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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	comes up all the time is needing a hash	whose values
       are lists.  Perl	has hashes, of course, but the values have to be
       scalars;	they can'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 couldn't be lists, 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.  Each human	being is a
       messy, inconvenient collection of cells.	But to refer to	a particular
       human, for instance the first computer programmer, it isn't necessary
       to describe each	of their cells;	all you	need is	the easy, convenient
       scalar string "Ada Lovelace".

       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 a	human name, 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   for my $country (sort keys %table) {
	   9	 print "$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   for my $country (sort keys %table) {
	   9	 print "$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
       the 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 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 v5.28.3			  2020-05-14			 PERLREFTUT(1)

NAME | DESCRIPTION | Who Needs Complicated Data Structures? | The Solution | Syntax | Solution | The Rest | Summary | Credits

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