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



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 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 "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 struc-
       tures 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 follow-
       ing:

       * 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-dimen-
       sional.  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 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 remem-
       ber 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 assign-
       ment.  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 con-
       tained 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 pro-
       duce 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 struc-
       tures.  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 illus-
       trating 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(1), perllol(1), perldata(1), perlobj(1)

AUTHOR
       Tom Christiansen <tchristATperl.com>

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



perl v5.8.8                       2006-01-07                        PERLDSC(1)