1Array::Unique(3) User Contributed Perl Documentation Array::Unique(3)
2
6 Array::Unique - Tie-able array that allows only unique values
7
9 use Array::Unique;
10 tie @a, 'Array::Unique';
11 Now use @a as a regular array.
13
15 This package lets you create an array which will allow only one
16 occurrence of any value.
17 In other words no matter how many times you put in 42 it will keep only
19 the first occurrence and the rest will be dropped.
20 You use the module via tie and once you tied your array to this module
22 it will behave correctly.
23 Uniqueness is checked with the 'eq' operator so among other things it
25 is case sensitive.
26 As a side effect the module does not allow undef as a value in the
28 array.
29
31 use Array::Unique;
32 tie @a, 'Array::Unique';
33 @a = qw(a b c a d e f);
35 push @a, qw(x b z);
36 print "@a\n"; # a b c d e f x z
37
39 When you are collecting a list of items and you want to make sure there
40 is only one occurrence of each item, you have several option:
41 1) using an array and extracting the unique elements later
43 You might use a regular array to hold this unique set of values and
44 either remove duplicates on each update by that keeping the array
45 always unique or remove duplicates just before you want to use the
46 uniqueness feature of the array. In either case you might run a
47 function you call @a = unique_value(@a);
48 The problem with this approach is that you have to implement the
50 unique_value function (see later) AND you have to make sure you
51 don't forget to call it. I would say don't rely on remembering
52 this.
53 There is good discussion about it in the 1st edition of the Perl
55 Cookbook of O'Reilly. I have copied the solutions here, you can see
56 further discussion in the book.
57 Extracting Unique Elements from a List (Section 4.6 in the Perl
59 Cookbook 1st ed.)
60 # Straightforward
62 %seen = ();
64 @uniq = ();
65 foreach $item (@list) [
66 unless ($seen{$item}) {
67 # if we get here we have not seen it before
68 $seen{$item} = 1;
69 push (@uniq, $item);
70 }
71 }
72 # Faster
74 %seen = ();
76 foreach $item (@list) {
77 push(@uniq, $item) unless $seen{$item}++;
78 }
79 # Faster but different
81 %seen;
83 foreach $item (@list) {
84 $seen{$item}++;
85 }
86 @uniq = keys %seen;
87 # Faster and even more different
89 %seen;
90 @uniq = grep {! $seen{$_}++} @list;
91 2) using a hash
93 Some people use the keys of a hash to keep the items and put an
94 arbitrary value as the values of the hash:
95 To build such a list:
97 %unique = map { $_ => 1 } qw( one two one two three four! );
99 To print it:
101 print join ", ", sort keys %unique;
103 To add values to it:
105 $unique{$_}=1 foreach qw( one after the nine oh nine );
107 To remove values:
109 delete @unique{ qw(oh nine) };
111 To check if a value is there:
113 $unique{ $value }; # which is why I like to use "1" as my value
115 (thanks to Gaal Yahas for the above examples)
117 There are three drawbacks I see:
119 1) You type more.
121 2) Your reader might not understand at first why did you use hash
122 and what will be the values.
123 3) You lose the order.
124 Usually non of them is critical but when I saw this the 10th time
126 in a code I had to understand with 0 documentation I got
127 frustrated.
128 3) using Array::Unique
130 So I decided to write this module because I got frustrated by my
131 lack of understanding what's going on in that code I mentioned.
132 In addition I thought it might be interesting to write this and
134 then benchmark it.
135 Additionally it is nice to have your name displayed in bright
137 lights all over CPAN ... or at least in a module.
138 Array::Unique lets you tie an array to hmmm, itself (?) and makes
140 sure the values of the array are always unique.
141 Since writing this I am not sure if I really recommend its usage.
143 I would say stick with the hash version and document that the
144 variable is aggregating a unique list of values.
145 4) Using real SET
147 There are modules on CPAN that let you create and maintain SETs. I
148 have not checked any of those but I guess they just as much of an
149 overkill for this functionality as Unique::Array.
150
152 use Array::Unique;
153 tie @a, 'Array::Unique';
154 @c = @a = qw(a b c a d e f b);
156 @c will contain the same as @a AND two undefs at the end because
158 @c you get the same length as the right most list.
159
161 Test:
162 Change size of the array Elements with false values ('', '0', 0)
164 splice:
166 splice @a;
167 splice @a, 3;
168 splice @a, -3;
169 splice @a, 3, 5;
170 splice @a, 3, -5;
171 splice @a, -3, 5;
172 splice @a, -3, -5;
173 splice @a, ?, ?, @b;
174 Benchmark speed
176 Add faster functions that don't check uniqueness so if I know part of
178 the data that comes from a unique source then I can speed up the
179 process, In short shoot myself in the leg.
180 Enable optional compare with other functions
182 Write even better implementations.
184
186 Gabor Szabo <gabor@pti.co.il>
187
189 Copyright (C) 2002-2008 Gabor Szabo <gabor@pti.co.il> All rights
190 reserved. http://www.pti.co.il/
191 You may distribute under the terms of either the GNU General Public
193 License or the Artistic License, as specified in the Perl README file.
194 No WARRANTY whatsoever.
196
198 Thanks for suggestions and bug reports to
199 Szabo Balazs (dLux)
200 Shlomo Yona
201 Gaal Yahas
202 Jeff 'japhy' Pinyan
203 Werner Weichselberger
204
206 Version: 0.08
207 Date: 2008 June 04
209perl v5.28.0 2008-06-04 Array::Unique(3)