`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.
+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%.
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 state names, like this:
+have a file of city and country names, like this:
- Chicago, Illinois
- New York, New York
- Albany, New York
- Springfield, Illinois
- Trenton, New Jersey
- Evanston, Illinois
+ Chicago, USA
+ Frankfurt, Germany
+ Berlin, Germany
+ Washington, USA
+ Helsinki, Finland
+ New York, USA
-and you want to produce an output like this, with each state mentioned
-once, and then an alphabetical list of the cities in that state:
+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:
- Illinois: Chicago, Evanston, Springfield.
- New Jersey: Trenton.
- New York: Albany, New York.
+ Finland: Helsinki.
+ Germany: Berlin, Frankfurt.
+ USA: Chicago, New York, Washington.
-The natural way to do this is to have a hash whose keys are state
-names. Associated with each state name key is a list of the cities in
-that state. Each time you read a line of input, split it into a state
+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
-state, and append the new city to the list. When you're done reading
+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.
=head1 The Solution
-Unfortunately, by the time Perl 5 rolled around, we were already stuck
-with this design: Hash values must be scalars. The solution to this is
+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 I<refers to> an entire array or an
-entire hash (or to just about anything else.) Names are one kind of
+entire hash (or to just about anything else). Names are one kind of
reference that you're already familiar with. Think of the President:
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,
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-state problem later, after we've seen
+We'll come back to this city-country problem later, after we've seen
some syntax for managing references.
=head1 Solution
-Here's the answer to the problem I posed the the beginning of the
-article, of reformatting a file of city and state names.
+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, $state) = split /, /;
- 4 push @{$table{$state}}, $city;
+ 3 my ($city, $country) = split /, /;
+ 4 push @{$table{$country}}, $city;
5 }
6
- 7 foreach $state (sort keys %table) {
- 8 print "$state: ";
- 9 my @cities = @{$table{$state}};
+ 7 foreach $country (sort keys %table) {
+ 8 print "$country: ";
+ 9 my @cities = @{$table{$country}};
10 print join ', ', sort @cities;
11 print ".\n";
12 }
report.
In the first part, line 4 is the important one. We're going to have a
-hash, C<%table>, whose keys are state names, and whose values are
+hash, C<%table>, whose keys are country names, and whose values are
(references to) arrays of city names. After acquiring a city and
-state name, the program looks up C<$table{$state}>, which holds (a
-reference to) the list of cities seen in that state so far. Line 4 is
+country name, the program looks up C<$table{$country}>, which holds (a
+reference to) the list of cities seen in that country so far. Line 4 is
totally analogous to
push @array, $city;
except that the name C<array> has been replaced by the reference
-C<{$table{$state}}>. The C<push> adds a city name to the end of the
+C<{$table{$country}}>. The C<push> adds a city name to the end of the
referred-to array.
In the second part, line 9 is the important one. Again,
-C<$table{$state}> is (a reference to) the list of cities in the state, so
+C<$table{$country}> is (a reference to) the list of cities in the country, so
we can recover the original list, and copy it into the array C<@cities>,
-by using C<@{$table{$state}}>. Line 9 is totally analogous to
+by using C<@{$table{$country}}>. Line 9 is totally analogous to
@cities = @array;
except that the name C<array> has been replaced by the reference
-C<{$table{$state}}>. The C<@> tells Perl to get the entire array.
+C<{$table{$country}}>. The C<@> tells Perl to get the entire array.
The rest of the program is just familiar uses of C<chomp>, C<split>, C<sort>,
C<print>, and doesn't involve references at all.
There's one fine point I skipped. Suppose the program has just read
-the first line in its input that happens to mention the state of Ohio.
-Control is at line 4, C<$state> is C<'Ohio'>, and C<$city> is
-C<'Cleveland'>. Since this is the first city in Ohio,
-C<$table{$state}> is undefined---in fact there isn't an C<'Ohio'> key
+the first line in its input that happens to mention Greece.
+Control is at line 4, C<$country> is C<'Greece'>, and C<$city> is
+C<'Athens'>. Since this is the first city in Greece,
+C<$table{$country}> is undefined---in fact there isn't an C<'Greece'> key
in C<%table> at all. What does line 4 do here?
- 4 push @{$table{$state}}, $city;
+ 4 push @{$table{$country}}, $city;
This is Perl, so it does the exact right thing. It sees that you want
-to push C<Cleveland> onto an array that doesn't exist, so it helpfully
+to push C<Athens> onto an array that doesn't exist, so it helpfully
makes a new, empty, anonymous array for you, installs it in the table,
-and then pushes C<Cleveland> onto it. This is called `autovivification'.
+and then pushes C<Athens> onto it. This is called `autovivification'.
=head1 The Rest
=item *
-In B<USE RULE 1>, you can often omit the curly braces. For example,
+In B<USE RULE 1>, you can omit the curly braces whenever the thing
+inside them is an atomic scalar variable like C<$aref>. For example,
C<@$aref> is the same as C<@{$aref}>, and C<$$aref[1]> is the same as
-C<${$aref}[1]>. If you're jsut starting out, you might want to adopt
+C<${$aref}[1]>. If you're just starting out, you may want to adopt
the habit of always including the curly braces.
=item *
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
-ARRAYfor array references.
+ARRAY for array references.
=item *
Author: Mark-Jason Dominus, Plover Systems (C<mjd-perl-ref@plover.com>)
-This article originally appeared in I<The Perl Journal> volume 3, #2.
-Reprinted with permission.
+This article originally appeared in I<The Perl Journal>
+(http://tpj.com) volume 3, #2. Reprinted with permission.
The original title was I<Understand References Today>.
+=head2 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.
-=cut
+
+
+=cut
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