3 perlreftut - Mark's very short tutorial about references
7 One of the most important new features in Perl 5 was the capability to
8 manage complicated data structures like multidimensional arrays and
9 nested hashes. To enable these, Perl 5 introduced a feature called
10 I<references>, and using references is the key to managing complicated,
11 structured data in Perl. Unfortunately, there's a lot of funny syntax
12 to learn, and the main manual page can be hard to follow. The manual
13 is quite complete, and sometimes people find that a problem, because
14 it can be hard to tell what is important and what isn't.
16 Fortunately, you only need to know 10% of what's in the main page to get
17 90% of the benefit. This page will show you that 10%.
19 =head1 Who Needs Complicated Data Structures?
21 One problem that comes up all the time is needing a hash whose values are
22 lists. Perl has hashes, of course, but the values have to be scalars;
25 Why would you want a hash of lists? Let's take a simple example: You
26 have a file of city and country names, like this:
35 and you want to produce an output like this, with each country mentioned
36 once, and then an alphabetical list of the cities in that country:
39 Germany: Berlin, Frankfurt.
40 USA: Chicago, New York, Washington.
42 The natural way to do this is to have a hash whose keys are country
43 names. Associated with each country name key is a list of the cities in
44 that country. Each time you read a line of input, split it into a country
45 and a city, look up the list of cities already known to be in that
46 country, and append the new city to the list. When you're done reading
47 the input, iterate over the hash as usual, sorting each list of cities
48 before you print it out.
50 If hash values couldn't be lists, you lose. You'd probably have to
51 combine all the cities into a single string somehow, and then when
52 time came to write the output, you'd have to break the string into a
53 list, sort the list, and turn it back into a string. This is messy
54 and error-prone. And it's frustrating, because Perl already has
55 perfectly good lists that would solve the problem if only you could
60 By the time Perl 5 rolled around, we were already stuck with this
61 design: Hash values must be scalars. The solution to this is
64 A reference is a scalar value that I<refers to> an entire array or an
65 entire hash (or to just about anything else). Names are one kind of
66 reference that you're already familiar with. Think of the President
67 of the United States: a messy, inconvenient bag of blood and bones.
68 But to talk about him, or to represent him in a computer program, all
69 you need is the easy, convenient scalar string "Barack Obama".
71 References in Perl are like names for arrays and hashes. They're
72 Perl's private, internal names, so you can be sure they're
73 unambiguous. Unlike "Barack Obama", a reference only refers to one
74 thing, and you always know what it refers to. If you have a reference
75 to an array, you can recover the entire array from it. If you have a
76 reference to a hash, you can recover the entire hash. But the
77 reference is still an easy, compact scalar value.
79 You can't have a hash whose values are arrays; hash values can only be
80 scalars. We're stuck with that. But a single reference can refer to
81 an entire array, and references are scalars, so you can have a hash of
82 references to arrays, and it'll act a lot like a hash of arrays, and
83 it'll be just as useful as a hash of arrays.
85 We'll come back to this city-country problem later, after we've seen
86 some syntax for managing references.
91 There are just two ways to make a reference, and just two ways to use
94 =head2 Making References
98 If you put a C<\> in front of a variable, you get a
99 reference to that variable.
101 $aref = \@array; # $aref now holds a reference to @array
102 $href = \%hash; # $href now holds a reference to %hash
103 $sref = \$scalar; # $sref now holds a reference to $scalar
105 Once the reference is stored in a variable like $aref or $href, you
106 can copy it or store it just the same as any other scalar value:
108 $xy = $aref; # $xy now holds a reference to @array
109 $p[3] = $href; # $p[3] now holds a reference to %hash
110 $z = $p[3]; # $z now holds a reference to %hash
113 These examples show how to make references to variables with names.
114 Sometimes you want to make an array or a hash that doesn't have a
115 name. This is analogous to the way you like to be able to use the
116 string C<"\n"> or the number 80 without having to store it in a named
119 =head3 B<Make Rule 2>
121 C<[ ITEMS ]> makes a new, anonymous array, and returns a reference to
122 that array. C<{ ITEMS }> makes a new, anonymous hash, and returns a
123 reference to that hash.
125 $aref = [ 1, "foo", undef, 13 ];
126 # $aref now holds a reference to an array
128 $href = { APR => 4, AUG => 8 };
129 # $href now holds a reference to a hash
132 The references you get from rule 2 are the same kind of
133 references that you get from rule 1:
138 # Does the same as this:
143 The first line is an abbreviation for the following two lines, except
144 that it doesn't create the superfluous array variable C<@array>.
146 If you write just C<[]>, you get a new, empty anonymous array.
147 If you write just C<{}>, you get a new, empty anonymous hash.
150 =head2 Using References
152 What can you do with a reference once you have it? It's a scalar
153 value, and we've seen that you can store it as a scalar and get it back
154 again just like any scalar. There are just two more ways to use it:
158 You can always use an array reference, in curly braces, in place of
159 the name of an array. For example, C<@{$aref}> instead of C<@array>.
161 Here are some examples of that:
167 reverse @a reverse @{$aref} Reverse the array
168 $a[3] ${$aref}[3] An element of the array
169 $a[3] = 17; ${$aref}[3] = 17 Assigning an element
172 On each line are two expressions that do the same thing. The
173 left-hand versions operate on the array C<@a>. The right-hand
174 versions operate on the array that is referred to by C<$aref>. Once
175 they find the array they're operating on, both versions do the same
176 things to the arrays.
178 Using a hash reference is I<exactly> the same:
181 keys %h keys %{$href} Get the keys from the hash
182 $h{'red'} ${$href}{'red'} An element of the hash
183 $h{'red'} = 17 ${$href}{'red'} = 17 Assigning an element
185 Whatever you want to do with a reference, B<Use Rule 1> tells you how
186 to do it. You just write the Perl code that you would have written
187 for doing the same thing to a regular array or hash, and then replace
188 the array or hash name with C<{$reference}>. "How do I loop over an
189 array when all I have is a reference?" Well, to loop over an array, you
192 for my $element (@array) {
196 so replace the array name, C<@array>, with the reference:
198 for my $element (@{$aref}) {
202 "How do I print out the contents of a hash when all I have is a
203 reference?" First write the code for printing out a hash:
205 for my $key (keys %hash) {
206 print "$key => $hash{$key}\n";
209 And then replace the hash name with the reference:
211 for my $key (keys %{$href}) {
212 print "$key => ${$href}{$key}\n";
217 L<B<Use Rule 1>|/B<Use Rule 1>> is all you really need, because it tells
218 you how to do absolutely everything you ever need to do with references.
219 But the most common thing to do with an array or a hash is to extract a
220 single element, and the L<B<Use Rule 1>|/B<Use Rule 1>> notation is
221 cumbersome. So there is an abbreviation.
223 C<${$aref}[3]> is too hard to read, so you can write C<< $aref->[3] >>
226 C<${$href}{red}> is too hard to read, so you can write
227 C<< $href->{red} >> instead.
229 If C<$aref> holds a reference to an array, then C<< $aref->[3] >> is
230 the fourth element of the array. Don't confuse this with C<$aref[3]>,
231 which is the fourth element of a totally different array, one
232 deceptively named C<@aref>. C<$aref> and C<@aref> are unrelated the
233 same way that C<$item> and C<@item> are.
235 Similarly, C<< $href->{'red'} >> is part of the hash referred to by
236 the scalar variable C<$href>, perhaps even one with no name.
237 C<$href{'red'}> is part of the deceptively named C<%href> hash. It's
238 easy to forget to leave out the C<< -> >>, and if you do, you'll get
239 bizarre results when your program gets array and hash elements out of
240 totally unexpected hashes and arrays that weren't the ones you wanted
246 Let's see a quick example of how all this is useful.
248 First, remember that C<[1, 2, 3]> makes an anonymous array containing
249 C<(1, 2, 3)>, and gives you a reference to that array.
258 C<@a> is an array with three elements, and each one is a reference to
261 C<$a[1]> is one of these references. It refers to an array, the array
262 containing C<(4, 5, 6)>, and because it is a reference to an array,
263 L<B<Use Rule 2>|/B<Use Rule 2>> says that we can write C<< $a[1]->[2] >>
264 to get the third element from that array. C<< $a[1]->[2] >> is the 6.
265 Similarly, C<< $a[0]->[1] >> is the 2. What we have here is like a
266 two-dimensional array; you can write C<< $a[ROW]->[COLUMN] >> to get or
267 set the element in any row and any column of the array.
269 The notation still looks a little cumbersome, so there's one more
274 In between two B<subscripts>, the arrow is optional.
276 Instead of C<< $a[1]->[2] >>, we can write C<$a[1][2]>; it means the
277 same thing. Instead of C<< $a[0]->[1] = 23 >>, we can write
278 C<$a[0][1] = 23>; it means the same thing.
280 Now it really looks like two-dimensional arrays!
282 You can see why the arrows are important. Without them, we would have
283 had to write C<${$a[1]}[2]> instead of C<$a[1][2]>. For
284 three-dimensional arrays, they let us write C<$x[2][3][5]> instead of
285 the unreadable C<${${$x[2]}[3]}[5]>.
289 Here's the answer to the problem I posed earlier, of reformatting a
290 file of city and country names.
296 4 my ($city, $country) = split /, /;
297 5 $table{$country} = [] unless exists $table{$country};
298 6 push @{$table{$country}}, $city;
301 8 for my $country (sort keys %table) {
302 9 print "$country: ";
303 10 my @cities = @{$table{$country}};
304 11 print join ', ', sort @cities;
309 The program has two pieces: Lines 2-7 read the input and build a data
310 structure, and lines 8-13 analyze the data and print out the report.
311 We're going to have a hash, C<%table>, whose keys are country names,
312 and whose values are references to arrays of city names. The data
313 structure will look like this:
318 | | | +-----------+--------+
319 |Germany| *---->| Frankfurt | Berlin |
320 | | | +-----------+--------+
323 |Finland| *---->| Helsinki |
326 | | | +---------+------------+----------+
327 | USA | *---->| Chicago | Washington | New York |
328 | | | +---------+------------+----------+
331 We'll look at output first. Supposing we already have this structure,
332 how do we print it out?
334 8 for my $country (sort keys %table) {
335 9 print "$country: ";
336 10 my @cities = @{$table{$country}};
337 11 print join ', ', sort @cities;
341 C<%table> is an ordinary hash, and we get a list of keys from it, sort
342 the keys, and loop over the keys as usual. The only use of references
343 is in line 10. C<$table{$country}> looks up the key C<$country> in the
344 hash and gets the value, which is a reference to an array of cities in
345 that country. L<B<Use Rule 1>|/B<Use Rule 1>> says that we can recover
346 the array by saying C<@{$table{$country}}>. Line 10 is just like
350 except that the name C<array> has been replaced by the reference
351 C<{$table{$country}}>. The C<@> tells Perl to get the entire array.
352 Having gotten the list of cities, we sort it, join it, and print it
355 Lines 2-7 are responsible for building the structure in the first
356 place. Here they are again:
360 4 my ($city, $country) = split /, /;
361 5 $table{$country} = [] unless exists $table{$country};
362 6 push @{$table{$country}}, $city;
365 Lines 2-4 acquire a city and country name. Line 5 looks to see if the
366 country is already present as a key in the hash. If it's not, the
367 program uses the C<[]> notation (L<B<Make Rule 2>|/B<Make Rule 2>>) to
368 manufacture a new, empty anonymous array of cities, and installs a
369 reference to it into the hash under the appropriate key.
371 Line 6 installs the city name into the appropriate array.
372 C<$table{$country}> now holds a reference to the array of cities seen
373 in that country so far. Line 6 is exactly like
377 except that the name C<array> has been replaced by the reference
378 C<{$table{$country}}>. The L<C<push>|perlfunc/push ARRAY,LIST> adds a
379 city name to the end of the referred-to array.
381 There's one fine point I skipped. Line 5 is unnecessary, and we can
386 4 my ($city, $country) = split /, /;
387 5 #### $table{$country} = [] unless exists $table{$country};
388 6 push @{$table{$country}}, $city;
391 If there's already an entry in C<%table> for the current C<$country>,
392 then nothing is different. Line 6 will locate the value in
393 C<$table{$country}>, which is a reference to an array, and push C<$city>
394 into the array. But what does it do when C<$country> holds a key, say
395 C<Greece>, that is not yet in C<%table>?
397 This is Perl, so it does the exact right thing. It sees that you want
398 to push C<Athens> onto an array that doesn't exist, so it helpfully
399 makes a new, empty, anonymous array for you, installs it into
400 C<%table>, and then pushes C<Athens> onto it. This is called
401 I<autovivification>--bringing things to life automatically. Perl saw
402 that the key wasn't in the hash, so it created a new hash entry
403 automatically. Perl saw that you wanted to use the hash value as an
404 array, so it created a new empty array and installed a reference to it
405 in the hash automatically. And as usual, Perl made the array one
406 element longer to hold the new city name.
410 I promised to give you 90% of the benefit with 10% of the details, and
411 that means I left out 90% of the details. Now that you have an
412 overview of the important parts, it should be easier to read the
413 L<perlref> manual page, which discusses 100% of the details.
415 Some of the highlights of L<perlref>:
421 You can make references to anything, including scalars, functions, and
426 In L<B<Use Rule 1>|/B<Use Rule 1>>, you can omit the curly brackets
427 whenever the thing inside them is an atomic scalar variable like
428 C<$aref>. For example, C<@$aref> is the same as C<@{$aref}>, and
429 C<$$aref[1]> is the same as C<${$aref}[1]>. If you're just starting
430 out, you may want to adopt the habit of always including the curly
435 This doesn't copy the underlying array:
439 You get two references to the same array. If you modify
440 C<< $aref1->[23] >> and then look at
441 C<< $aref2->[23] >> you'll see the change.
443 To copy the array, use
445 $aref2 = [@{$aref1}];
447 This uses C<[...]> notation to create a new anonymous array, and
448 C<$aref2> is assigned a reference to the new array. The new array is
449 initialized with the contents of the array referred to by C<$aref1>.
451 Similarly, to copy an anonymous hash, you can use
453 $href2 = {%{$href1}};
457 To see if a variable contains a reference, use the
458 L<C<ref>|perlfunc/ref EXPR> function. It returns true if its argument
459 is a reference. Actually it's a little better than that: It returns
460 C<HASH> for hash references and C<ARRAY> for array references.
464 If you try to use a reference like a string, you get strings like
466 ARRAY(0x80f5dec) or HASH(0x826afc0)
468 If you ever see a string that looks like this, you'll know you
469 printed out a reference by mistake.
471 A side effect of this representation is that you can use
472 L<C<eq>|perlop/Equality Operators> to see if two references refer to the
473 same thing. (But you should usually use
474 L<C<==>|perlop/Equality Operators> instead because it's much faster.)
478 You can use a string as if it were a reference. If you use the string
479 C<"foo"> as an array reference, it's taken to be a reference to the
480 array C<@foo>. This is called a I<symbolic reference>. The declaration
481 L<C<use strict 'refs'>|strict> disables this feature, which can cause
482 all sorts of trouble if you use it by accident.
486 You might prefer to go on to L<perllol> instead of L<perlref>; it
487 discusses lists of lists and multidimensional arrays in detail. After
488 that, you should move on to L<perldsc>; it's a Data Structure Cookbook
489 that shows recipes for using and printing out arrays of hashes, hashes
490 of arrays, and other kinds of data.
494 Everyone needs compound data structures, and in Perl the way you get
495 them is with references. There are four important rules for managing
496 references: Two for making references and two for using them. Once
497 you know these rules you can do most of the important things you need
498 to do with references.
502 Author: Mark Jason Dominus, Plover Systems (C<mjd-perl-ref+@plover.com>)
504 This article originally appeared in I<The Perl Journal>
505 ( http://www.tpj.com/ ) volume 3, #2. Reprinted with permission.
507 The original title was I<Understand References Today>.
509 =head2 Distribution Conditions
511 Copyright 1998 The Perl Journal.
513 This documentation is free; you can redistribute it and/or modify it
514 under the same terms as Perl itself.
516 Irrespective of its distribution, all code examples in these files are
517 hereby placed into the public domain. You are permitted and
518 encouraged to use this code in your own programs for fun or for profit
519 as you see fit. A simple comment in the code giving credit would be
520 courteous but is not required.