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

       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 com-
       plicated, 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 some-
       times 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 bene-
       fit.  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 read-
       ing 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 "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 man-
       aging references.

       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

       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

	       # 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 cre-
       ate the superfluous array variable @array.

       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

       If $aref contains a reference to an array, then you can put "{$aref}" anywhere you would
       normally put the name of an array.  For example, "@{$aref}" instead of @array.

       Here are some examples of that:


	       @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, and the right-hand versions operate on the array that is referred to by
       $aref, but once they find the array they're operating on, they do the same things to the

       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

       Use Rule 2

       "${$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.

       Most often, when you have an array or a hash, you want to get or set a single element from
       it.  "${$aref}[3]" and "${$href}{'red'}" have too much punctuation, and Perl lets you

       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 differ-
       ent 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], we can write $a[0][1]; 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 coun-
       try names.

	   1   while (<>) {
	   2	 chomp;
	   3	 my ($city, $country) = split /, /;
	   4	 push @{$table{$country}}, $city;
	   5   }
	   7   foreach $country (sort keys %table) {
	   8	 print "$country: ";
	   9	 my @cities = @{$table{$country}};
	  10	 print join ', ', sort @cities;
	  11	 print ".\n";
	  12   }

       The program has two pieces:  Lines 1--5 read the input and build a data structure, and
       lines 7--12 analyze the data and print out the report.

       In the first part, line 4 is the important one.	We're going to have a hash, %table, whose
       keys are country names, and whose values are (references to) arrays of city names.  After
       acquiring a city and country name, the program looks up $table{$country}, which holds (a
       reference to) the list of cities seen in that country so far.  Line 4 is totally analogous

	       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.

       In the second part, line 9 is the important one.  Again, $table{$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 @cities, by using "@{$table{$country}}".	Line 9 is totally analogous to

	       @cities = @array;

       except that the name "array" has been replaced by the reference "{$table{$country}}".  The
       "@" tells Perl to get the entire array.

       The rest of the program is just familiar uses of "chomp", "split", "sort", "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 Greece.  Control is at line 4, $country is 'Greece', and
       $city is 'Athens'.  Since this is the first city in Greece, $table{$country} is unde-
       fined---in fact there isn't an 'Greece' key in %table at all.  What does line 4 do here?

	4      push @{$table{$country}}, $city;

       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 in the table, and then pushes "Athens" onto it.  This is called `autoviv-

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 refer-

       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   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

       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.

       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.

       Everyone needs compound data structures, and in Perl the way you get them is with refer-
       ences.  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.

       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.

       When included as part of the Standard Version of Perl, or as part of its complete documen-
       tation 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.

perl v5.8.0				    2003-02-18				    PERLREFTUT(1)
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