`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.
B<Use Rule 2>
-C<${$aref}[3]> is too hard to read, so you can write C<$aref-E<gt>[3]>
+C<${$aref}[3]> is too hard to read, so you can write C<< $aref->[3] >>
instead.
C<${$href}{red}> is too hard to read, so you can write
-C<$href-E<gt>{red}> instead.
+C<< $href->{red} >> instead.
Most often, when you have an array or a hash, you want to get or set a
single element from it. C<${$aref}[3]> and C<${$href}{'red'}> have
too much punctuation, and Perl lets you abbreviate.
-If C<$aref> holds a reference to an array, then C<$aref-E<gt>[3]> is
+If C<$aref> holds a reference to an array, then C<< $aref->[3] >> is
the fourth element of the array. Don't confuse this with C<$aref[3]>,
which is the fourth element of a totally different array, one
deceptively named C<@aref>. C<$aref> and C<@aref> are unrelated the
same way that C<$item> and C<@item> are.
-Similarly, C<$href-E<gt>{'red'}> is part of the hash referred to by
+Similarly, C<< $href->{'red'} >> is part of the hash referred to by
the scalar variable C<$href>, perhaps even one with no name.
C<$href{'red'}> is part of the deceptively named C<%href> hash. It's
-easy to forget to leave out the C<-E<gt>>, and if you do, you'll get
+easy to forget to leave out the C<< -> >>, and if you do, you'll get
bizarre results when your program gets array and hash elements out of
totally unexpected hashes and arrays that weren't the ones you wanted
to use.
C<$a[1]> is one of these references. It refers to an array, the array
containing C<(4, 5, 6)>, and because it is a reference to an array,
-B<USE RULE 2> says that we can write C<$a[1]-E<gt>[2]> to get the
-third element from that array. C<$a[1]-E<gt>[2]> is the 6.
-Similarly, C<$a[0]-E<gt>[1]> is the 2. What we have here is like a
-two-dimensional array; you can write C<$a[ROW]-E<gt>[COLUMN]> to get
+B<USE RULE 2> says that we can write C<< $a[1]->[2] >> to get the
+third element from that array. C<< $a[1]->[2] >> is the 6.
+Similarly, C<< $a[0]->[1] >> is the 2. What we have here is like a
+two-dimensional array; you can write C<< $a[ROW]->[COLUMN] >> to get
or set the element in any row and any column of the array.
The notation still looks a little cumbersome, so there's one more
In between two B<subscripts>, the arrow is optional.
-Instead of C<$a[1]-E<gt>[2]>, we can write C<$a[1][2]>; it means the
-same thing. Instead of C<$a[0]-E<gt>[1]>, we can write C<$a[0][1]>;
+Instead of C<< $a[1]->[2] >>, we can write C<$a[1][2]>; it means the
+same thing. Instead of C<< $a[0]->[1] >>, we can write C<$a[0][1]>;
it means the same thing.
Now it really looks like two-dimensional arrays!
=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}}, $city;
This is Perl, so it does the exact right thing. It sees that you want
-to push C<Cleveland> onto an array that doesn't exist, so it helpfully
+to push C<Athens> onto an array that doesn't exist, so it helpfully
makes a new, empty, anonymous array for you, installs it in the table,
-and then pushes C<Cleveland> onto it. This is called `autovivification'.
+and then pushes C<Athens> onto it. This is called `autovivification'.
=head1 The Rest
=item *
-In B<USE RULE 1>, you can often omit the curly braces. For example,
+In B<USE RULE 1>, you can omit the curly brackets whenever the thing
+inside them is an atomic scalar variable like C<$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
-the habit of always including the curly braces.
+C<${$aref}[1]>. If you're just starting out, you may want to adopt
+the habit of always including the curly brackets.
=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 *
=head1 Credits
-Author: Mark-Jason Dominus, Plover Systems (C<mjd-perl-ref@plover.com>)
+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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