1PERLREFTUT(1)          Perl Programmers Reference Guide          PERLREFTUT(1)
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3
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NAME

6       perlreftut - Mark's very short tutorial about references
7

DESCRIPTION

9       One of the most important new features in Perl 5 was the capability to
10       manage complicated data structures like multidimensional arrays and
11       nested hashes.  To enable these, Perl 5 introduced a feature called
12       references, and using references is the key to managing complicated,
13       structured data in Perl.  Unfortunately, there's a lot of funny syntax
14       to learn, and the main manual page can be hard to follow.  The manual
15       is quite complete, and sometimes people find that a problem, because it
16       can be hard to tell what is important and what isn't.
17
18       Fortunately, you only need to know 10% of what's in the main page to
19       get 90% of the benefit.  This page will show you that 10%.
20

Who Needs Complicated Data Structures?

22       One problem that comes up all the time is needing a hash whose values
23       are lists.  Perl has hashes, of course, but the values have to be
24       scalars; they can't be lists.
25
26       Why would you want a hash of lists?  Let's take a simple example: You
27       have a file of city and country names, like this:
28
29               Chicago, USA
30               Frankfurt, Germany
31               Berlin, Germany
32               Washington, USA
33               Helsinki, Finland
34               New York, USA
35
36       and you want to produce an output like this, with each country
37       mentioned once, and then an alphabetical list of the cities in that
38       country:
39
40               Finland: Helsinki.
41               Germany: Berlin, Frankfurt.
42               USA:  Chicago, New York, Washington.
43
44       The natural way to do this is to have a hash whose keys are country
45       names.  Associated with each country name key is a list of the cities
46       in that country.  Each time you read a line of input, split it into a
47       country and a city, look up the list of cities already known to be in
48       that country, and append the new city to the list.  When you're done
49       reading the input, iterate over the hash as usual, sorting each list of
50       cities before you print it out.
51
52       If hash values couldn't be lists, you lose.  You'd probably have to
53       combine all the cities into a single string somehow, and then when time
54       came to write the output, you'd have to break the string into a list,
55       sort the list, and turn it back into a string.  This is messy and
56       error-prone.  And it's frustrating, because Perl already has perfectly
57       good lists that would solve the problem if only you could use them.
58

The Solution

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
62       references.
63
64       A reference is a scalar value that 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.  Each human being is a
67       messy, inconvenient collection of cells. But to refer to a particular
68       human, for instance the first computer programmer, it isn't necessary
69       to describe each of their cells; all you need is the easy, convenient
70       scalar string "Ada Lovelace".
71
72       References in Perl are like names for arrays and hashes.  They're
73       Perl's private, internal names, so you can be sure they're unambiguous.
74       Unlike a human name, a reference only refers to one thing, and you
75       always know what it refers to.  If you have a reference to an array,
76       you can recover the entire array from it.  If you have a reference to a
77       hash, you can recover the entire hash.  But the reference is still an
78       easy, compact scalar value.
79
80       You can't have a hash whose values are arrays; hash values can only be
81       scalars.  We're stuck with that.  But a single reference can refer to
82       an entire array, and references are scalars, so you can have a hash of
83       references to arrays, and it'll act a lot like a hash of arrays, and
84       it'll be just as useful as a hash of arrays.
85
86       We'll come back to this city-country problem later, after we've seen
87       some syntax for managing references.
88

Syntax

90       There are just two ways to make a reference, and just two ways to use
91       it once you have it.
92
93   Making References
94       Make Rule 1
95
96       If you put a "\" in front of a variable, you get a reference to that
97       variable.
98
99           $aref = \@array;         # $aref now holds a reference to @array
100           $href = \%hash;          # $href now holds a reference to %hash
101           $sref = \$scalar;        # $sref now holds a reference to $scalar
102
103       Once the reference is stored in a variable like $aref or $href, you can
104       copy it or store it just the same as any other scalar value:
105
106           $xy = $aref;             # $xy now holds a reference to @array
107           $p[3] = $href;           # $p[3] now holds a reference to %hash
108           $z = $p[3];              # $z now holds a reference to %hash
109
110       These examples show how to make references to variables with names.
111       Sometimes you want to make an array or a hash that doesn't have a name.
112       This is analogous to the way you like to be able to use the string "\n"
113       or the number 80 without having to store it in a named variable first.
114
115       Make Rule 2
116
117       "[ ITEMS ]" makes a new, anonymous array, and returns a reference to
118       that array.  "{ ITEMS }" makes a new, anonymous hash, and returns a
119       reference to that hash.
120
121           $aref = [ 1, "foo", undef, 13 ];
122           # $aref now holds a reference to an array
123
124           $href = { APR => 4, AUG => 8 };
125           # $href now holds a reference to a hash
126
127       The references you get from rule 2 are the same kind of references that
128       you get from rule 1:
129
130               # This:
131               $aref = [ 1, 2, 3 ];
132
133               # Does the same as this:
134               @array = (1, 2, 3);
135               $aref = \@array;
136
137       The first line is an abbreviation for the following two lines, except
138       that it doesn't create the superfluous array variable @array.
139
140       If you write just "[]", you get a new, empty anonymous array.  If you
141       write just "{}", you get a new, empty anonymous hash.
142
143   Using References
144       What can you do with a reference once you have it?  It's a scalar
145       value, and we've seen that you can store it as a scalar and get it back
146       again just like any scalar.  There are just two more ways to use it:
147
148       Use Rule 1
149
150       You can always use an array reference, in curly braces, in place of the
151       name of an array.  For example, "@{$aref}" instead of @array.
152
153       Here are some examples of that:
154
155       Arrays:
156
157               @a              @{$aref}                An array
158               reverse @a      reverse @{$aref}        Reverse the array
159               $a[3]           ${$aref}[3]             An element of the array
160               $a[3] = 17;     ${$aref}[3] = 17        Assigning an element
161
162       On each line are two expressions that do the same thing.  The left-hand
163       versions operate on the array @a.  The right-hand versions operate on
164       the array that is referred to by $aref.  Once they find the array
165       they're operating on, both versions do the same things to the arrays.
166
167       Using a hash reference is exactly the same:
168
169               %h              %{$href}              A hash
170               keys %h         keys %{$href}         Get the keys from the hash
171               $h{'red'}       ${$href}{'red'}       An element of the hash
172               $h{'red'} = 17  ${$href}{'red'} = 17  Assigning an element
173
174       Whatever you want to do with a reference, Use Rule 1 tells you how to
175       do it.  You just write the Perl code that you would have written for
176       doing the same thing to a regular array or hash, and then replace the
177       array or hash name with "{$reference}".  "How do I loop over an array
178       when all I have is a reference?"  Well, to loop over an array, you
179       would write
180
181               for my $element (@array) {
182                 ...
183               }
184
185       so replace the array name, @array, with the reference:
186
187               for my $element (@{$aref}) {
188                 ...
189               }
190
191       "How do I print out the contents of a hash when all I have is a
192       reference?"  First write the code for printing out a hash:
193
194               for my $key (keys %hash) {
195                 print "$key => $hash{$key}\n";
196               }
197
198       And then replace the hash name with the reference:
199
200               for my $key (keys %{$href}) {
201                 print "$key => ${$href}{$key}\n";
202               }
203
204       Use Rule 2
205
206       Use Rule 1 is all you really need, because it tells you how to do
207       absolutely everything you ever need to do with references.  But the
208       most common thing to do with an array or a hash is to extract a single
209       element, and the Use Rule 1 notation is cumbersome.  So there is an
210       abbreviation.
211
212       "${$aref}[3]" is too hard to read, so you can write "$aref->[3]"
213       instead.
214
215       "${$href}{red}" is too hard to read, so you can write "$href->{red}"
216       instead.
217
218       If $aref holds a reference to an array, then "$aref->[3]" is the fourth
219       element of the array.  Don't confuse this with $aref[3], which is the
220       fourth element of a totally different array, one deceptively named
221       @aref.  $aref and @aref are unrelated the same way that $item and @item
222       are.
223
224       Similarly, "$href->{'red'}" is part of the hash referred to by the
225       scalar variable $href, perhaps even one with no name.  $href{'red'} is
226       part of the deceptively named %href hash.  It's easy to forget to leave
227       out the "->", and if you do, you'll get bizarre results when your
228       program gets array and hash elements out of totally unexpected hashes
229       and arrays that weren't the ones you wanted to use.
230
231   An Example
232       Let's see a quick example of how all this is useful.
233
234       First, remember that "[1, 2, 3]" makes an anonymous array containing
235       "(1, 2, 3)", and gives you a reference to that array.
236
237       Now think about
238
239               @a = ( [1, 2, 3],
240                      [4, 5, 6],
241                      [7, 8, 9]
242                    );
243
244       @a is an array with three elements, and each one is a reference to
245       another array.
246
247       $a[1] is one of these references.  It refers to an array, the array
248       containing "(4, 5, 6)", and because it is a reference to an array, Use
249       Rule 2 says that we can write $a[1]->[2] to get the third element from
250       that array.  $a[1]->[2] is the 6.  Similarly, $a[0]->[1] is the 2.
251       What we have here is like a two-dimensional array; you can write
252       $a[ROW]->[COLUMN] to get or set the element in any row and any column
253       of the array.
254
255       The notation still looks a little cumbersome, so there's one more
256       abbreviation:
257
258   Arrow Rule
259       In between two subscripts, the arrow is optional.
260
261       Instead of $a[1]->[2], we can write $a[1][2]; it means the same thing.
262       Instead of "$a[0]->[1] = 23", we can write "$a[0][1] = 23"; it means
263       the same thing.
264
265       Now it really looks like two-dimensional arrays!
266
267       You can see why the arrows are important.  Without them, we would have
268       had to write "${$a[1]}[2]" instead of $a[1][2].  For three-dimensional
269       arrays, they let us write $x[2][3][5] instead of the unreadable
270       "${${$x[2]}[3]}[5]".
271

Solution

273       Here's the answer to the problem I posed earlier, of reformatting a
274       file of city and country names.
275
276           1   my %table;
277
278           2   while (<>) {
279           3     chomp;
280           4     my ($city, $country) = split /, /;
281           5     $table{$country} = [] unless exists $table{$country};
282           6     push @{$table{$country}}, $city;
283           7   }
284
285           8   for my $country (sort keys %table) {
286           9     print "$country: ";
287          10     my @cities = @{$table{$country}};
288          11     print join ', ', sort @cities;
289          12     print ".\n";
290          13   }
291
292       The program has two pieces: Lines 2-7 read the input and build a data
293       structure, and lines 8-13 analyze the data and print out the report.
294       We're going to have a hash, %table, whose keys are country names, and
295       whose values are references to arrays of city names.  The data
296       structure will look like this:
297
298                  %table
299               +-------+---+
300               |       |   |   +-----------+--------+
301               |Germany| *---->| Frankfurt | Berlin |
302               |       |   |   +-----------+--------+
303               +-------+---+
304               |       |   |   +----------+
305               |Finland| *---->| Helsinki |
306               |       |   |   +----------+
307               +-------+---+
308               |       |   |   +---------+------------+----------+
309               |  USA  | *---->| Chicago | Washington | New York |
310               |       |   |   +---------+------------+----------+
311               +-------+---+
312
313       We'll look at output first.  Supposing we already have this structure,
314       how do we print it out?
315
316           8   for my $country (sort keys %table) {
317           9     print "$country: ";
318          10     my @cities = @{$table{$country}};
319          11     print join ', ', sort @cities;
320          12     print ".\n";
321          13   }
322
323       %table is an ordinary hash, and we get a list of keys from it, sort the
324       keys, and loop over the keys as usual.  The only use of references is
325       in line 10.  $table{$country} looks up the key $country in the hash and
326       gets the value, which is a reference to an array of cities in that
327       country.  Use Rule 1 says that we can recover the array by saying
328       "@{$table{$country}}".  Line 10 is just like
329
330               @cities = @array;
331
332       except that the name "array" has been replaced by the reference
333       "{$table{$country}}".  The "@" tells Perl to get the entire array.
334       Having gotten the list of cities, we sort it, join it, and print it out
335       as usual.
336
337       Lines 2-7 are responsible for building the structure in the first
338       place.  Here they are again:
339
340           2   while (<>) {
341           3     chomp;
342           4     my ($city, $country) = split /, /;
343           5     $table{$country} = [] unless exists $table{$country};
344           6     push @{$table{$country}}, $city;
345           7   }
346
347       Lines 2-4 acquire a city and country name.  Line 5 looks to see if the
348       country is already present as a key in the hash.  If it's not, the
349       program uses the "[]" notation (Make Rule 2) to manufacture a new,
350       empty anonymous array of cities, and installs a reference to it into
351       the hash under the appropriate key.
352
353       Line 6 installs the city name into the appropriate array.
354       $table{$country} now holds a reference to the array of cities seen in
355       that country so far.  Line 6 is exactly like
356
357               push @array, $city;
358
359       except that the name "array" has been replaced by the reference
360       "{$table{$country}}".  The "push" adds a city name to the end of the
361       referred-to array.
362
363       There's one fine point I skipped.  Line 5 is unnecessary, and we can
364       get rid of it.
365
366           2   while (<>) {
367           3     chomp;
368           4     my ($city, $country) = split /, /;
369           5   ####  $table{$country} = [] unless exists $table{$country};
370           6     push @{$table{$country}}, $city;
371           7   }
372
373       If there's already an entry in %table for the current $country, then
374       nothing is different.  Line 6 will locate the value in
375       $table{$country}, which is a reference to an array, and push $city into
376       the array.  But what does it do when $country holds a key, say
377       "Greece", that is not yet in %table?
378
379       This is Perl, so it does the exact right thing.  It sees that you want
380       to push "Athens" onto an array that doesn't exist, so it helpfully
381       makes a new, empty, anonymous array for you, installs it into %table,
382       and then pushes "Athens" onto it.  This is called
383       autovivification--bringing things to life automatically.  Perl saw that
384       the key wasn't in the hash, so it created a new hash entry
385       automatically. Perl saw that you wanted to use the hash value as an
386       array, so it created a new empty array and installed a reference to it
387       in the hash automatically.  And as usual, Perl made the array one
388       element longer to hold the new city name.
389

The Rest

391       I promised to give you 90% of the benefit with 10% of the details, and
392       that means I left out 90% of the details.  Now that you have an
393       overview of the important parts, it should be easier to read the
394       perlref manual page, which discusses 100% of the details.
395
396       Some of the highlights of perlref:
397
398       •   You can make references to anything, including scalars, functions,
399           and other references.
400
401       •   In Use Rule 1, you can omit the curly brackets whenever the thing
402           inside them is an atomic scalar variable like $aref.  For example,
403           @$aref is the same as "@{$aref}", and $$aref[1] is the same as
404           "${$aref}[1]".  If you're just starting out, you may want to adopt
405           the habit of always including the curly brackets.
406
407       •   This doesn't copy the underlying array:
408
409                   $aref2 = $aref1;
410
411           You get two references to the same array.  If you modify
412           "$aref1->[23]" and then look at "$aref2->[23]" you'll see the
413           change.
414
415           To copy the array, use
416
417                   $aref2 = [@{$aref1}];
418
419           This uses "[...]" notation to create a new anonymous array, and
420           $aref2 is assigned a reference to the new array.  The new array is
421           initialized with the contents of the array referred to by $aref1.
422
423           Similarly, to copy an anonymous hash, you can use
424
425                   $href2 = {%{$href1}};
426
427       •   To see if a variable contains a reference, use the "ref" function.
428           It returns true if its argument is a reference.  Actually it's a
429           little better than that: It returns "HASH" for hash references and
430           "ARRAY" for array references.
431
432       •   If you try to use a reference like a string, you get strings like
433
434                   ARRAY(0x80f5dec)   or    HASH(0x826afc0)
435
436           If you ever see a string that looks like this, you'll know you
437           printed out a reference by mistake.
438
439           A side effect of this representation is that you can use "eq" to
440           see if two references refer to the same thing.  (But you should
441           usually use "==" instead because it's much faster.)
442
443       •   You can use a string as if it were a reference.  If you use the
444           string "foo" as an array reference, it's taken to be a reference to
445           the array @foo.  This is called a symbolic reference.  The
446           declaration "use strict 'refs'" disables this feature, which can
447           cause all sorts of trouble if you use it by accident.
448
449       You might prefer to go on to perllol instead of perlref; it discusses
450       lists of lists and multidimensional arrays in detail.  After that, you
451       should move on to perldsc; it's a Data Structure Cookbook that shows
452       recipes for using and printing out arrays of hashes, hashes of arrays,
453       and other kinds of data.
454

Summary

456       Everyone needs compound data structures, and in Perl the way you get
457       them is with references.  There are four important rules for managing
458       references: Two for making references and two for using them.  Once you
459       know these rules you can do most of the important things you need to do
460       with references.
461

Credits

463       Author: Mark Jason Dominus, Plover Systems ("mjd-perl-ref+@plover.com")
464
465       This article originally appeared in The Perl Journal (
466       <http://www.tpj.com/> ) volume 3, #2.  Reprinted with permission.
467
468       The original title was Understand References Today.
469
470   Distribution Conditions
471       Copyright 1998 The Perl Journal.
472
473       This documentation is free; you can redistribute it and/or modify it
474       under the same terms as Perl itself.
475
476       Irrespective of its distribution, all code examples in these files are
477       hereby placed into the public domain.  You are permitted and encouraged
478       to use this code in your own programs for fun or for profit as you see
479       fit.  A simple comment in the code giving credit would be courteous but
480       is not required.
481
482
483
484perl v5.34.1                      2022-03-15                     PERLREFTUT(1)
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