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perldsc (1)
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    NAME

         perldsc - Perl Data Structures Cookbook
    
    
    

    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 `$m{$AoA,$b}' notation
         borrowed from awk in which the keys are actually more like a
         single concatenated string `"$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 a 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 `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 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.
    
    
    

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

    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 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 `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 $i (1..10) {
                 @array = somefunc($i);
                 $AoA[$i] = [ @array ];
             }
    
         The square brackets make a reference to a new array with a
         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 copy of the data.  Something else could
         be going on in this new case with the `@{$AoA[$i]}}'
         dereference on the left-hand-side of the assignment.  It all
         depends on whether `$AoA[$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 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 `[]' 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 $i (1..10) {
                 my @array = somefunc($i);
                 $AoA[$i] = \@array;
             }
    
         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:
    
             $AoA[$i] = [ @array ];      # usually best
             $AoA[$i] = \@array;         # perilous; just how my() was that array?
             @{ $AoA[$i] } = @array;     # way too tricky for most programmers
    
    
    

    CAVEAT ON PRECEDENCE

         Speaking of things like `@{$AoA[$i]}', the following are
         actually the same thing:
    
             $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: `$ @ * %
         &') 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, `$$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'th value of the array pointed to by
         $AoA. If you wanted the C notion, you'd have to write
         `${$AoA[$i]}' to force the `$AoA[$i]' to get evaluated first
         before the leading `$' dereferencer.
    
    
    

    WHY YOU SHOULD ALWAYS `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 at
         compile time, because you were accidentally accessing
         `@aref', an undeclared variable, and it would thereby remind
         you to write instead:
    
    
             print $aref->[2][2]
    
    
    
    

    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 `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'
    
    
    
    

    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.
    
    
    

    ARRAYS OF ARRAYS

         Declaration of a ARRAY OF ARRAYS
    
          @AoA = (
                 [ "fred", "barney" ],
                 [ "george", "jane", "elroy" ],
                 [ "homer", "marge", "bart" ],
               );
    
    
         Generation of a 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";
    
    
         Access and Printing of a 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";
              }
          }
    
    
    
    

    HASHES OF ARRAYS

         Declaration of a HASH OF ARRAYS
    
          %HoA = (
                 flintstones        => [ "fred", "barney" ],
                 jetsons            => [ "george", "jane", "elroy" ],
                 simpsons           => [ "homer", "marge", "bart" ],
               );
    
    
    
         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";
    
    
         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";
          }
    
    
    
    

    ARRAYS OF HASHES

         Declaration of a ARRAY OF HASHES
    
          @AoH = (
                 {
                     Lead     => "fred",
                     Friend   => "barney",
                 },
                 {
                     Lead     => "george",
                     Wife     => "jane",
                     Son      => "elroy",
                 },
                 {
                     Lead     => "homer",
                     Wife     => "marge",
                     Son      => "bart",
                 }
           );
    
    
         Generation of a 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";
    
    
         Access and Printing of a 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";
              }
          }
    
    
    
    

    HASHES OF HASHES

         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",
                 },
          );
    
    
         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};
          }
    
    
         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";
          }
    
    
    
    

    MORE ELABORATE RECORDS

         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");
    
    
         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, },
                     ],
                  },
               );
    
    
    
         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";
              }
    
    
    
    

    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 the
         perlmodlib manpage for source code to MLDBM.
    
    
    

    SEE ALSO

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

    AUTHOR

         Tom Christiansen <tchrist@perl.com>
    
         Last update:  Wed Oct 23 04:57:50 MET DST 1996
    
    
    
    


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