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PERLREFTUT(1)
NAME
perlreftut - 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 multidimensional 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 syntax to learn, and the main
manual page can be hard to follow. 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 solution 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 familiar with. Think of the President of the United
States: a messy, inconvenient bag of blood and bones. But to talk about
him, or to represent him in a computer program, all you need is the easy,
convenient scalar string "George Bush".
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
"George Bush", 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 reference 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 reference 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, anonymous 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 $aref, but once they find the array
they're operating on, they do the same things to the arrays.
Using a hash reference is exactly the same:
%h %{$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 program gets array and
hash elements out of totally unexpected 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 reference 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 program 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, $city;
except that the name "array" has been replaced by the reference
"{$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 reference
"{$table{$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 mention 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 perlref manual page, which
discusses 100% of the details.
Some of the highlights of perlref:
· You can make references to anything, including scalars, functions, and
other references.
· In USE RULE 1, you can omit the curly brackets whenever the thing
inside them is an atomic scalar variable 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 perllol instead of perlref; it discusses lists
of lists and multidimensional arrays in detail. After that, you should
move on to perldsc; 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://www.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.
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