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,$b} notation borrowed from awk in which the keys are actually more like a sin-gle single
gle concatenated string "$AoA$b", but traversal and sorting were difficult. More desperate program-mers programmers
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 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 docu-mentation documentation
mentation 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 multidimen-sional multidimensional
sional arrays--is that even though they might appear otherwise, Perl @ARRAYs and %HASHes are all
internally one-dimensional. They can hold only scalar values (meaning a string, number, or a refer-ence). reference).
ence). 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 $AoA[1][2];
7
print @AoA;
ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)
That's because Perl doesn't (ever) implicitly dereference your variables. If you want to get at the
thing a reference is referring to, then you have to do this yourself using either prefix typing indi-cators, indicators,
cators, like "${$blah}", "@{$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 acci-dentally accidentally
dentally 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 prob-lem problem
lem 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[$i]} = @array;
}
Is it the same? Well, maybe so--and maybe not. The subtle difference is that when you assign some-thing something
thing in square brackets, you know for sure it's always a brand new reference with a new copy of the
data. Something else could be going on in this new case with the "@{$AoA[$i]}}" dereference on the
left-hand-side of the assignment. It all depends on whether $AoA[$i] had been undefined to start
with, or whether it already contained a reference. If you had already populated @AoA with refer-ences, references,
ences, 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 produce more efficient code at the risk of misleading all but the most
experienced of programmers. So I usually advise against teaching it to beginners. In fact, except
for passing arguments to functions, I seldom like to see the gimme-a-reference operator (backslash)
used much at all in code. Instead, I advise beginners that they (and most of the rest of us) should
try to use the much more easily understood constructors "[]" and "{}" instead of relying upon lexical
(or dynamic) scoping and hidden reference-counting to do the right thing behind the scenes.
In summary:
$AoA[$i] = [ @array ]; # usually best
$AoA[$i] = \@array; # perilous; just how my() was that array?
@{ $AoA[$i] } = @array; # way too tricky for most programmers
CAVEAT ON PRECEDENCE
Speaking of things like "@{$AoA[$i]}", the following are actually the same thing:
$aref->[2][2] # clear
$$aref[2][2] # confusing
That's because Perl's precedence rules on its five prefix dereferencers (which look like someone
swearing: "$ @ * % &") make them bind more tightly than the postfix subscripting brackets or braces!
This will no doubt come as a great shock to the C or C++ programmer, who is quite accustomed to using
*a[i] to mean what's pointed to by the i'th element of "a". That is, they first take the subscript,
and only then dereference the thing at that subscript. That's fine in C, but this isn't C.
The seemingly equivalent construct in Perl, $$aref[$i] first does the deref of $aref, making it take
$aref as a reference to an array, and then dereference that, and finally tell you the i'th value of
the array pointed to by $AoA. If you wanted the C notion, you'd have to write "${$AoA[$i]}" to force
the $AoA[$i] to get evaluated first before the leading "$" dereferencer.
WHY YOU SHOULD ALWAYS "use strict"
If this is starting to sound scarier than it's worth, relax. Perl has some features to help you
avoid its most common pitfalls. The best way to avoid getting confused is to start every program
like this:
#!/usr/bin/perl -w
use strict;
This way, you'll be forced to declare all your variables with my() and also disallow accidental "sym-bolic "symbolic
bolic 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 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 exam-ples examples
ples illustrating access of various types of data structures.
ARRAYS OF ARRAYS
Declaration of an ARRAY OF ARRAYS
@AoA = (
[ "fred", "barney" ],
[ "george", "jane", "elroy" ],
[ "homer", "marge", "bart" ],
);
Generation of an ARRAY OF ARRAYS
# reading from file
while ( <> ) {
push @AoA, [ split ];
}
# calling a function
for $i ( 1 .. 10 ) {
$AoA[$i] = [ somefunc($i) ];
}
# using temp vars
for $i ( 1 .. 10 ) {
@tmp = somefunc($i);
$AoA[$i] = [ @tmp ];
}
# add to an existing row
push @{ $AoA[0] }, "wilma", "betty";
Access and Printing of an ARRAY OF ARRAYS
# one element
$AoA[0][0] = "Fred";
# another element
$AoA[1][1] =~ s/(\w)/\u$1/;
# print the whole thing with refs
for $aref ( @AoA ) {
print "\t [ @$aref ],\n";
}
# print the whole thing with indices
for $i ( 0 .. $#AoA ) {
print "\t [ @{$AoA[$i]} ],\n";
}
# print the whole thing one at a time
for $i ( 0 .. $#AoA ) {
for $j ( 0 .. $#{ $AoA[$i] } ) {
print "elt $i $j is $AoA[$i][$j]\n";
}
}
HASHES OF ARRAYS
Declaration of a HASH OF ARRAYS
%HoA = (
flintstones => [ "fred", "barney" ],
jetsons => [ "george", "jane", "elroy" ],
simpsons => [ "homer", "marge", "bart" ],
);
Generation of a HASH OF ARRAYS
# reading from file
# flintstones: fred barney wilma dino
while ( <> ) {
next unless s/^(.*?):\s*//;
$HoA{$1} = [ split ];
}
# reading from file; more temps
# flintstones: fred barney wilma dino
while ( $line = <> ) {
($who, $rest) = split /:\s*/, $line, 2;
@fields = split ' ', $rest;
$HoA{$who} = [ @fields ];
}
# calling a function that returns a list
for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoA{$group} = [ get_family($group) ];
}
# likewise, but using temps
for $group ( "simpsons", "jetsons", "flintstones" ) {
@members = get_family($group);
$HoA{$group} = [ @members ];
}
# append new members to an existing family
push @{ $HoA{"flintstones"} }, "wilma", "betty";
Access and Printing of a HASH OF ARRAYS
# one element
$HoA{flintstones}[0] = "Fred";
# another element
$HoA{simpsons}[1] =~ s/(\w)/\u$1/;
# print the whole thing
foreach $family ( keys %HoA ) {
print "$family: @{ $HoA{$family} }\n"
}
# print the whole thing with indices
foreach $family ( keys %HoA ) {
print "family: ";
foreach $i ( 0 .. $#{ $HoA{$family} } ) {
print " $i = $HoA{$family}[$i]";
}
print "\n";
}
# print the whole thing sorted by number of members
foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
print "$family: @{ $HoA{$family} }\n"
}
# print the whole thing sorted by number of members and name
foreach $family ( sort {
@{$HoA{$b}} <=> @{$HoA{$a}}
||
$a cmp $b
} keys %HoA )
{
print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n";
}
ARRAYS OF HASHES
Declaration of an ARRAY OF HASHES
@AoH = (
{
Lead => "fred",
Friend => "barney",
},
{
Lead => "george",
Wife => "jane",
Son => "elroy",
},
{
Lead => "homer",
Wife => "marge",
Son => "bart",
}
);
Generation of an ARRAY OF HASHES
# reading from file
# format: LEAD=fred FRIEND=barney
while ( <> ) {
$rec = {};
for $field ( split ) {
($key, $value) = split /=/, $field;
$rec->{$key} = $value;
}
push @AoH, $rec;
}
# reading from file
# format: LEAD=fred FRIEND=barney
# no temp
while ( <> ) {
push @AoH, { split /[\s+=]/ };
}
# calling a function that returns a key/value pair list, like
# "lead","fred","daughter","pebbles"
while ( %fields = getnextpairset() ) {
push @AoH, { %fields };
}
# likewise, but using no temp vars
while (<>) {
push @AoH, { parsepairs($_) };
}
# add key/value to an element
$AoH[0]{pet} = "dino";
$AoH[2]{pet} = "santa's little helper";
Access and Printing of an ARRAY OF HASHES
# one element
$AoH[0]{lead} = "fred";
# another element
$AoH[1]{lead} =~ s/(\w)/\u$1/;
# print the whole thing with refs
for $href ( @AoH ) {
print "{ ";
for $role ( keys %$href ) {
print "$role=$href->{$role} ";
}
print "}\n";
}
# print the whole thing with indices
for $i ( 0 .. $#AoH ) {
print "$i is { ";
for $role ( keys %{ $AoH[$i] } ) {
print "$role=$AoH[$i]{$role} ";
}
print "}\n";
}
# print the whole thing one at a time
for $i ( 0 .. $#AoH ) {
for $role ( keys %{ $AoH[$i] } ) {
print "elt $i $role is $AoH[$i]{$role}\n";
}
}
HASHES OF HASHES
Declaration of a HASH OF HASHES
%HoH = (
flintstones => {
lead => "fred",
pal => "barney",
},
jetsons => {
lead => "george",
wife => "jane",
"his boy" => "elroy",
},
simpsons => {
lead => "homer",
wife => "marge",
kid => "bart",
},
);
Generation of a HASH OF HASHES
# reading from file
# flintstones: lead=fred pal=barney wife=wilma pet=dino
while ( <> ) {
next unless s/^(.*?):\s*//;
$who = $1;
for $field ( split ) {
($key, $value) = split /=/, $field;
$HoH{$who}{$key} = $value;
}
# reading from file; more temps
while ( <> ) {
next unless s/^(.*?):\s*//;
$who = $1;
$rec = {};
$HoH{$who} = $rec;
for $field ( split ) {
($key, $value) = split /=/, $field;
$rec->{$key} = $value;
}
}
# calling a function that returns a key,value hash
for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoH{$group} = { get_family($group) };
}
# likewise, but using temps
for $group ( "simpsons", "jetsons", "flintstones" ) {
%members = get_family($group);
$HoH{$group} = { %members };
}
# append new members to an existing family
%new_folks = (
wife => "wilma",
pet => "dino",
);
for $what (keys %new_folks) {
$HoH{flintstones}{$what} = $new_folks{$what};
}
Access and Printing of a HASH OF HASHES
# one element
$HoH{flintstones}{wife} = "wilma";
# another element
$HoH{simpsons}{lead} =~ s/(\w)/\u$1/;
# print the whole thing
foreach $family ( keys %HoH ) {
print "$family: { ";
for $role ( keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# print the whole thing somewhat sorted
foreach $family ( sort keys %HoH ) {
print "$family: { ";
for $role ( sort keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# print the whole thing sorted by number of members
foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) {
print "$family: { ";
for $role ( sort keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# establish a sort order (rank) for each role
$i = 0;
for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }
# now print the whole thing sorted by number of members
foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) {
print "$family: { ";
# and print these according to rank order
for $role ( sort { $rank{$a} <=> $rank{$b} } keys %{ $HoH{$family} } ) {
print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
MORE ELABORATE RECORDS
Declaration of MORE ELABORATE RECORDS
Here's a sample showing how to create and use a record whose fields are of many different sorts:
$rec = {
TEXT => $string,
SEQUENCE => [ @old_values ],
LOOKUP => { %some_table },
THATCODE => \&some_function,
THISCODE => sub { $_[0] ** $_[1] },
HANDLE => \*STDOUT,
};
print $rec->{TEXT};
print $rec->{SEQUENCE}[0];
$last = pop @ { $rec->{SEQUENCE} };
print $rec->{LOOKUP}{"key"};
($first_k, $first_v) = each %{ $rec->{LOOKUP} };
$answer = $rec->{THATCODE}->($arg);
$answer = $rec->{THISCODE}->($arg1, $arg2);
# careful of extra block braces on fh ref
print { $rec->{HANDLE} } "a string\n";
use FileHandle;
$rec->{HANDLE}->autoflush(1);
$rec->{HANDLE}->print(" a string\n");
Declaration of a HASH OF COMPLEX RECORDS
%TV = (
flintstones => {
series => "flintstones",
nights => [ qw(monday thursday friday) ],
members => [
{ name => "fred", role => "lead", age => 36, },
{ name => "wilma", role => "wife", age => 31, },
{ name => "pebbles", role => "kid", age => 4, },
],
},
jetsons => {
series => "jetsons",
nights => [ qw(wednesday saturday) ],
members => [
{ name => "george", role => "lead", age => 41, },
{ name => "jane", role => "wife", age => 39, },
{ name => "elroy", role => "kid", age => 9, },
],
},
simpsons => {
series => "simpsons",
nights => [ qw(monday) ],
members => [
{ name => "homer", role => "lead", age => 34, },
{ name => "marge", role => "wife", age => 37, },
{ name => "bart", role => "kid", age => 11, },
],
},
);
Generation of a HASH OF COMPLEX RECORDS
# reading from file
# this is most easily done by having the file itself be
# in the raw data format as shown above. perl is happy
# to parse complex data structures if declared as data, so
# sometimes it's easiest to do that
# here's a piece by piece build up
$rec = {};
$rec->{series} = "flintstones";
$rec->{nights} = [ find_days() ];
@members = ();
# assume this file in field=value syntax
while (<>) {
%fields = split /[\s=]+/;
push @members, { %fields };
}
$rec->{members} = [ @members ];
# now remember the whole thing
$TV{ $rec->{series} } = $rec;
###########################################################
# now, you might want to make interesting extra fields that
# include pointers back into the same data structure so if
# change one piece, it changes everywhere, like for example
# if you wanted a {kids} field that was a reference
# to an array of the kids' records without having duplicate
# records and thus update problems.
###########################################################
foreach $family (keys %TV) {
$rec = $TV{$family}; # temp pointer
@kids = ();
for $person ( @{ $rec->{members} } ) {
if ($person->{role} =~ /kid|son|daughter/) {
push @kids, $person;
}
}
# REMEMBER: $rec and $TV{$family} point to same data!!
$rec->{kids} = [ @kids ];
}
# you copied the array, but the array itself contains pointers
# to uncopied objects. this means that if you make bart get
# older via
$TV{simpsons}{kids}[0]{age}++;
# then this would also change in
print $TV{simpsons}{members}[2]{age};
# because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
# both point to the same underlying anonymous hash table
# print the whole thing
foreach $family ( keys %TV ) {
print "the $family";
print " is on during @{ $TV{$family}{nights} }\n";
print "its members are:\n";
for $who ( @{ $TV{$family}{members} } ) {
print " $who->{name} ($who->{role}), age $who->{age}\n";
}
print "it turns out that $TV{$family}{lead} has ";
print scalar ( @{ $TV{$family}{kids} } ), " kids named ";
print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } );
print "\n";
}
Database Ties
You cannot easily tie a multilevel data structure (such as a hash of hashes) to a dbm file. The
first problem is that all but GDBM and Berkeley DB have size limitations, but beyond that, you also
have problems with how references are to be represented on disk. One experimental module that does
partially attempt to address this need is the MLDBM module. Check your nearest CPAN site as
described in perlmodlib for source code to MLDBM.
SEE ALSO
perlref(1), perllol(1), perldata(1), perlobj(1)
AUTHOR
Tom Christiansen <tchrist@perl.com>
Last update: Wed Oct 23 04:57:50 MET DST 1996
perl v5.8.8 2006-01-07 PERLDSC(1)
|