perlreftut - Mark's very short tutorial about references
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.
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.
There are just two ways to make a reference, and just two
ways to use it once you have it.
Making References [Toc] [Back]
Make Rule 1
If you put a "
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 "0 or the number
80 without having to store it in a named variable first.
Make Rule 2 [Toc] [Back]
"[ 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);
$aref = @array;
The first line is an abbreviation for the following two
lines, except that it doesn't create the superfluous array
variable @array.
If you write just "[]", you get a new, empty anonymous
array. If you write just "{}", you get a new, empty
anonymous hash.
Using References [Toc] [Back]
What can you do with a reference once you have it? It's a
scalar value, and we've seen that you can store it as a
scalar and get it back again just like any scalar. There
are just two more ways to use it:
Use Rule 1
You can always use an array reference, in curly braces, in
place of the name of an array. For example, "@{$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. The
right-hand versions operate on the array that is referred
to by $aref. Once they find the array they're operating
on, both versions do the same things to the arrays.
Using a hash reference is exactly the same:
%h %{$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
Whatever you want to do with a reference, Use Rule 1 tells
you how to do it. You just write the Perl code that you
would have written for doing the same thing to a regular
array or hash, and then replace the array or hash name
with "{$reference}". "How do I loop over an array when
all I have is a reference?" Well, to loop over an array,
you would write
for my $element (@array) {
...
}
so replace the array name, @array, with the reference:
for my $element (@{$aref}) {
...
}
"How do I print out the contents of a hash when all I have
is a reference?" First write the code for printing out a
hash:
for my $key (keys %hash) {
print "$key => $hash{$key}0;
}
And then replace the hash name with the reference:
for my $key (keys %{$href}) {
print "$key => ${$href}{$key}0;
}
Use Rule 2
Use Rule 1 is all you really need, because it tells you
how to to absolutely everything you ever need to do with
references. But the most common thing to do with an array
or a hash is to extract a single element, and the Use Rule
1 notation is cumbersome. So there is an abbreviation.
"${$aref}[3]" is too hard to read, so you can write
"$aref->[3]" instead.
"${$href}{red}" is too hard to read, so you can write
"$href->{red}" instead.
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 [Toc] [Back]
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 [Toc] [Back]
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] = 23", we can write
"$a[0][1] = 23"; it means the same thing.
Now it really looks like two-dimensional arrays!
You can see why the arrows are important. Without them,
we would have had to write "${$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]".
Here's the answer to the problem I posed earlier, of
reformatting a file of city and country names.
1 my %table;
2 while (<>) {
3 chomp;
4 my ($city, $country) = split /, /;
5 $table{$country} = [] unless exists
$table{$country};
6 push @{$table{$country}}, $city;
7 }
8 foreach $country (sort keys %table) {
9 print "$country: ";
10 my @cities = @{$table{$country}};
11 print join ', ', sort @cities;
12 print ".0;
13 }
The program has two pieces: Lines 2--7 read the input and
build a data structure, and lines 8-13 analyze the data
and print out the report. We're going to have a hash,
%table, whose keys are country names, and whose values are
references to arrays of city names. The data structure
will look like this:
%table
+-------+---+
| | | +-----------+--------+
|Germany| *---->| Frankfurt | Berlin |
| | | +-----------+--------+
+-------+---+
| | | +----------+
|Finland| *---->| Helsinki |
| | | +----------+
+-------+---+
| | |
+---------+------------+----------+
| USA | *---->| Chicago | Washington | New York
|
| | |
+---------+------------+----------+
+-------+---+
We'll look at output first. Supposing we already have
this structure, how do we print it out?
8 foreach $country (sort keys %table) {
9 print "$country: ";
10 my @cities = @{$table{$country}};
11 print join ', ', sort @cities;
12 print ".0;
13 }
%table is an ordinary hash, and we get a list of keys from
it, sort the keys, and loop over the keys as usual. The
only use of references is in line 10. $table{$country}
looks up the key $country in the hash and gets the value,
which is a reference to an array of cities in that country.
Use Rule 1 says that we can recover the array by
saying "@{$table{$country}}". Line 10 is just like
@cities = @array;
except that the name "array" has been replaced by the reference
"{$table{$country}}". The "@" tells Perl to get
the entire array. Having gotten the list of cities, we
sort it, join it, and print it out as usual.
Lines 2-7 are responsible for building the structure in
the first place. Here they are again:
2 while (<>) {
3 chomp;
4 my ($city, $country) = split /, /;
5 $table{$country} = [] unless exists
$table{$country};
6 push @{$table{$country}}, $city;
7 }
Lines 2-4 acquire a city and country name. Line 5 looks
to see if the country is already present as a key in the
hash. If it's not, the program uses the "[]" notation
(Make Rule 2) to manufacture a new, empty anonymous array
of cities, and installs a reference to it into the hash
under the appropriate key.
Line 6 installs the city name into the appropriate array.
$table{$country} now holds a reference to the array of
cities seen in that country so far. Line 6 is exactly
like
push @array, $city;
except that the name "array" has been replaced by the reference
"{$table{$country}}". The "push" adds a city name
to the end of the referred-to array.
There's one fine point I skipped. Line 5 is unnecessary,
and we can get rid of it.
2 while (<>) {
3 chomp;
4 my ($city, $country) = split /, /;
5 #### $table{$country} = [] unless exists
$table{$country};
6 push @{$table{$country}}, $city;
7 }
If there's already an entry in %table for the current
$country, then nothing is different. Line 6 will locate
the value in $table{$country}, which is a reference to an
array, and push $city into the array. But what does it do
when $country holds a key, say "Greece", that is not yet
in %table?
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 into %table, and then pushes "Athens"
onto it. This is called `autovivification'--bringing
things to life automatically. Perl saw that they key
wasn't in the hash, so it created a new hash entry automatically.
Perl saw that you wanted to use the hash value
as an array, so it created a new empty array and installed
a reference to it in the hash automatically. And as
usual, Perl made the array one element longer to hold the
new city name.
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:
o You can make references to anything, including
scalars, functions, and other references.
o 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.
o This doesn't copy the underlying array:
$aref2 = $aref1;
You get two references to the same array. If you modify
"$aref1->[23]" and then look at "$aref2->[23]"
you'll see the change.
To copy the array, use
$aref2 = [@{$aref1}];
This uses "[...]" notation to create a new anonymous
array, and $aref2 is assigned a reference to the new
array. The new array is initialized with the contents
of the array referred to by $aref1.
Similarly, to copy an anonymous hash, you can use
$href2 = {%{$href1}};
o 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.
o 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.)
o 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. The
declaration "use strict 'refs'" disables this feature,
which can cause all sorts of trouble if you use it by
accident.
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.
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.
Author: Mark Jason Dominus, Plover Systems
("[email protected]")
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 [Toc] [Back]
Copyright 1998 The Perl Journal.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
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.
perl v5.8.5 2002-11-06 10 [ Back ] |
