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perlreftut (1)
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    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:  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 "Bill Clinton".
    
         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 "Bill Clinton", 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
    
         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'.
    
         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:
    
         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', 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 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
    
         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 abbreviate.
    
         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 `$a[0]->[1]', we can write
         `$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]'.
    
    
    

    Solution

         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, $country) = split /, /;
             4     push @{$table{$country}}, $city;
             5   }
             6
             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 to
    
                 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 undefined---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 `autovivification'.
    
    
    

    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 the perlref manpage manual page, which
         discusses 100% of the details.
    
         Some of the highlights of the perlref manpage:
    
         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   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.
    
         You might prefer to go on to the perllol manpage instead of
         the perlref manpage; it discusses lists of lists and
         multidimensional arrays in detail.  After that, you should
         move on to the perldsc manpage; 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://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 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.
    
    
    
    


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