-=head1 TITLE
+=head1 NAME
-perldsc - Manipulating Complex Data Structures in Perl
+perldsc - Perl Data Structures Cookbook
-=head1 INTRODUCTION
+=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<$m{$
LoL,$b}> notation borrowed from I<awk> in which the
-keys are actually more like a single concatenated string C<"$LoL$b">, but
+get away with the C<$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.
for $x (1 .. 10) {
for $y (1 .. 10) {
for $z (1 .. 10) {
- $LoL[$x][$y][$z] =
+ $AoA[$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 just say C<print @LoL>? How do
+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 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?
+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.
+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
-documents on each of the following:
+sections on each of the following:
=over 5
=item * more elaborate constructs
-=item * recursive and self-referential data structures
-
-=item * objects
-
=back
-But for now, let's look at some of the general issues common to all
-of these types of data structures.
+But for now, let's look at general issues common to all
+these types of data structures.
=head1 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 C<@ARRAY>s and C<%HASH>es are all internally
-one-dimensional. They can only hold scalar values (meaning a string,
+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.
-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'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.) That means that when you have something that
-looks to you like an access to two-or-more-dimensional array and/or hash,
-that what's really going on is that in all these cases, the base type is
+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.
- $list[7][12] # array of arrays
- $list[7]{string} # array of hashes
+ $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 only contains references, if you try to print
+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];
+ @AoA = ( [2, 3], [4, 5, 7], [0] );
+ print $AoA[1][2];
7
- print @LoL;
+ print @AoA;
ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)
of a nested array:
for $i (1..10) {
- @list = somefunc($i);
- $LoL[$i] = @list; # WRONG!
- }
+ @array = somefunc($i);
+ $AoA[$i] = @array; # WRONG!
+ }
-That's just the simple case of assigning a list to a scalar and getting
+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 $i (1..10) {
- @list = somefunc($i);
- $counts[$i] = scalar @list;
- }
+ @array = somefunc($i);
+ $counts[$i] = scalar @array;
+ }
Here's the case of taking a reference to the same memory location
again and again:
for $i (1..10) {
- @list = somefunc($i);
- $LoL[$i] = \@list; # WRONG!
- }
+ @array = somefunc($i);
+ $AoA[$i] = \@array; # WRONG!
+ }
-So, just what's the big problem with that? It looks right, doesn't it?
+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 I<very same place>, and they will therefore all hold
-whatever was last in @list! It's similar to the problem demonstrated in
+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>
rp = getpwnam("root");
dp = getpwnam("daemon");
- printf("daemon name is %s\nroot name is %s\n",
+ 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
+ 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
+broken code fragments:
for $i (1..10) {
- @list = somefunc($i);
- $LoL[$i] = [ @list ];
- }
+ @array = somefunc($i);
+ $AoA[$i] = [ @array ];
+ }
The square brackets make a reference to a new array with a I<copy>
-of what's in @list at the time of the assignment. This is what
-you want.
+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) {
- @list = 0 .. $i;
- @{$LoL[$i]} = @list;
- }
+ @array = 0 .. $i;
+ @{$AoA[$i]} = @array;
+ }
Is it the same? Well, maybe so--and maybe not. The subtle difference
is that when you assign something in square brackets, you know for sure
it's always a brand new reference with a new I<copy> of the data.
-Something else could be going on in this new case with the C<@{$LoL[$i]}}>
+Something else could be going on in this new case with the C<@{$AoA[$i]}}>
dereference on the left-hand-side of the assignment. It all depends on
-whether C<$LoL[$i]> had been undefined to start with, or whether it
-already contained a reference. If you had already populated @LoL with
+whether C<$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
- $LoL[3] = \@another_list;
+ $AoA[3] = \@another_array;
Then the assignment with the indirection on the left-hand-side would
use the existing reference that was already there:
- @{$LoL[3]} = @list;
+ @{$AoA[3]} = @array;
Of course, this I<would> have the "interesting" effect of clobbering
-@another_list. (Have you ever noticed how when a programmer says
+@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 to always use the array or hash constructors with C<[]>
+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.
+efficient.
Surprisingly, the following dangerous-looking construct will
actually work out fine:
for $i (1..10) {
- my @list = somefunc($i);
- $LoL[$i] = \@list;
- }
+ 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
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
+ $AoA[$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
+=head1 CAVEAT ON PRECEDENCE
-Speaking of things like C<@{$LoL[$i]}>, the following are actually the
+Speaking of things like C<@{$AoA[$i]}>, the following are actually the
same thing:
- $listref->[2][2] # clear
- $$listref[2][2] # confusing
+ $aref->[2][2] # clear
+ $$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
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<$$listref[$i]> first does
-the deref of C<$listref>, making it take $listref as a reference to an
+The seemingly equivalent construct in Perl, C<$$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 $LoL. If you wanted the C notion, you'd have to
-write C<${$LoL[$i]}> to force the C<$LoL[$i]> to get evaluated first
+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>
also disallow accidental "symbolic dereferencing". Therefore if you'd done
this:
- my $listref = [
+ my $aref = [
[ "fred", "barney", "pebbles", "bambam", "dino", ],
[ "homer", "bart", "marge", "maggie", ],
- [ "george", "jane", "alroy", "judy", ],
+ [ "george", "jane", "elroy", "judy", ],
];
- print $listref[2][2];
+ print $aref[2][2];
The compiler would immediately flag that as an error I<at compile time>,
-because you were accidentally accessing C<@listref>, an undeclared
-variable, and it would thereby remind you to instead write:
+because you were accidentally accessing C<@aref>, an undeclared
+variable, and it would thereby remind you to write instead:
- print $listref->[2][2]
+ print $aref->[2][2]
=head1 DEBUGGING
-The standard Perl debugger in 5.001 doesn't do a very nice job of
-printing out complex data structures. However, the perl5db that
-Ilya Zakharevich E<lt>F<ilya@math.ohio-state.edu>E<gt>
-wrote, which is accessible at
-
- ftp://ftp.perl.com/pub/perl/ext/perl5db-kit-0.9.tar.gz
+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:
-has 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 $LoL above, here's the debugger output:
-
- DB<1> X $LoL
- $LoL = ARRAY(0x13b5a0)
+ DB<1> x $AoA
+ $AoA = ARRAY(0x13b5a0)
0 ARRAY(0x1f0a24)
0 'fred'
1 'barney'
2 ARRAY(0x13b540)
0 'george'
1 'jane'
- 2 'alroy'
+ 2 'elroy'
3 'judy'
-There's also a lower-case B<x> command which is nearly the same.
+=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
+
+=head2 Declaration of a ARRAY OF ARRAYS
+
+ @AoA = (
+ [ "fred", "barney" ],
+ [ "george", "jane", "elroy" ],
+ [ "homer", "marge", "bart" ],
+ );
+
+=head2 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";
+
+=head2 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";
+ }
+ }
+
+=head1 HASHES OF ARRAYS
+
+=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
+
+=head2 Declaration of a ARRAY OF HASHES
+
+ @AoH = (
+ {
+ Lead => "fred",
+ Friend => "barney",
+ },
+ {
+ Lead => "george",
+ Wife => "jane",
+ Son => "elroy",
+ },
+ {
+ Lead => "homer",
+ Wife => "marge",
+ Son => "bart",
+ }
+ );
+
+=head2 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";
+
+=head2 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";
+ }
+ }
+
+=head1 HASHES OF HASHES
+
+=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
+
+=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
-perlref(1), perldata(1)
+perlref(1), perllol(1), perldata(1), perlobj(1)
=head1 AUTHOR
-Tom Christiansen E<lt>F<tchrist@perl.com>E<gt>
-
-Last update:
-Sat Oct 7 22:41:09 MDT 1995
+Tom Christiansen <F<tchrist@perl.com>>
+Last update:
+Wed Oct 23 04:57:50 MET DST 1996
https://perl5.git.perl.org / perl5.git / blobdiff