PERLDSC(1) Perl Programmers Reference Guide PERLDSC(1)NAMEperldsc - Perl Data Structures Cookbook
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,$b}" nota
tion borrowed from awk in which the keys are actually more
like a single concatenated string ""$AoA$b"", but traver
sal and sorting were difficult. More desperate program
mers 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 a array with three dimensions!
for $x (1 .. 10) {
for $y (1 .. 10) {
for $z (1 .. 10) {
$AoA[$x][$y][$z] =
$x ** $y + $z;
}
}
}
Alas, however simple this may appear, underneath it's a
much more elaborate construct than meets the eye!
How do you print it out? Why can't you say just "print
@AoA"? How do you sort it? How can you pass it to a
function or get one of these back from a function? Is is
an object? Can you save it to disk to read back later?
How do you access whole rows or columns of that matrix?
Do all the values have to be numeric?
As you see, it's quite easy to become confused. While
some small portion of the blame for this can be attributed
to the reference-based implementation, it's really more
due to a lack of existing documentation with examples
designed for the beginner.
This document is meant to be a detailed but understandable
treatment of the many different sorts of data structures
you might want to develop. It should also serve as a
cookbook of examples. That way, when you need to create
one of these complex data structures, you can just pinch,
pilfer, or purloin a drop-in example from here.
Let's look at each of these possible constructs in detail.
There are separate sections on each of the following:
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 "@ARRAY"s and "%HASH"es 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 a array or hash in quite the
same way that you would a real array or hash. For C or
C++ programmers unused to distinguishing between arrays
and pointers to the same, this can be confusing. If so,
just think of it as the difference between a structure and
a pointer to a structure.
You can (and should) read more about references in the
perlref(1) man page. Briefly, references are rather like
pointers that know what they point to. (Objects are also
a kind of reference, but we won't be needing them right
away--if ever.) This means that when you have something
which looks to you like an access to a two-or-more-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
thing like an array of arrays is either accidentally
counting the number of elements or else taking a reference
to the same memory location repeatedly. Here's the case
where you just get the count instead of a nested array:
for $i (1..10) {
@array = somefunc($i);
$AoA[$i] = @array; # WRONG!
}
That's just the simple case of assigning an array to a
scalar and getting its element count. If that's what you
really and truly want, then you might do well to consider
being a tad more explicit about it, like this:
for $i (1..10) {
@array = somefunc($i);
$counts[$i] = scalar @array;
}
Here's the case of taking a reference to the same memory
location again and again:
for $i (1..10) {
@array = somefunc($i);
$AoA[$i] = \@array; # WRONG!
}
So, what's the big problem with that? It looks right,
doesn't it? After all, I just told you that you need an
array of references, so by golly, you've made me one!
Unfortunately, while this is true, it's still broken. All
the references in @AoA refer to the very same place, and
they will therefore all hold whatever was last in @array!
It's similar to the problem demonstrated in the following
C program:
#include <pwd.h>
main() {
struct passwd *getpwnam(), *rp, *dp;
rp = getpwnam("root");
dp = getpwnam("daemon");
printf("daemon name is %s\nroot name is %s\n",
dp->pw_name, rp->pw_name);
}
Which will print
daemon name is daemon
root name is daemon
The problem is that both "rp" and "dp" are pointers to the
same location in memory! In C, you'd have to remember to
malloc() yourself some new memory. In Perl, you'll want
to use the array constructor "[]" or the hash constructor
"{}" instead. Here's the right way to do the preceding
broken code fragments:
for $i (1..10) {
@array = somefunc($i);
$AoA[$i] = [ @array ];
}
The square brackets make a reference to a new array with a
copy of what's in @array at the time of the assignment.
This is what you want.
Note that this will produce something similar, but it's
much harder to read:
for $i (1..10) {
@array = 0 .. $i;
@{$AoA[$i]} = @array;
}
Is it the same? Well, maybe so--and maybe not. The sub
tle difference is that when you assign something in square
brackets, you know for sure it's always a brand new refer
ence with a new copy of the data. Something else could be
going on in this new case with the "@{$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.
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
$$aref[2][2] # confusing
That's because Perl's precedence rules on its five prefix
dereferencers (which look like someone swearing: "$ @ * %
&") make them bind more tightly than the postfix sub
scripting brackets or braces! This will no doubt come as
a great shock to the C or C++ programmer, who is quite
accustomed to using "*a[i]" to mean what's pointed to by
the i'th element of "a". That is, they first take the
subscript, and only then dereference the thing at that
subscript. That's fine in C, but this isn't C.
The seemingly equivalent construct in Perl, "$$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 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
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 illustrat
ing access of various types of data structures.
ARRAYS OF ARRAYS
Declaration of a ARRAY OF ARRAYS
@AoA = (
[ "fred", "barney" ],
[ "george", "jane", "elroy" ],
[ "homer", "marge", "bart" ],
);
Generation of a ARRAY OF ARRAYS
# reading from file
while ( <> ) {
push @AoA, [ split ];
}
# calling a function
for $i ( 1 .. 10 ) {
$AoA[$i] = [ somefunc($i) ];
}
# using temp vars
for $i ( 1 .. 10 ) {
@tmp = somefunc($i);
$AoA[$i] = [ @tmp ];
}
# add to an existing row
push @{ $AoA[0] }, "wilma", "betty";
Access and Printing of a ARRAY OF ARRAYS
# one element
$AoA[0][0] = "Fred";
# another element
$AoA[1][1] =~ s/(\w)/\u$1/;
# print the whole thing with refs
for $aref ( @AoA ) {
print "\t [ @$aref ],\n";
}
# print the whole thing with indices
for $i ( 0 .. $#AoA ) {
print "\t [ @{$AoA[$i]} ],\n";
}
# print the whole thing one at a time
for $i ( 0 .. $#AoA ) {
for $j ( 0 .. $#{ $AoA[$i] } ) {
print "elt $i $j is $AoA[$i][$j]\n";
}
}
HASHES OF ARRAYS
Declaration of a HASH OF ARRAYS
%HoA = (
flintstones => [ "fred", "barney" ],
jetsons => [ "george", "jane", "elroy" ],
simpsons => [ "homer", "marge", "bart" ],
);
Generation of a HASH OF ARRAYS
# reading from file
# flintstones: fred barney wilma dino
while ( <> ) {
next unless s/^(.*?):\s*//;
$HoA{$1} = [ split ];
}
# reading from file; more temps
# flintstones: fred barney wilma dino
while ( $line = <> ) {
($who, $rest) = split /:\s*/, $line, 2;
@fields = split ' ', $rest;
$HoA{$who} = [ @fields ];
}
# calling a function that returns a list
for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoA{$group} = [ get_family($group) ];
}
# likewise, but using temps
for $group ( "simpsons", "jetsons", "flintstones" ) {
@members = get_family($group);
$HoA{$group} = [ @members ];
}
# append new members to an existing family
push @{ $HoA{"flintstones"} }, "wilma", "betty";
Access and Printing of a HASH OF ARRAYS
# one element
$HoA{flintstones}[0] = "Fred";
# another element
$HoA{simpsons}[1] =~ s/(\w)/\u$1/;
# print the whole thing
foreach $family ( keys %HoA ) {
print "$family: @{ $HoA{$family} }\n"
}
# print the whole thing with indices
foreach $family ( keys %HoA ) {
print "family: ";
foreach $i ( 0 .. $#{ $HoA{$family} } ) {
print " $i = $HoA{$family}[$i]";
}
print "\n";
}
# print the whole thing sorted by number of members
foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
print "$family: @{ $HoA{$family} }\n"
}
# print the whole thing sorted by number of members and name
foreach $family ( sort {
@{$HoA{$b}} <=> @{$HoA{$a}}
||
$a cmp $b
} keys %HoA )
{
print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n";
}
ARRAYS OF HASHES
Declaration of a ARRAY OF HASHES
@AoH = (
{
Lead => "fred",
Friend => "barney",
},
{
Lead => "george",
Wife => "jane",
Son => "elroy",
},
{
Lead => "homer",
Wife => "marge",
Son => "bart",
}
);
Generation of a ARRAY OF HASHES
# reading from file
# format: LEAD=fred FRIEND=barney
while ( <> ) {
$rec = {};
for $field ( split ) {
($key, $value) = split /=/, $field;
$rec->{$key} = $value;
}
push @AoH, $rec;
}
# reading from file
# format: LEAD=fred FRIEND=barney
# no temp
while ( <> ) {
push @AoH, { split /[\s+=]/ };
}
# calling a function that returns a key/value pair list, like
# "lead","fred","daughter","pebbles"
while ( %fields = getnextpairset() ) {
push @AoH, { %fields };
}
# likewise, but using no temp vars
while (<>) {
push @AoH, { parsepairs($_) };
}
# add key/value to an element
$AoH[0]{pet} = "dino";
$AoH[2]{pet} = "santa's little helper";
Access and Printing of a ARRAY OF HASHES
# one element
$AoH[0]{lead} = "fred";
# another element
$AoH[1]{lead} =~ s/(\w)/\u$1/;
# print the whole thing with refs
for $href ( @AoH ) {
print "{ ";
for $role ( keys %$href ) {
print "$role=$href->{$role} ";
}
print "}\n";
}
# print the whole thing with indices
for $i ( 0 .. $#AoH ) {
print "$i is { ";
for $role ( keys %{ $AoH[$i] } ) {
print "$role=$AoH[$i]{$role} ";
}
print "}\n";
}
# print the whole thing one at a time
for $i ( 0 .. $#AoH ) {
for $role ( keys %{ $AoH[$i] } ) {
print "elt $i $role is $AoH[$i]{$role}\n";
}
}
HASHES OF HASHES
Declaration of a HASH OF HASHES
%HoH = (
flintstones => {
lead => "fred",
pal => "barney",
},
jetsons => {
lead => "george",
wife => "jane",
"his boy" => "elroy",
},
simpsons => {
lead => "homer",
wife => "marge",
kid => "bart",
},
);
Generation of a HASH OF HASHES
# reading from file
# flintstones: lead=fred pal=barney wife=wilma pet=dino
while ( <> ) {
next unless s/^(.*?):\s*//;
$who = $1;
for $field ( split ) {
($key, $value) = split /=/, $field;
$HoH{$who}{$key} = $value;
}
# reading from file; more temps
while ( <> ) {
next unless s/^(.*?):\s*//;
$who = $1;
$rec = {};
$HoH{$who} = $rec;
for $field ( split ) {
($key, $value) = split /=/, $field;
$rec->{$key} = $value;
}
}
# calling a function that returns a key,value hash
for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoH{$group} = { get_family($group) };
}
# likewise, but using temps
for $group ( "simpsons", "jetsons", "flintstones" ) {
%members = get_family($group);
$HoH{$group} = { %members };
}
# append new members to an existing family
%new_folks = (
wife => "wilma",
pet => "dino",
);
for $what (keys %new_folks) {
$HoH{flintstones}{$what} = $new_folks{$what};
}
Access and Printing of a HASH OF HASHES
# one element
$HoH{flintstones}{wife} = "wilma";
# another element
$HoH{simpsons}{lead} =~ s/(\w)/\u$1/;
# print the whole thing
foreach $family ( keys %HoH ) {
print "$family: { ";
for $role ( keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# print the whole thing somewhat sorted
foreach $family ( sort keys %HoH ) {
print "$family: { ";
for $role ( sort keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# print the whole thing sorted by number of members
foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) {
print "$family: { ";
for $role ( sort keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# establish a sort order (rank) for each role
$i = 0;
for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }
# now print the whole thing sorted by number of members
foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) {
print "$family: { ";
# and print these according to rank order
for $role ( sort { $rank{$a} <=> $rank{$b} } keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
MORE ELABORATE RECORDS
Declaration of MORE ELABORATE RECORDS
Here's a sample showing how to create and use a record
whose fields are of many different sorts:
$rec = {
TEXT => $string,
SEQUENCE => [ @old_values ],
LOOKUP => { %some_table },
THATCODE => \&some_function,
THISCODE => sub { $_[0] ** $_[1] },
HANDLE => \*STDOUT,
};
print $rec->{TEXT};
print $rec->{SEQUENCE}[0];
$last = pop @ { $rec->{SEQUENCE} };
print $rec->{LOOKUP}{"key"};
($first_k, $first_v) = each %{ $rec->{LOOKUP} };
$answer = $rec->{THATCODE}->($arg);
$answer = $rec->{THISCODE}->($arg1, $arg2);
# careful of extra block braces on fh ref
print { $rec->{HANDLE} } "a string\n";
use FileHandle;
$rec->{HANDLE}->autoflush(1);
$rec->{HANDLE}->print(" a string\n");
Declaration of a HASH OF COMPLEX RECORDS
%TV = (
flintstones => {
series => "flintstones",
nights => [ qw(monday thursday friday) ],
members => [
{ name => "fred", role => "lead", age => 36, },
{ name => "wilma", role => "wife", age => 31, },
{ name => "pebbles", role => "kid", age => 4, },
],
},
jetsons => {
series => "jetsons",
nights => [ qw(wednesday saturday) ],
members => [
{ name => "george", role => "lead", age => 41, },
{ name => "jane", role => "wife", age => 39, },
{ name => "elroy", role => "kid", age => 9, },
],
},
simpsons => {
series => "simpsons",
nights => [ qw(monday) ],
members => [
{ name => "homer", role => "lead", age => 34, },
{ name => "marge", role => "wife", age => 37, },
{ name => "bart", role => "kid", age => 11, },
],
},
);
Generation of a HASH OF COMPLEX RECORDS
# reading from file
# this is most easily done by having the file itself be
# in the raw data format as shown above. perl is happy
# to parse complex data structures if declared as data, so
# sometimes it's easiest to do that
# here's a piece by piece build up
$rec = {};
$rec->{series} = "flintstones";
$rec->{nights} = [ find_days() ];
@members = ();
# assume this file in field=value syntax
while (<>) {
%fields = split /[\s=]+/;
push @members, { %fields };
}
$rec->{members} = [ @members ];
# now remember the whole thing
$TV{ $rec->{series} } = $rec;
###########################################################
# now, you might want to make interesting extra fields that
# include pointers back into the same data structure so if
# change one piece, it changes everywhere, like for example
# if you wanted a {kids} field that was a reference
# to an array of the kids' records without having duplicate
# records and thus update problems.
###########################################################
foreach $family (keys %TV) {
$rec = $TV{$family}; # temp pointer
@kids = ();
for $person ( @{ $rec->{members} } ) {
if ($person->{role} =~ /kid|son|daughter/) {
push @kids, $person;
}
}
# REMEMBER: $rec and $TV{$family} point to same data!!
$rec->{kids} = [ @kids ];
}
# you copied the array, but the array itself contains pointers
# to uncopied objects. this means that if you make bart get
# older via
$TV{simpsons}{kids}[0]{age}++;
# then this would also change in
print $TV{simpsons}{members}[2]{age};
# because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
# both point to the same underlying anonymous hash table
# print the whole thing
foreach $family ( keys %TV ) {
print "the $family";
print " is on during @{ $TV{$family}{nights} }\n";
print "its members are:\n";
for $who ( @{ $TV{$family}{members} } ) {
print " $who->{name} ($who->{role}), age $who->{age}\n";
}
print "it turns out that $TV{$family}{lead} has ";
print scalar ( @{ $TV{$family}{kids} } ), " kids named ";
print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } );
print "\n";
}
Database Ties
You cannot easily tie a multilevel data structure (such as
a hash of hashes) to a dbm file. The first problem is
that all but GDBM and Berkeley DB have size limitations,
but beyond that, you also have problems with how refer
ences are to be represented on disk. One experimental
module that does partially attempt to address this need is
the MLDBM module. Check your nearest CPAN site as
described in the perlmodlib manpage for source code to
MLDBM.
SEE ALSOperlref(1), perllol(1), perldata(1), perlobj(1)AUTHOR
Tom Christiansen <tchrist@perl.com>
Last update: Wed Oct 23 04:57:50 MET DST 1996
2001-04-07 perl v5.6.1 PERLDSC(1)
