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+=head1 NAME
+X<data structure> X<complex data structure> X<struct>
+
+perldsc - Perl Data Structures Cookbook
+
+=head1 DESCRIPTION
+
+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<$m{$AoA,$b}> notation borrowed from B<awk> in which the
+keys are actually more like a single concatenated string C<"$AoA$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 an array
+with three dimensions!
+
+ for $x (1 .. 10) {
+ for $y (1 .. 10) {
+ for $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:
+
+ @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 $i (1..10) {
+ @array = somefunc($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 $i (1..10) {
+ @array = somefunc($i);
+ $counts[$i] = scalar @array;
+ }
+
+Here's the case of taking a reference to the same memory location
+again and again:
+
+ for $i (1..10) {
+ @array = somefunc($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<{}>
+
+ for $i (1..10) {
+ @array = somefunc($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:
+
+ for $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 $i (1..10) {
+ my @array = somefunc($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>
+
+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 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>>
+
+Last update:
+Wed Oct 23 04:57:50 MET DST 1996