PERLREFTUT(1) Perl Programmers Reference Guide PERLREFTUT(1)NAMEperlreftut - Mark's very short tutorial about references
DESCRIPTION
One of the most important new features in Perl 5 was the
capability to manage complicated data structures like mul
tidimensional arrays and nested hashes. To enable these,
Perl 5 introduced a feature called `references', and using
references is the key to managing complicated, structured
data in Perl. Unfortunately, there's a lot of funny syn
tax to learn, and the main manual page can be hard to fol
low. The manual is quite complete, and sometimes people
find that a problem, because it can be hard to tell what
is important and what isn't.
Fortunately, you only need to know 10% of what's in the
main page to get 90% of the benefit. This page will show
you that 10%.
Who Needs Complicated Data Structures?
One problem that came up all the time in Perl 4 was how to
represent a hash whose values were lists. Perl 4 had
hashes, of course, but the values had to be scalars; they
couldn't be lists.
Why would you want a hash of lists? Let's take a simple
example: You have a file of city and country names, like
this:
Chicago, USA
Frankfurt, Germany
Berlin, Germany
Washington, USA
Helsinki, Finland
New York, USA
and you want to produce an output like this, with each
country mentioned once, and then an alphabetical list of
the cities in that country:
Finland: Helsinki.
Germany: Berlin, Frankfurt.
USA: Chicago, New York, Washington.
The natural way to do this is to have a hash whose keys
are country names. Associated with each country name key
is a list of the cities in that country. Each time you
read a line of input, split it into a country and a city,
look up the list of cities already known to be in that
country, and append the new city to the list. When you're
done reading the input, iterate over the hash as usual,
sorting each list of cities before you print it out.
If hash values can't be lists, you lose. In Perl 4, hash
values can't be lists; they can only be strings. You
lose. You'd probably have to combine all the cities into
a single string somehow, and then when time came to write
the output, you'd have to break the string into a list,
sort the list, and turn it back into a string. This is
messy and error-prone. And it's frustrating, because Perl
already has perfectly good lists that would solve the
problem if only you could use them.
The Solution
By the time Perl 5 rolled around, we were already stuck
with this design: Hash values must be scalars. The solu
tion to this is references.
A reference is a scalar value that refers to an entire
array or an entire hash (or to just about anything else).
Names are one kind of reference that you're already famil
iar with. Think of the President: a messy, inconvenient
bag of blood and bones. But to talk about him, or to rep
resent him in a computer program, all you need is the
easy, convenient scalar string "Bill Clinton".
References in Perl are like names for arrays and hashes.
They're Perl's private, internal names, so you can be sure
they're unambiguous. Unlike "Bill Clinton", a reference
only refers to one thing, and you always know what it
refers to. If you have a reference to an array, you can
recover the entire array from it. If you have a reference
to a hash, you can recover the entire hash. But the ref
erence is still an easy, compact scalar value.
You can't have a hash whose values are arrays; hash values
can only be scalars. We're stuck with that. But a single
reference can refer to an entire array, and references are
scalars, so you can have a hash of references to arrays,
and it'll act a lot like a hash of arrays, and it'll be
just as useful as a hash of arrays.
We'll come back to this city-country problem later, after
we've seen some syntax for managing references.
Syntax
There are just two ways to make a reference, and just two
ways to use it once you have it.
Making References
Make Rule 1
If you put a "\" in front of a variable, you get a refer
ence to that variable.
$aref = \@array; # $aref now holds a reference to @array
$href = \%hash; # $href now holds a reference to %hash
Once the reference is stored in a variable like $aref or
$href, you can copy it or store it just the same as any
other scalar value:
$xy = $aref; # $xy now holds a reference to @array
$p[3] = $href; # $p[3] now holds a reference to %hash
$z = $p[3]; # $z now holds a reference to %hash
These examples show how to make references to variables
with names. Sometimes you want to make an array or a hash
that doesn't have a name. This is analogous to the way
you like to be able to use the string ""\n"" or the number
80 without having to store it in a named variable first.
Make Rule 2
"[ ITEMS ]" makes a new, anonymous array, and returns a
reference to that array. "{ ITEMS }" makes a new, anony
mous hash. and returns a reference to that hash.
$aref = [ 1, "foo", undef, 13 ];
# $aref now holds a reference to an array
$href = { APR => 4, AUG => 8 };
# $href now holds a reference to a hash
The references you get from rule 2 are the same kind of
references that you get from rule 1:
# This:
$aref = [ 1, 2, 3 ];
# Does the same as this:
@array = (1, 2, 3);
$aref = \@array;
The first line is an abbreviation for the following two
lines, except that it doesn't create the superfluous array
variable "@array".
Using References
What can you do with a reference once you have it? It's a
scalar value, and we've seen that you can store it as a
scalar and get it back again just like any scalar. There
are just two more ways to use it:
Use Rule 1
If "$aref" contains a reference to an array, then you can
put "{$aref}" anywhere you would normally put the name of
an array. For example, "@{$aref}" instead of "@array".
Here are some examples of that:
Arrays:
@a @{$aref} An array
reverse @a reverse @{$aref} Reverse the array
$a[3] ${$aref}[3] An element of the array
$a[3] = 17; ${$aref}[3] = 17 Assigning an element
On each line are two expressions that do the same thing.
The left-hand versions operate on the array "@a", and the
right-hand versions operate on the array that is referred
to by "$aref", but once they find the array they're oper
ating on, they do the same things to the arrays.
Using a hash reference is exactly the same:
%h %{$href} A hash
keys %h keys %{$href} Get the keys from the hash
$h{'red'} ${$href}{'red'} An element of the hash
$h{'red'} = 17 ${$href}{'red'} = 17 Assigning an element
Use Rule 2
"${$aref}[3]" is too hard to read, so you can write
"$aref->[3]" instead.
"${$href}{red}" is too hard to read, so you can write
"$href->{red}" instead.
Most often, when you have an array or a hash, you want to
get or set a single element from it. "${$aref}[3]" and
"${$href}{'red'}" have too much punctuation, and Perl lets
you abbreviate.
If "$aref" holds a reference to an array, then
"$aref->[3]" is the fourth element of the array. Don't
confuse this with "$aref[3]", which is the fourth element
of a totally different array, one deceptively named
"@aref". "$aref" and "@aref" are unrelated the same way
that "$item" and "@item" are.
Similarly, "$href->{'red'}" is part of the hash referred
to by the scalar variable "$href", perhaps even one with
no name. "$href{'red'}" is part of the deceptively named
"%href" hash. It's easy to forget to leave out the "->",
and if you do, you'll get bizarre results when your pro
gram gets array and hash elements out of totally unex
pected hashes and arrays that weren't the ones you wanted
to use.
An Example
Let's see a quick example of how all this is useful.
First, remember that "[1, 2, 3]" makes an anonymous array
containing "(1, 2, 3)", and gives you a reference to that
array.
Now think about
@a = ( [1, 2, 3],
[4, 5, 6],
[7, 8, 9]
);
@a is an array with three elements, and each one is a ref
erence to another array.
"$a[1]" is one of these references. It refers to an
array, the array containing "(4, 5, 6)", and because it is
a reference to an array, USE RULE 2 says that we can write
"$a[1]->[2]" to get the third element from that array.
"$a[1]->[2]" is the 6. Similarly, "$a[0]->[1]" is the 2.
What we have here is like a two-dimensional array; you can
write "$a[ROW]->[COLUMN]" to get or set the element in any
row and any column of the array.
The notation still looks a little cumbersome, so there's
one more abbreviation:
Arrow Rule
In between two subscripts, the arrow is optional.
Instead of "$a[1]->[2]", we can write "$a[1][2]"; it means
the same thing. Instead of "$a[0]->[1]", we can write
"$a[0][1]"; it means the same thing.
Now it really looks like two-dimensional arrays!
You can see why the arrows are important. Without them,
we would have had to write "${$a[1]}[2]" instead of
"$a[1][2]". For three-dimensional arrays, they let us
write "$x[2][3][5]" instead of the unreadable
"${${$x[2]}[3]}[5]".
Solution
Here's the answer to the problem I posed earlier, of
reformatting a file of city and country names.
1 while (<>) {
2 chomp;
3 my ($city, $country) = split /, /;
4 push @{$table{$country}}, $city;
5 }
6
7 foreach $country (sort keys %table) {
8 print "$country: ";
9 my @cities = @{$table{$country}};
10 print join ', ', sort @cities;
11 print ".\n";
12 }
The program has two pieces: Lines 1--5 read the input and
build a data structure, and lines 7--12 analyze the data
and print out the report.
In the first part, line 4 is the important one. We're
going to have a hash, "%table", whose keys are country
names, and whose values are (references to) arrays of city
names. After acquiring a city and country name, the pro
gram looks up "$table{$country}", which holds (a reference
to) the list of cities seen in that country so far. Line
4 is totally analogous to
push @array, $city;
except that the name "array" has been replaced by the ref
erence "{$table{$country}}". The "push" adds a city name
to the end of the referred-to array.
In the second part, line 9 is the important one. Again,
"$table{$country}" is (a reference to) the list of cities
in the country, so we can recover the original list, and
copy it into the array "@cities", by using
"@{$table{$country}}". Line 9 is totally analogous to
@cities = @array;
except that the name "array" has been replaced by the ref
erence "{$table{$country}}". The "@" tells Perl to get
the entire array.
The rest of the program is just familiar uses of "chomp",
"split", "sort", "print", and doesn't involve references
at all.
There's one fine point I skipped. Suppose the program has
just read the first line in its input that happens to men
tion Greece. Control is at line 4, "$country" is
"'Greece'", and "$city" is "'Athens'". Since this is the
first city in Greece, "$table{$country}" is undefined---in
fact there isn't an "'Greece'" key in "%table" at all.
What does line 4 do here?
4 push @{$table{$country}}, $city;
This is Perl, so it does the exact right thing. It sees
that you want to push "Athens" onto an array that doesn't
exist, so it helpfully makes a new, empty, anonymous array
for you, installs it in the table, and then pushes
"Athens" onto it. This is called `autovivification'.
The Rest
I promised to give you 90% of the benefit with 10% of the
details, and that means I left out 90% of the details.
Now that you have an overview of the important parts, it
should be easier to read the the perlref manpage manual
page, which discusses 100% of the details.
Some of the highlights of the perlref manpage:
You can make references to anything, including
scalars, functions, and other references.
In USE RULE 1, you can omit the curly brackets when
ever the thing inside them is an atomic scalar vari
able like "$aref". For example, "@$aref" is the same
as "@{$aref}", and "$$aref[1]" is the same as
"${$aref}[1]". If you're just starting out, you may
want to adopt the habit of always including the curly
brackets.
To see if a variable contains a reference, use the
`ref' function. It returns true if its argument is a
reference. Actually it's a little better than that:
It returns HASH for hash references and ARRAY for
array references.
If you try to use a reference like a string, you get
strings like
ARRAY(0x80f5dec) or HASH(0x826afc0)
If you ever see a string that looks like this, you'll
know you printed out a reference by mistake.
A side effect of this representation is that you can
use "eq" to see if two references refer to the same
thing. (But you should usually use "==" instead
because it's much faster.)
You can use a string as if it were a reference. If
you use the string ""foo"" as an array reference, it's
taken to be a reference to the array "@foo". This is
called a soft reference or symbolic reference.
You might prefer to go on to the perllol manpage instead
of the perlref manpage; it discusses lists of lists and
multidimensional arrays in detail. After that, you should
move on to the perldsc manpage; it's a Data Structure
Cookbook that shows recipes for using and printing out
arrays of hashes, hashes of arrays, and other kinds of
data.
Summary
Everyone needs compound data structures, and in Perl the
way you get them is with references. There are four
important rules for managing references: Two for making
references and two for using them. Once you know these
rules you can do most of the important things you need to
do with references.
Credits
Author: Mark-Jason Dominus, Plover Systems
("mjd-perl-ref+@plover.com")
This article originally appeared in The Perl Journal
(http://tpj.com) volume 3, #2. Reprinted with permission.
The original title was Understand References Today.
Distribution Conditions
Copyright 1998 The Perl Journal.
When included as part of the Standard Version of Perl, or
as part of its complete documentation whether printed or
otherwise, this work may be distributed only under the
terms of Perl's Artistic License. Any distribution of
this file or derivatives thereof outside of that package
require that special arrangements be made with copyright
holder.
Irrespective of its distribution, all code examples in
these files are hereby placed into the public domain. You
are permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but
is not required.
2001-03-18 perl v5.6.1 PERLREFTUT(1)
