perldsc(1) - Perl Data Structures Cookbook



  • PERLDSC(1)	       Perl Programmers Reference Guide 	   PERLDSC(1)
    
    NAME
           perldsc - Perl Data Structures Cookbook
    
    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 "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:
    
           ·    arrays of arrays
    
           ·    hashes of arrays
    
           ·    arrays of hashes
    
           ·    hashes of hashes
    
           ·    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 @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 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 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 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 "${$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 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 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:
    
    	   # 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 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 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 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 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
           You can use the debugger's "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 an ARRAY OF ARRAYS
    	@AoA = (
    	       [ "fred", "barney" ],
    	       [ "george", "jane", "elroy" ],
    	       [ "homer", "marge", "bart" ],
    	     );
    
       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";
    
       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";
    	    }
    	}
    
    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 an ARRAY OF HASHES
    	@AoH = (
    	       {
    		   Lead     => "fred",
    		   Friend   => "barney",
    	       },
    	       {
    		   Lead     => "george",
    		   Wife     => "jane",
    		   Son	    => "elroy",
    	       },
    	       {
    		   Lead     => "homer",
    		   Wife     => "marge",
    		   Son	    => "bart",
    	       }
    	 );
    
       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";
    
       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";
    	    }
    	}
    
    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
           perlmodlib for source code to MLDBM.
    
    SEE ALSO
           perlref, perllol, perldata, perlobj
    
    AUTHOR
           Tom Christiansen <[email protected]>
    
    perl v5.22.1			  2018-06-12			   PERLDSC(1)
    

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