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PERLREFTUT(1)			 Perl Programmers Reference Guide		    PERLREFTUT(1)

NAME
       perlreftut - Mark's very short tutorial about references

DESCRIPTION
       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.

The Solution
       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 "Barack Obama".

       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 "Barack Obama", 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.

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

   Making References
       Make Rule 1

       If you put a "\" in front of a variable, you get a reference 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, 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 "\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 ];
	   # $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
       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}\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 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.

   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 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
       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]".

Solution
       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};
	   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.

The Rest
       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.

Summary
       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.

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
       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 v5.16.3				    2013-03-04				    PERLREFTUT(1)
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