PERLDSC(1)       Perl Programmers Reference Guide      PERLDSC(1)

NNAAMMEE
       perldsc - Perl Data Structures Cookbook

DDEESSCCRRIIPPTTIIOONN
       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 $m{$LoL,$b} notation
       borrowed from awk in which the keys are actually more like
       a single concatenated string "$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 say just 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 sections on each of the following:

       o arrays of arrays

       o hashes of arrays

       o arrays of hashes

       o hashes of hashes

       o more elaborate constructs

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

RREEFFEERREENNCCEESS
       The most important thing to understand about all data
       structures in Perl -- including multidimensional
       arrays--is that even though they might appear otherwise,
       Perl @ARRAYs and %HASHes 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
       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.)  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 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 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:

           @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
       ${$blah}, @{$blah}, @{$blah[$i]}, or else postfix pointer
       arrows, like $a->[3], $h->{fred}, or even
       $ob->method()->[3].

CCOOMMMMOONN MMIISSTTAAKKEESS
       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, 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 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 rp and 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 [] or the hash constructor {}
       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
       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 copy of the data.  Something else
       could be going on in this new case with the @{$LoL[$i]}}
       dereference on the left-hand-side of the assignment.  It
       all depends on whether $LoL[$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 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 always to use the array or hash
       constructors with [] or {}, 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 per se.  This means that
       the my() variable is remade afresh each time through the
       loop.  So even though it 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 [] and {} 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

CCAAVVEEAATT OONN PPRREECCEEDDEENNCCEE
       Speaking of things like @{$LoL[$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: $ @ * %
       &) 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 *a[i] to mean what's pointed to by the
       i'th element of 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, $$listref[$i]
       first does the deref of $listref, making it take $listref
       as a reference to an array, and then dereference that, and
       finally tell you the i'th value of the array pointed to by
       $LoL. If you wanted the C notion, you'd have to write
       ${$LoL[$i]} to force the $LoL[$i] to get evaluated first
       before the leading $ dereferencer.

WWHHYY YYOOUU SSHHOOUULLDD AALLWWAAYYSS uussee ssttrriicctt
       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", "elroy", "judy", ],
           ];

           print $listref[2][2];

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

           print $listref->[2][2]

DDEEBBUUGGGGIINNGG
       Before version 5.002, the standard Perl debugger didn't do
       a very nice job of printing out complex data structures.
       With 5.002 or above, the debugger includes several new
       features, including command line editing as well as the 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  'elroy'
                 3  'judy'

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

LLIISSTTSS OOFF LLIISSTTSS
       DDeeccllaarraattiioonn ooff aa LLIISSTT OOFF LLIISSTTSS

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

       GGeenneerraattiioonn ooff aa LLIISSTT OOFF LLIISSTTSS

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

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

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

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

       AAcccceessss aanndd PPrriinnttiinngg ooff aa LLIISSTT OOFF LLIISSTTSS

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

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

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

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

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

HHAASSHHEESS OOFF LLIISSTTSS
       DDeeccllaarraattiioonn ooff aa HHAASSHH OOFF LLIISSTTSS

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

       GGeenneerraattiioonn ooff aa HHAASSHH OOFF LLIISSTTSS

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

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

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

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

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

       AAcccceessss aanndd PPrriinnttiinngg ooff aa HHAASSHH OOFF LLIISSTTSS

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

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

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

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

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

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

LLIISSTTSS OOFF HHAASSHHEESS
       DDeeccllaarraattiioonn ooff aa LLIISSTT OOFF HHAASSHHEESS

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

       GGeenneerraattiioonn ooff aa LLIISSTT OOFF HHAASSHHEESS

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

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

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

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

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

       AAcccceessss aanndd PPrriinnttiinngg ooff aa LLIISSTT OOFF HHAASSHHEESS

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

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

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

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

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

HHAASSHHEESS OOFF HHAASSHHEESS
       DDeeccllaarraattiioonn ooff aa HHAASSHH OOFF HHAASSHHEESS

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

       GGeenneerraattiioonn ooff aa HHAASSHH OOFF HHAASSHHEESS

        # 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};
        }

       AAcccceessss aanndd PPrriinnttiinngg ooff aa HHAASSHH OOFF HHAASSHHEESS

        # 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";
        }

MMOORREE EELLAABBOORRAATTEE RREECCOORRDDSS
       DDeeccllaarraattiioonn ooff MMOORREE EELLAABBOORRAATTEE RREECCOORRDDSS

       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");

       DDeeccllaarraattiioonn ooff aa HHAASSHH OOFF CCOOMMPPLLEEXX RREECCOORRDDSS

            %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, },
                   ],
                },
             );

       GGeenneerraattiioonn ooff aa HHAASSHH OOFF CCOOMMPPLLEEXX RREECCOORRDDSS

            # 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 examples
            # if you wanted a {kids} field that was an array reference
            # to a list 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 list, but the list 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";
            }

DDaattaabbaassee TTiieess
       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 the perlmodlib manpage for source
       code to MLDBM.

SSEEEE AALLSSOO
       perlref(1), perllol(1), perldata(1), perlobj(1)

AAUUTTHHOORR
       Tom Christiansen <tchrist@perl.com>

       Last update: Wed Oct 23 04:57:50 MET DST 1996

27/Mar/1999            perl 5.005, patch 03                     1