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perldsc



DESCRIPTION

       The single feature most sorely lacking in the Perl pro­
       gramming 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 struc­
       tures.  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 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:
       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 refer­
       ences 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-dimen­
       sional array and/or hash, what's really going on is that
       the base type is merely a one-dimensional entity that con­
       tains 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 refer­
       ence 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 some­
       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 ];
           }

       tle difference is that when you assign something in square
       brackets, you know for sure it's always a brand new refer­
       ence with a new copy of the data.  Something else could be
       going on in this new case with the "@{$AoA[$i]}}" derefer­
       ence 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 construc­
       tors 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 effi­
       cient 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 under­
       stood constructors "[]" and "{}" instead of relying upon
       lexical (or dynamic) scoping and hidden reference-counting
       to do the right thing behind the scenes.
       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 sub­
       scripting 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 sub­
       script, and only then dereference the thing at that sub­
       script.  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 derefer­
       encing".  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
                 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 illustrat­
       ing 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";

        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
        }

        # 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
        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

                       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) {
            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->{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() ];

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

perl v5.8.1                 2003-09-02                 PERLDSC(1)

An undefined database error occurred. SELECT distinct pages.pagepath,pages.pageid FROM pages, page2command WHERE pages.pageid = page2command.pageid AND commandid =


  




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