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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%.
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.
If you put a \ in front of a variable, you get a
reference to that variable.
$aref = \@array;
$href = \%hash;
$sref = \$scalar;
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;
$p[3] = $href;
$z = $p[3];
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 ];
$href = { APR => 4, AUG => 8 };
The references you get from rule 2 are the same kind of
references that you get from rule 1:
$aref = [ 1, 2, 3 ];
@array = (1, 2, 3);
$aref = \@array;
The first line is an abbreviation for the following two lines, except
that it doesn't create the superfluous array variable @array.
If you write just [], you get a new, empty anonymous array.
If you write just {}, you get a new, empty anonymous hash.
What can you do with a reference once you have it? It's a scalar
value, and we've seen that you can store it as a scalar and get it back
again just like any scalar. There are just two more ways to use it:
You can always use an array reference, in curly braces, in place of
the name of an array. For example, @{$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}\n";
}
And then replace the hash name with the reference:
for my $key (keys %{$href}) {
print "$key => ${$href}{$key}\n";
}
Use Rule 1 is all you really need, because it tells you how to do
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.
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:
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 ".\n";
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 ".\n";
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};
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