PERLDSC(1) Perl Programmers Reference Guide PERLDSC(1)NAMEperldsc - Perl Data Structures Cookbook
DESCRIPTION
The single feature most sorely lacking in the Perl program-
ming 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 occa-
sionally 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 func-
tion 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 pur-
loin a drop-in example from here.
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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 struc-
tures in Perl -- including multidimensional 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). 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() func-
tion, you'll get something that doesn't look very nice, like
this:
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@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:
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#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[$i]} = @array;
}
Is it the same? Well, maybe so--and maybe not. The subtle
difference is that when you assign something in square
brackets, you know for sure it's always a brand new refer-
ence with a new copy of the data. Something else could be
going on in this new case with the "@{$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
$AoA[3] = \@another_array;
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Then the assignment with the indirection on the left-hand-
side would use the existing reference that was already
there:
@{$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 vari-
able 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 pro-
grammers. So I usually advise against teaching it to
beginners. In fact, except for passing arguments to func-
tions, 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:
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$aref->[2][2] # clear
$$aref[2][2] # confusing
That's because Perl's precedence rules on its five prefix
dereferencers (which look like someone swearing: "$ @ * %
&") make them bind more tightly than the postfix subscript-
ing brackets or braces! This will no doubt come as a great
shock to the C or C++ programmer, who is quite accustomed to
using *a[i] to mean what's pointed to by the i'th element of
"a". That is, they first take the subscript, and only then
dereference the thing at that subscript. That's fine in C,
but this isn't C.
The seemingly equivalent construct in Perl, $$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 dereferenc-
ing". 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 com-
pile 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
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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 man-
pages someday) here are short code examples 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) ];
}
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# 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 ];
}
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# 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"
}
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# 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, $rec;
}
# reading from file
# format: LEAD=fred FRIEND=barney
# no temp
while ( <> ) {
push @AoH, { split /[\s+=]/ };
}
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# calling a function that returns a key/value pair list, like
# "lead","fred","daughter","pebbles"
while ( %fields = getnextpairset() ) {
push @AoH, { %fields };
}
# likewise, but using no temp vars
while (<>) {
push @AoH, { parsepairs($_) };
}
# 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 HASHESperl v5.8.8 2006-06-30 11
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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) };
}
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# 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";
}
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# 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
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%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 ];
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# 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
perl v5.8.8 2006-06-30 16
PERLDSC(1) Perl Programmers Reference Guide PERLDSC(1)
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 perl-
modlib 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
perl v5.8.8 2006-06-30 17
