MirOS Manual: perlreftut(1)


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 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 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 coun-
     try 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.

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     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, incon-
     venient 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 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.

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         $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

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     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:

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             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 refer-
     ences.  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]
                  );

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     @a is an array with three elements, and each one is a refer-
     ence 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 refer-
     ence 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 refor-
     matting 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

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     print out the report. We're going to have a hash, %table,
     whose keys are country names, and whose values are refer-
     ences 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 refer-
     ence "{$table{$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:

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         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 refer-
     ence "{$table{$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 $coun-
     try, 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.

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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 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.

     +   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}};

     +   To see if a variable contains a reference, use the "ref"
         function.  It returns true if its argument is a refer-
         ence.  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

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         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 permis-
     sion.

     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 pro-
     grams for fun or for profit as you see fit.  A simple com-
     ment in the code giving credit would be courteous but is not
     required.

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