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perlreftut



DESCRIPTION

       One of the most important new features in Perl 5 was the
       capability to manage complicated data structures like mul­
       tidimensional 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 syn­
       tax to learn, and the main manual page can be hard to fol­
       low.  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
       array or an entire hash (or to just about anything else).
       Names are one kind of reference that you're already famil­
       iar 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 ref­
       erence 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

       MMaakkee RRuullee 11

       If you put a "\" in front of a variable, you get a refer­
       ence 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

       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:

       UUssee RRuullee 11

       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

       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";
               }

       UUssee RRuullee 22

       Use Rule 1 is all you really need, because it tells you
       how to to 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.


       $a[1] is one of these references.  It refers to an array,
       the array containing "(4, 5, 6)", and because it is a ref­
       erence 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
               |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 $country in the hash and gets the value,
       which is a reference to an array of cities in that coun­
       try.  Use Rule 1 says that we can recover the array by
       saying "@{$table{$country}}".  Line 10 is just like

               @cities = @array;

       except that the name "array" has been replaced by the ref­
       erence "{$table{$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
           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 auto­
       matically. 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:

       ·   You can make references to anything, including
           scalars, functions, and other references.

       ·   In Use Rule 1, you can omit the curly brackets when­
           ever the thing inside them is an atomic scalar vari­
           able 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.

       ·   This doesn't copy the underlying array:

                   $aref2 = $aref1;

           You get two references to the same array.  If you mod­
           ify "$aref1->[23]" and then look at "$aref2->[23]"
           you'll see the change.

       ·   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.

       ·   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.)

       ·   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 per­
       mission.

       The original title was Understand References Today.


perl v5.8.1                 2003-09-02              PERLREFTUT(1)
  




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