
perluniprops.pod	285306	0644	edit dl rm
perlunitut.pod	7951	0644	edit dl rm
perlutil.pod	7640	0644	edit dl rm
perlvar.pod	78364	0644	edit dl rm
perlvms.pod	50823	0644	edit dl rm
perlvos.pod	3843	0644	edit dl rm
perlwin32.pod	39298	0644	edit dl rm
perlxs.pod	78920	0644	edit dl rm
perlxstut.pod	50095	0644	edit dl rm
perlxstypemap.pod	24001	0644	edit dl rm

Edit: /usr/share/perl5/pod/perldsc.pod (25614B)

=head1 NAME
X<data structure> X<complex data structure> X<struct>

perldsc - Perl Data Structures Cookbook

=head1 DESCRIPTION

Perl lets us have complex data structures.  You can write something like
this and all of a sudden, you'd have an array with three dimensions!

for my $x (1 .. 10) {
        for my $y (1 .. 10) {
            for my $z (1 .. 10) {
                $AoA[$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 say just C<print @AoA>?  How do
you sort it?  How can you pass it to a function or get one of these back
from a function?  Is it 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
sections 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

=back

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

=head1 REFERENCES
X<reference> X<dereference> X<dereferencing> X<pointer>

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 hold only 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.
X<multidimensional array> X<array, multidimensional>

You can't use a reference to an 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 L<perlref>.
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.)  This means that when you have something which
looks to you like an access to a two-or-more-dimensional array and/or hash,
what's really going on is that 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.

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

Now, because the top level contains only 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:

my @AoA = ( [2, 3], [4, 5, 7], [0] );
    print $AoA[1][2];
  7
    print @AoA;
  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 my $i (1..10) {
        my @array = somefunc($i);
        $AoA[$i] = @array;      # WRONG!
    }

That's just the simple case of assigning an array 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 my $i (1..10) {
        my @array = somefunc($i);
        $counts[$i] = scalar @array;
    }

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

# Either without strict or having an outer-scope my @array;
    # declaration.

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

So, 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 @AoA refer to the I<very same place>, and they will therefore all hold
whatever was last in @array!  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:
X<[]> X<{}>

# Either without strict or having an outer-scope my @array;
    # declaration.

for my $i (1..10) {
        @array = somefunc($i);
        $AoA[$i] = [ @array ];
    }

The square brackets make a reference to a new array with a I<copy>
of what's in @array 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:

# Either without strict or having an outer-scope my @array;
    # declaration.
    for my $i (1..10) {
        @array = 0 .. $i;
        @{$AoA[$i]} = @array;
    }

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<@{$AoA[$i]}>
dereference on the left-hand-side of the assignment.  It all depends on
whether C<$AoA[$i]> had been undefined to start with, or whether it
already contained a reference.  If you had already populated @AoA with
references, as in

$AoA[3] = \@another_array;

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

@{$AoA[3]} = @array;

Of course, this I<would> have the "interesting" effect of clobbering
@another_array.  (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 always to 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 my $i (1..10) {
        my @array = somefunc($i);
        $AoA[$i] = \@array;
    }

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:

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

=head1 CAVEAT ON PRECEDENCE
X<dereference, precedence> X<dereferencing, precedence>

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

$aref->[2][2]       # clear
    $$aref[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<$$aref[$i]> first does
the deref of $aref, making it take $aref 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 $AoA. If you wanted the C notion, you'd have to
write C<${$AoA[$i]}> to force the C<$AoA[$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 $aref = [
        [ "fred", "barney", "pebbles", "bambam", "dino", ],
        [ "homer", "bart", "marge", "maggie", ],
        [ "george", "jane", "elroy", "judy", ],
    ];

print $aref[2][2];

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

print $aref->[2][2]

=head1 DEBUGGING
X<data structure, debugging> X<complex data structure, debugging>
X<AoA, debugging> X<HoA, debugging> X<AoH, debugging> X<HoH, debugging>
X<array of arrays, debugging> X<hash of arrays, debugging>
X<array of hashes, debugging> X<hash of hashes, debugging>

You can use the debugger's C<x> command to dump out complex data structures.
For example, given the assignment to $AoA above, here's the debugger output:

DB<1> x $AoA
    $AoA = 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  'elroy'
          3  'judy'

=head1 CODE EXAMPLES

Presented with little comment (these will get their own manpages someday)
here are short code examples illustrating access of various
types of data structures.

=head1 ARRAYS OF ARRAYS
X<array of arrays> X<AoA>

=head2 Declaration of an ARRAY OF ARRAYS

@AoA = (
        [ "fred", "barney" ],
        [ "george", "jane", "elroy" ],
        [ "homer", "marge", "bart" ],
      );

=head2 Generation of an ARRAY OF ARRAYS

# reading from file
 while ( <> ) {
     push @AoA, [ split ];
 }

# calling a function
 for $i ( 1 .. 10 ) {
     $AoA[$i] = [ somefunc($i) ];
 }

# using temp vars
 for $i ( 1 .. 10 ) {
     @tmp = somefunc($i);
     $AoA[$i] = [ @tmp ];
 }

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

=head2 Access and Printing of an ARRAY OF ARRAYS

# one element
 $AoA[0][0] = "Fred";

# another element
 $AoA[1][1] =~ s/(\w)/\u$1/;

# print the whole thing with refs
 for $aref ( @AoA ) {
     print "\t [ @$aref ],\n";
 }

# print the whole thing with indices
 for $i ( 0 .. $#AoA ) {
     print "\t [ @{$AoA[$i]} ],\n";
 }

# print the whole thing one at a time
 for $i ( 0 .. $#AoA ) {
     for $j ( 0 .. $#{ $AoA[$i] } ) {
         print "elt $i $j is $AoA[$i][$j]\n";
     }
 }

=head1 HASHES OF ARRAYS
X<hash of arrays> X<HoA>

=head2 Declaration of a HASH OF ARRAYS

%HoA = (
        flintstones        => [ "fred", "barney" ],
        jetsons            => [ "george", "jane", "elroy" ],
        simpsons           => [ "homer", "marge", "bart" ],
      );

=head2 Generation of a HASH OF ARRAYS

# reading from file
 # flintstones: fred barney wilma dino
 while ( <> ) {
     next unless s/^(.*?):\s*//;
     $HoA{$1} = [ split ];
 }

# reading from file; more temps
 # flintstones: fred barney wilma dino
 while ( $line = <> ) {
     ($who, $rest) = split /:\s*/, $line, 2;
     @fields = split ' ', $rest;
     $HoA{$who} = [ @fields ];
 }

# calling a function that returns a list
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     $HoA{$group} = [ get_family($group) ];
 }

# likewise, but using temps
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     @members = get_family($group);
     $HoA{$group} = [ @members ];
 }

# append new members to an existing family
 push @{ $HoA{"flintstones"} }, "wilma", "betty";

=head2 Access and Printing of a HASH OF ARRAYS

# one element
 $HoA{flintstones}[0] = "Fred";

# another element
 $HoA{simpsons}[1] =~ s/(\w)/\u$1/;

# print the whole thing
 foreach $family ( keys %HoA ) {
     print "$family: @{ $HoA{$family} }\n"
 }

# print the whole thing with indices
 foreach $family ( keys %HoA ) {
     print "family: ";
     foreach $i ( 0 .. $#{ $HoA{$family} } ) {
         print " $i = $HoA{$family}[$i]";
     }
     print "\n";
 }

# print the whole thing sorted by number of members
 foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
     print "$family: @{ $HoA{$family} }\n"
 }

# print the whole thing sorted by number of members and name
 foreach $family ( sort {
                            @{$HoA{$b}} <=> @{$HoA{$a}}
                                        ||
                                    $a cmp $b
            } keys %HoA )
 {
     print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n";
 }

=head1 ARRAYS OF HASHES
X<array of hashes> X<AoH>

=head2 Declaration of an ARRAY OF HASHES

@AoH = (
        {
            Lead     => "fred",
            Friend   => "barney",
        },
        {
            Lead     => "george",
            Wife     => "jane",
            Son      => "elroy",
        },
        {
            Lead     => "homer",
            Wife     => "marge",
            Son      => "bart",
        }
  );

=head2 Generation of an ARRAY OF HASHES

# reading from file
 # format: LEAD=fred FRIEND=barney
 while ( <> ) {
     $rec = {};
     for $field ( split ) {
         ($key, $value) = split /=/, $field;
         $rec->{$key} = $value;
     }
     push @AoH, $rec;
 }

# reading from file
 # format: LEAD=fred FRIEND=barney
 # no temp
 while ( <> ) {
     push @AoH, { split /[\s+=]/ };
 }

# calling a function  that returns a key/value pair list, like
 # "lead","fred","daughter","pebbles"
 while ( %fields = getnextpairset() ) {
     push @AoH, { %fields };
 }

# likewise, but using no temp vars
 while (<>) {
     push @AoH, { parsepairs($_) };
 }

# add key/value to an element
 $AoH[0]{pet} = "dino";
 $AoH[2]{pet} = "santa's little helper";

=head2 Access and Printing of an ARRAY OF HASHES

# one element
 $AoH[0]{lead} = "fred";

# another element
 $AoH[1]{lead} =~ s/(\w)/\u$1/;

# print the whole thing with refs
 for $href ( @AoH ) {
     print "{ ";
     for $role ( keys %$href ) {
         print "$role=$href->{$role} ";
     }
     print "}\n";
 }

# print the whole thing with indices
 for $i ( 0 .. $#AoH ) {
     print "$i is { ";
     for $role ( keys %{ $AoH[$i] } ) {
         print "$role=$AoH[$i]{$role} ";
     }
     print "}\n";
 }

# print the whole thing one at a time
 for $i ( 0 .. $#AoH ) {
     for $role ( keys %{ $AoH[$i] } ) {
         print "elt $i $role is $AoH[$i]{$role}\n";
     }
 }

=head1 HASHES OF HASHES
X<hash of hashes> X<HoH>

=head2 Declaration of a HASH OF HASHES

%HoH = (
        flintstones => {
                lead      => "fred",
                pal       => "barney",
        },
        jetsons     => {
                lead      => "george",
                wife      => "jane",
                "his boy" => "elroy",
        },
        simpsons    => {
                lead      => "homer",
                wife      => "marge",
                kid       => "bart",
        },
 );

=head2 Generation of a HASH OF HASHES

# reading from file
 # flintstones: lead=fred pal=barney wife=wilma pet=dino
 while ( <> ) {
     next unless s/^(.*?):\s*//;
     $who = $1;
     for $field ( split ) {
         ($key, $value) = split /=/, $field;
         $HoH{$who}{$key} = $value;
     }

# reading from file; more temps
 while ( <> ) {
     next unless s/^(.*?):\s*//;
     $who = $1;
     $rec = {};
     $HoH{$who} = $rec;
     for $field ( split ) {
         ($key, $value) = split /=/, $field;
         $rec->{$key} = $value;
     }
 }

# calling a function  that returns a key,value hash
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     $HoH{$group} = { get_family($group) };
 }

# likewise, but using temps
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     %members = get_family($group);
     $HoH{$group} = { %members };
 }

# append new members to an existing family
 %new_folks = (
     wife => "wilma",
     pet  => "dino",
 );

for $what (keys %new_folks) {
     $HoH{flintstones}{$what} = $new_folks{$what};
 }

=head2 Access and Printing of a HASH OF HASHES

# one element
 $HoH{flintstones}{wife} = "wilma";

# another element
 $HoH{simpsons}{lead} =~ s/(\w)/\u$1/;

# print the whole thing
 foreach $family ( keys %HoH ) {
     print "$family: { ";
     for $role ( keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }

# print the whole thing  somewhat sorted
 foreach $family ( sort keys %HoH ) {
     print "$family: { ";
     for $role ( sort keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }

# print the whole thing sorted by number of members
 foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} }
                                                             keys %HoH )
 {
     print "$family: { ";
     for $role ( sort keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }

# establish a sort order (rank) for each role
 $i = 0;
 for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }

# now print the whole thing sorted by number of members
 foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } }
                                                             keys %HoH )
 {
     print "$family: { ";
     # and print these according to rank order
     for $role ( sort { $rank{$a} <=> $rank{$b} }
                                               keys %{ $HoH{$family} } )
     {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }

=head1 MORE ELABORATE RECORDS
X<record> X<structure> X<struct>

=head2 Declaration of MORE ELABORATE RECORDS

Here's a sample showing how to create and use a record whose fields are of
many different sorts:

$rec = {
         TEXT      => $string,
         SEQUENCE  => [ @old_values ],
         LOOKUP    => { %some_table },
         THATCODE  => \&some_function,
         THISCODE  => sub { $_[0] ** $_[1] },
         HANDLE    => \*STDOUT,
     };

print $rec->{TEXT};

print $rec->{SEQUENCE}[0];
     $last = pop @ { $rec->{SEQUENCE} };

print $rec->{LOOKUP}{"key"};
     ($first_k, $first_v) = each %{ $rec->{LOOKUP} };

$answer = $rec->{THATCODE}->($arg);
     $answer = $rec->{THISCODE}->($arg1, $arg2);

# careful of extra block braces on fh ref
     print { $rec->{HANDLE} } "a string\n";

use FileHandle;
     $rec->{HANDLE}->autoflush(1);
     $rec->{HANDLE}->print(" a string\n");

=head2 Declaration of a HASH OF COMPLEX RECORDS

%TV = (
        flintstones => {
            series   => "flintstones",
            nights   => [ qw(monday thursday friday) ],
            members  => [
                { name => "fred",    role => "lead", age  => 36, },
                { name => "wilma",   role => "wife", age  => 31, },
                { name => "pebbles", role => "kid",  age  =>  4, },
            ],
        },

jetsons     => {
            series   => "jetsons",
            nights   => [ qw(wednesday saturday) ],
            members  => [
                { name => "george",  role => "lead", age  => 41, },
                { name => "jane",    role => "wife", age  => 39, },
                { name => "elroy",   role => "kid",  age  =>  9, },
            ],
         },

simpsons    => {
            series   => "simpsons",
            nights   => [ qw(monday) ],
            members  => [
                { name => "homer", role => "lead", age  => 34, },
                { name => "marge", role => "wife", age => 37, },
                { name => "bart",  role => "kid",  age  =>  11, },
            ],
         },
      );

=head2 Generation of a HASH OF COMPLEX RECORDS

# reading from file
     # this is most easily done by having the file itself be
     # in the raw data format as shown above.  perl is happy
     # to parse complex data structures if declared as data, so
     # sometimes it's easiest to do that

# here's a piece by piece build up
     $rec = {};
     $rec->{series} = "flintstones";
     $rec->{nights} = [ find_days() ];

@members = ();
     # assume this file in field=value syntax
     while (<>) {
         %fields = split /[\s=]+/;
         push @members, { %fields };
     }
     $rec->{members} = [ @members ];

# now remember the whole thing
     $TV{ $rec->{series} } = $rec;

###########################################################
     # now, you might want to make interesting extra fields that
     # include pointers back into the same data structure so if
     # change one piece, it changes everywhere, like for example
     # if you wanted a {kids} field that was a reference
     # to an array of the kids' records without having duplicate
     # records and thus update problems.
     ###########################################################
     foreach $family (keys %TV) {
         $rec = $TV{$family}; # temp pointer
         @kids = ();
         for $person ( @{ $rec->{members} } ) {
             if ($person->{role} =~ /kid|son|daughter/) {
                 push @kids, $person;
             }
         }
         # REMEMBER: $rec and $TV{$family} point to same data!!
         $rec->{kids} = [ @kids ];
     }

# you copied the array, but the array itself contains pointers
     # to uncopied objects. this means that if you make bart get
     # older via

$TV{simpsons}{kids}[0]{age}++;

# then this would also change in
     print $TV{simpsons}{members}[2]{age};

# because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
     # both point to the same underlying anonymous hash table

# print the whole thing
     foreach $family ( keys %TV ) {
         print "the $family";
         print " is on during @{ $TV{$family}{nights} }\n";
         print "its members are:\n";
         for $who ( @{ $TV{$family}{members} } ) {
             print " $who->{name} ($who->{role}), age $who->{age}\n";
         }
         print "it turns out that $TV{$family}{lead} has ";
         print scalar ( @{ $TV{$family}{kids} } ), " kids named ";
         print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } );
         print "\n";
     }

=head1 Database Ties

You cannot easily tie a multilevel data structure (such as a hash of
hashes) to a dbm file.  The first problem is that all but GDBM and
Berkeley DB have size limitations, but beyond that, you also have problems
with how references are to be represented on disk.  One experimental
module that does partially attempt to address this need is the MLDBM
module.  Check your nearest CPAN site as described in L<perlmodlib> for
source code to MLDBM.

=head1 SEE ALSO

L<perlref>, L<perllol>, L<perldata>, L<perlobj>

=head1 AUTHOR

Tom Christiansen <F<tchrist@perl.com>>

cmd: