One of the most important new features in Perl 5 was the capability tomanage complicated data structures like multidimensional arrays andnested 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 syntaxto learn, and the main manual page can be hard to follow. The manualis quite complete, and sometimes people find that a problem, becauseit 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 get90% 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 ahash whose values were lists. Perl 4 had hashes, of course, but thevalues had to be scalars; they couldn't be lists.

Why would you want a hash of lists? Let's take a simple example: Youhave 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 mentionedonce, 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 countrynames. Associated with each country name key is a list of the cities inthat country. Each time you read a line of input, split it into a countryand a city, look up the list of cities already known to be in thatcountry, and append the new city to the list. When you're done readingthe input, iterate over the hash as usual, sorting each list of citiesbefore you print it out.

If hash values can't be lists, you lose. In Perl 4, hash values can'tbe lists; they can only be strings. You lose. You'd probably have tocombine all the cities into a single string somehow, and then whentime came to write the output, you'd have to break the string into alist, sort the list, and turn it back into a string. This is messyand error-prone. And it's frustrating, because Perl already hasperfectly good lists that would solve the problem if only you coulduse them.

The Solution

By the time Perl 5 rolled around, we were already stuck with thisdesign: Hash values must be scalars. The solution to this isreferences.

A reference is a scalar value that refers to an entire array or anentire hash (or to just about anything else). Names are one kind ofreference that you're already familiar with. Think of the Presidentof the United States: a messy, inconvenient bag of blood and bones.But to talk about him, or to represent him in a computer program, allyou need is the easy, convenient scalar string "Barack Obama".

References in Perl are like names for arrays and hashes. They'rePerl's private, internal names, so you can be sure they'reunambiguous. Unlike "Barack Obama", a reference only refers to onething, and you always know what it refers to. If you have a referenceto an array, you can recover the entire array from it. If you have areference to a hash, you can recover the entire hash. But thereference is still an easy, compact scalar value.

You can't have a hash whose values are arrays; hash values can only bescalars. We're stuck with that. But a single reference can refer toan entire array, and references are scalars, so you can have a hash ofreferences to arrays, and it'll act a lot like a hash of arrays, andit'll be just as useful as a hash of arrays.

We'll come back to this city-country problem later, after we've seensome syntax for managing references.

Syntax

There are just two ways to make a reference, and just two ways to useit once you have it.

Making References

Make Rule 1

If you put a \ in front of a variable, you get areference to that variable.

 $aref = \@array; # $aref now holds a reference to @array
  $href = \%hash;  # $href now holds a reference to %hash
  $sref = \$scalar; # $sref now holds a reference to $scalar

Once the reference is stored in a variable like $aref or $href, youcan 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 aname. This is analogous to the way you like to be able to use thestring "\n" or the number 80 without having to store it in a namedvariable first.

Make Rule 2

[ ITEMS ] makes a new, anonymous array, and returns a reference tothat array. { ITEMS } makes a new, anonymous hash, and returns areference 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 ofreferences 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, exceptthat 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

What can you do with a reference once you have it? It's a scalarvalue, and we've seen that you can store it as a scalar and get it backagain 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 ofthe name of an array. For example, @{$aref} instead of @array.

Here are some examples of that:

Arrays:

@a@{$aref}An array
 reverse @areverse @{$aref}Reverse the array
 $a[3]${$aref}[3]An element of the array
 $a[3] = 17;${$aref}[3] = 17Assigning an element

On each line are two expressions that do the same thing. Theleft-hand versions operate on the array @a. The right-handversions operate on the array that is referred to by $aref. Oncethey find the array they're operating on, both versions do the samethings to the arrays.

Using a hash reference is exactly the same:

%h%{$href}  A hash
 keys %hkeys %{$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 howto do it. You just write the Perl code that you would have writtenfor doing the same thing to a regular array or hash, and then replacethe array or hash name with {$reference}. "How do I loop over anarray when all I have is a reference?" Well, to loop over an array, youwould 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 areference?" 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 2

Use Rule 1 is all you really need, because it tells you how to doabsolutely everything you ever need to do with references. But themost common thing to do with an array or a hash is to extract a singleelement, and the Use Rule 1 notation is cumbersome. So there is anabbreviation.

${$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] isthe fourth element of the array. Don't confuse this with $aref[3],which is the fourth element of a totally different array, onedeceptively named @aref. $aref and @aref are unrelated thesame way that $item and @item are.

Similarly, $href->{'red'} is part of the hash referred to bythe scalar variable $href, perhaps even one with no name.$href{'red'} is part of the deceptively named %href hash. It'seasy to forget to leave out the ->, and if you do, you'll getbizarre results when your program gets array and hash elements out oftotally unexpected hashes and arrays that weren't the ones you wantedto 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 toanother array.

$a[1] is one of these references. It refers to an array, the arraycontaining (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 thethird element from that array. $a[1]->[2] is the 6.Similarly, $a[0]->[1] is the 2. What we have here is like atwo-dimensional array; you can write $a[ROW]->[COLUMN] to getor set the element in any row and any column of the array.

The notation still looks a little cumbersome, so there's one moreabbreviation:

Arrow Rule

In between two subscripts, the arrow is optional.

Instead of $a[1]->[2], we can write $a[1][2]; it means thesame 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 havehad to write ${$a[1]}[2] instead of $a[1][2]. Forthree-dimensional arrays, they let us write $x[2][3][5] instead ofthe unreadable ${${$x[2]}[3]}[5].

Solution

Here's the answer to the problem I posed earlier, of reformatting afile 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 datastructure, 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 datastructure 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 anordinary hash, and we get a list of keys from it, sort the keys, andloop over the keys as usual. The only use of references is in line 10.$table{$country} looks up the key $country in the hashand gets the value, which is a reference to an array of cities in that country.Use Rule 1 says thatwe can recover the array by saying@{$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 itout as usual.

Lines 2-7 are responsible for building the structure in the firstplace. 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 thecountry is already present as a key in the hash. If it's not, theprogram uses the [] notation (Make Rule 2) to manufacture a new,empty anonymous array of cities, and installs a reference to it intothe 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 seenin 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 thereferred-to array.

There's one fine point I skipped. Line 5 is unnecessary, and we canget 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. Butwhat 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 wantto push Athens onto an array that doesn't exist, so it helpfullymakes 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 sawthat they key wasn't in the hash, so it created a new hash entryautomatically. Perl saw that you wanted to use the hash value as anarray, so it created a new empty array and installed a reference to itin the hash automatically. And as usual, Perl made the array oneelement longer to hold the new city name.

The Rest

I promised to give you 90% of the benefit with 10% of the details, andthat means I left out 90% of the details. Now that you have anoverview of the important parts, it should be easier to read theperlref manual page, which discusses 100% of the details.

Some of the highlights of perlref:

You can make references to anything, including scalars, functions, andother references.
In Use Rule 1, you can omit the curly brackets whenever the thinginside 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 adoptthe habit of always including the curly brackets.
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 isinitialized with the contents of the array referred to by $aref1.
Similarly, to copy an anonymous hash, you can use
```
 $href2 = {%{$href1}};
 
```
To see if a variable contains a reference, use the ref function. Itreturns true if its argument is a reference. Actually it's a littlebetter than that: It returns HASH for hash references and ARRAYfor 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 youprinted out a reference by mistake.
A side effect of this representation is that you can use eq to seeif 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 thearray @foo. This is called a soft reference or symbolicreference. The declaration use strict 'refs' disables thisfeature, which can cause all sorts of trouble if you use it by accident.

You might prefer to go on to perllol instead of perlref; itdiscusses lists of lists and multidimensional arrays in detail. Afterthat, you should move on to perldsc; it's a Data Structure Cookbookthat shows recipes for using and printing out arrays of hashes, hashesof arrays, and other kinds of data.

Summary

Everyone needs compound data structures, and in Perl the way you getthem is with references. There are four important rules for managingreferences: Two for making references and two for using them. Onceyou know these rules you can do most of the important things you needto 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

This documentation is free; you can redistribute it and/or modify itunder the same terms as Perl itself.

Irrespective of its distribution, all code examples in these files arehereby placed into the public domain. You are permitted andencouraged to use this code in your own programs for fun or for profitas you see fit. A simple comment in the code giving credit would becourteous but is not required.

Page index

NAME
DESCRIPTION
Who Needs Complicated Data Structures?
The Solution
Syntax
Solution
The Rest
Summary
Credits
- Distribution Conditions

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