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perldsc - Perl Data Structures Cookbook
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 $m{$LoL,$b} notation borrowed from awk in
which the keys are actually more like a single concatenated string
"$LoL$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 a array with
three dimensions!
for $x (1 .. 10) {
for $y (1 .. 10) {
for $z (1 .. 10) {
$LoL[$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 @LoL? 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:
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o arrays of arrays
o hashes of arrays
o arrays of hashes
o hashes of hashes
o more elaborate constructs
But for now, let's look at some of the general issues common to all of
these types of data structures.
The most important thing to understand about all data structures in Perl
-- including multidimensional arrays--is that even though they might
appear otherwise, Perl @ARRAYs and %HASHes are all internally onedimensional.
They can hold only scalar values (meaning a string, number,
or a reference). They cannot directly contain other arrays or hashes,
but instead contain references to other arrays or hashes.
You can't use a reference to 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-dimensional array and/or
hash, what's really going on is that the base type is merely a onedimensional
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.
$list[7][12] # array of arrays
$list[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:
@LoL = ( [2, 3], [4, 5, 7], [0] );
print $LoL[1][2];
7
print @LoL;
ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)
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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].
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 $i (1..10) {
@list = somefunc($i);
$LoL[$i] = @list; # WRONG!
}
That's just the simple case of assigning a list 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 $i (1..10) {
@list = somefunc($i);
$counts[$i] = scalar @list;
}
Here's the case of taking a reference to the same memory location again
and again:
for $i (1..10) {
@list = somefunc($i);
$LoL[$i] = \@list; # 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 @LoL refer to the very same place, and they will therefore all hold
whatever was last in @list! 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);
}
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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 $i (1..10) {
@list = somefunc($i);
$LoL[$i] = [ @list ];
}
The square brackets make a reference to a new array with a copy of what's
in @list 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) {
@list = 0 .. $i;
@{$LoL[$i]} = @list;
}
Is it the same? Well, maybe so--and maybe not. The subtle difference is
that when you assign something in square brackets, you know for sure it's
always a brand new reference with a new copy of the data. Something else
could be going on in this new case with the @{$LoL[$i]}} dereference on
the left-hand-side of the assignment. It all depends on whether $LoL[$i]
had been undefined to start with, or whether it already contained a
reference. If you had already populated @LoL with references, as in
$LoL[3] = \@another_list;
Then the assignment with the indirection on the left-hand-side would use
the existing reference that was already there:
@{$LoL[3]} = @list;
Of course, this would have the "interesting" effect of clobbering
@another_list. (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.
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Surprisingly, the following dangerous-looking construct will actually
work out fine:
for $i (1..10) {
my @list = somefunc($i);
$LoL[$i] = \@list;
}
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-areference
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:
$LoL[$i] = [ @list ]; # usually best
$LoL[$i] = \@list; # perilous; just how my() was that list?
@{ $LoL[$i] } = @list; # way too tricky for most programmers
Speaking of things like @{$LoL[$i]}, the following are actually the same
thing:
$listref->[2][2] # clear
$$listref[2][2] # confusing
That's because Perl's precedence rules on its five prefix dereferencers
(which look like someone swearing: $ @ * % &) make them bind more tightly
than the postfix subscripting brackets or braces! This will no doubt
come as a great shock to the C or C++ programmer, who is quite accustomed
to using *a[i] to mean what's pointed to by the i'th element of a. That
is, they first take the subscript, and only then dereference the thing at
that subscript. That's fine in C, but this isn't C.
The seemingly equivalent construct in Perl, $$listref[$i] first does the
deref of $listref, making it take $listref as a reference to an array,
and then dereference that, and finally tell you the i'th value of the
array pointed to by $LoL. If you wanted the C notion, you'd have to write
${$LoL[$i]} to force the $LoL[$i] to get evaluated first before the
leading $ dereferencer.
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WHY YOU SHOULD ALWAYS use strict
If this is starting to sound scarier than it's worth, relax. Perl has
some features to help you avoid its most common pitfalls. The best way
to avoid getting confused is to start every program like this:
#!/usr/bin/perl -w
use strict;
This way, you'll be forced to declare all your variables with my() and
also disallow accidental "symbolic dereferencing". Therefore if you'd
done this:
my $listref = [
[ "fred", "barney", "pebbles", "bambam", "dino", ],
[ "homer", "bart", "marge", "maggie", ],
[ "george", "jane", "elroy", "judy", ],
];
print $listref[2][2];
The compiler would immediately flag that as an error at compile time,
because you were accidentally accessing @listref, an undeclared variable,
and it would thereby remind you to write instead:
print $listref->[2][2]
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 $LoL above, here's the debugger output:
DB<1> X $LoL
$LoL = 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'
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There's also a lowercase x command which is nearly the same.
Presented with little comment (these will get their own manpages someday)
here are short code examples illustrating access of various types of data
structures.
Declaration of a LIST OF LISTS
@LoL = (
[ "fred", "barney" ],
[ "george", "jane", "elroy" ],
[ "homer", "marge", "bart" ],
);
Generation of a LIST OF LISTS
# reading from file
while ( <> ) {
push @LoL, [ split ];
}
# calling a function
for $i ( 1 .. 10 ) {
$LoL[$i] = [ somefunc($i) ];
}
# using temp vars
for $i ( 1 .. 10 ) {
@tmp = somefunc($i);
$LoL[$i] = [ @tmp ];
}
# add to an existing row
push @{ $LoL[0] }, "wilma", "betty";
Access and Printing of a LIST OF LISTS
# one element
$LoL[0][0] = "Fred";
# another element
$LoL[1][1] =~ s/(\w)/\u$1/;
# print the whole thing with refs
for $aref ( @LoL ) {
print "\t [ @$aref ],\n";
}
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# print the whole thing with indices
for $i ( 0 .. $#LoL ) {
print "\t [ @{$LoL[$i]} ],\n";
}
# print the whole thing one at a time
for $i ( 0 .. $#LoL ) {
for $j ( 0 .. $#{ $LoL[$i] } ) {
print "elt $i $j is $LoL[$i][$j]\n";
}
}
Declaration of a HASH OF LISTS
%HoL = (
flintstones => [ "fred", "barney" ],
jetsons => [ "george", "jane", "elroy" ],
simpsons => [ "homer", "marge", "bart" ],
);
Generation of a HASH OF LISTS
# reading from file
# flintstones: fred barney wilma dino
while ( <> ) {
next unless s/^(.*?):\s*//;
$HoL{$1} = [ split ];
}
# reading from file; more temps
# flintstones: fred barney wilma dino
while ( $line = <> ) {
($who, $rest) = split /:\s*/, $line, 2;
@fields = split ' ', $rest;
$HoL{$who} = [ @fields ];
}
# calling a function that returns a list
for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoL{$group} = [ get_family($group) ];
}
# likewise, but using temps
for $group ( "simpsons", "jetsons", "flintstones" ) {
@members = get_family($group);
$HoL{$group} = [ @members ];
}
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# append new members to an existing family
push @{ $HoL{"flintstones"} }, "wilma", "betty";
Access and Printing of a HASH OF LISTS
# one element
$HoL{flintstones}[0] = "Fred";
# another element
$HoL{simpsons}[1] =~ s/(\w)/\u$1/;
# print the whole thing
foreach $family ( keys %HoL ) {
print "$family: @{ $HoL{$family} }\n"
}
# print the whole thing with indices
foreach $family ( keys %HoL ) {
print "family: ";
foreach $i ( 0 .. $#{ $HoL{$family} } ) {
print " $i = $HoL{$family}[$i]";
}
print "\n";
}
# print the whole thing sorted by number of members
foreach $family ( sort { @{$HoL{$b}} <=> @{$HoL{$a}} } keys %HoL ) {
print "$family: @{ $HoL{$family} }\n"
}
# print the whole thing sorted by number of members and name
foreach $family ( sort {
@{$HoL{$b}} <=> @{$HoL{$a}}
||
$a cmp $b
} keys %HoL )
{
print "$family: ", join(", ", sort @{ $HoL{$family}), "\n";
}
Declaration of a LIST OF HASHES
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@LoH = (
{
Lead => "fred",
Friend => "barney",
},
{
Lead => "george",
Wife => "jane",
Son => "elroy",
},
{
Lead => "homer",
Wife => "marge",
Son => "bart",
}
);
Generation of a LIST OF HASHES
# reading from file
# format: LEAD=fred FRIEND=barney
while ( <> ) {
$rec = {};
for $field ( split ) {
($key, $value) = split /=/, $field;
$rec->{$key} = $value;
}
push @LoH, $rec;
}
# reading from file
# format: LEAD=fred FRIEND=barney
# no temp
while ( <> ) {
push @LoH, { split /[\s+=]/ };
}
# calling a function that returns a key,value list, like
# "lead","fred","daughter","pebbles"
while ( %fields = getnextpairset() ) {
push @LoH, { %fields };
}
# likewise, but using no temp vars
while (<>) {
push @LoH, { parsepairs($_) };
}
# add key/value to an element
$LoH[0]{pet} = "dino";
$LoH[2]{pet} = "santa's little helper";
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Access and Printing of a LIST OF HASHES
# one element
$LoH[0]{lead} = "fred";
# another element
$LoH[1]{lead} =~ s/(\w)/\u$1/;
# print the whole thing with refs
for $href ( @LoH ) {
print "{ ";
for $role ( keys %$href ) {
print "$role=$href->{$role} ";
}
print "}\n";
}
# print the whole thing with indices
for $i ( 0 .. $#LoH ) {
print "$i is { ";
for $role ( keys %{ $LoH[$i] } ) {
print "$role=$LoH[$i]{$role} ";
}
print "}\n";
}
# print the whole thing one at a time
for $i ( 0 .. $#LoH ) {
for $role ( keys %{ $LoH[$i] } ) {
print "elt $i $role is $LoH[$i]{$role}\n";
}
}
Declaration of a HASH OF HASHES
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%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 };
}
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# 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 }
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# 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 [Toc] [Back] 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->{LIST}[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 [Toc] [Back]
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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() ];
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@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 examples
# if you wanted a {kids} field that was an array reference
# to a list 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 list, but the list 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
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# 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";
}
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 the
perlmodlib manpage for source code to MLDBM.
perlref(1), perllol(1), perldata(1), perlobj(1)
Tom Christiansen <tchrist@perl.com>
Last update: Wed Oct 23 04:57:50 MET DST 1996
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