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Annotated edit history of perllol(1) version 1, including all changes. View license author blame.
Rev Author # Line
1 perry 1 PERLLOL
2 !!!PERLLOL
3 NAME
4 DESCRIPTION
5 SEE ALSO
6 AUTHOR
7 ----
8 !!NAME
9
10
11 perllol - Manipulating Arrays of Arrays in Perl
12 !!DESCRIPTION
13
14
15 __Declaration and Access of Arrays of
16 Arrays__
17
18
19 The simplest thing to build an array of arrays (sometimes
20 imprecisely called a list of lists). It's reasonably easy to
21 understand, and almost everything that applies here will
22 also be applicable later on with the fancier data
23 structures.
24
25
26 An array of an array is just a regular old array
27 @AoA that you can get at with two subscripts, like
28 $AoA[[3][[2]. Here's a declaration of the
29 array:
30
31
32 # assign to our array, an array of array references
33 @AoA = (
34 [[
35 print $AoA[[2][[2];
36 bart
37 Now you should be very careful that the outer bracket type is a round one, that is, a parenthesis. That's because you're assigning to an @array, so you need parentheses. If you wanted there ''not'' to be an @AoA, but rather just a reference to it, you could do something more like this:
38
39
40 # assign a reference to array of array references
41 $ref_to_AoA = [[
42 [[
43 print $ref_to_AoA-
44 Notice that the outer bracket type has changed, and so our access syntax has also changed. That's because unlike C, in perl you can't freely interchange arrays and references thereto. $ref_to_AoA is a reference to an array, whereas @AoA is an array proper. Likewise, $AoA[[2] is not an array, but an array ref. So how come you can write these:
45
46
47 $AoA[[2][[2]
48 $ref_to_AoA-
49 instead of having to write these:
50
51
52 $AoA[[2]-
53 Well, that's because the rule is that on adjacent brackets only (whether square or curly), you are free to omit the pointer dereferencing arrow. But you cannot do so for the very first one if it's a scalar containing a reference, which means that $ref_to_AoA always needs it.
54
55
56 __Growing Your Own__
57
58
59 That's all well and good for declaration of a fixed data
60 structure, but what if you wanted to add new elements on the
61 fly, or build it up entirely from scratch?
62
63
64 First, let's look at reading it in from a file. This is
65 something like adding a row at a time. We'll assume that
66 there's a flat file in which each line is a row and each
67 word an element. If you're trying to develop an
68 @AoA array containing all these, here's the right
69 way to do that:
70
71
72 while (
73 You might also have loaded that from a function:
74
75
76 for $i ( 1 .. 10 ) {
77 $AoA[[$i] = [[ somefunc($i) ];
78 }
79 Or you might have had a temporary variable sitting around with the array in it.
80
81
82 for $i ( 1 .. 10 ) {
83 @tmp = somefunc($i);
84 $AoA[[$i] = [[ @tmp ];
85 }
86 It's very important that you make sure to use the [[] array reference constructor. That's because this will be very wrong:
87
88
89 $AoA[[$i] = @tmp;
90 You see, assigning a named array like that to a scalar just counts the number of elements in @tmp, which probably isn't what you want.
91
92
93 If you are running under use strict, you'll have to
94 add some declarations to make it happy:
95
96
97 use strict;
98 my(@AoA, @tmp);
99 while (
100 Of course, you don't need the temporary array to have a name at all:
101
102
103 while (
104 You also don't have to use ''push()''. You could just make a direct assignment if you knew where you wanted to put it:
105
106
107 my (@AoA, $i, $line);
108 for $i ( 0 .. 10 ) {
109 $line =
110 or even just
111
112
113 my (@AoA, $i);
114 for $i ( 0 .. 10 ) {
115 $AoA[[$i] = [[ split ' ',
116 You should in general be leery of using functions that could potentially return lists in scalar context without explicitly stating such. This would be clearer to the casual reader:
117
118
119 my (@AoA, $i);
120 for $i ( 0 .. 10 ) {
121 $AoA[[$i] = [[ split ' ', scalar(
122 If you wanted to have a $ref_to_AoA variable as a reference to an array, you'd have to do something like this:
123
124
125 while (
126 Now you can add new rows. What about adding new columns? If you're dealing with just matrices, it's often easiest to use simple assignment:
127
128
129 for $x (1 .. 10) {
130 for $y (1 .. 10) {
131 $AoA[[$x][[$y] = func($x, $y);
132 }
133 }
134 for $x ( 3, 7, 9 ) {
135 $AoA[[$x][[20] += func2($x);
136 }
137 It doesn't matter whether those elements are already there or not: it'll gladly create them for you, setting intervening elements to undef as need be.
138
139
140 If you wanted just to append to a row, you'd have to do
141 something a bit funnier looking:
142
143
144 # add new columns to an existing row
145 push @{ $AoA[[0] },
146 Notice that I ''couldn't'' say just:
147
148
149 push $AoA[[0],
150 In fact, that wouldn't even compile. How come? Because the argument to ''push()'' must be a real array, not just a reference to such.
151
152
153 __Access and Printing__
154
155
156 Now it's time to print your data structure out. How are you
157 going to do that? Well, if you want only one of the
158 elements, it's trivial:
159
160
161 print $AoA[[0][[0];
162 If you want to print the whole thing, though, you can't say
163
164
165 print @AoA; # WRONG
166 because you'll get just references listed, and perl will never automatically dereference things for you. Instead, you have to roll yourself a loop or two. This prints the whole structure, using the shell-style ''for()'' construct to loop across the outer set of subscripts.
167
168
169 for $aref ( @AoA ) {
170 print
171 If you wanted to keep track of subscripts, you might do this:
172
173
174 for $i ( 0 .. $#AoA ) {
175 print
176 or maybe even this. Notice the inner loop.
177
178
179 for $i ( 0 .. $#AoA ) {
180 for $j ( 0 .. $#{$AoA[[$i]} ) {
181 print
182 As you can see, it's getting a bit complicated. That's why sometimes is easier to take a temporary on your way through:
183
184
185 for $i ( 0 .. $#AoA ) {
186 $aref = $AoA[[$i];
187 for $j ( 0 .. $#{$aref} ) {
188 print
189 Hmm... that's still a bit ugly. How about this:
190
191
192 for $i ( 0 .. $#AoA ) {
193 $aref = $AoA[[$i];
194 $n = @$aref - 1;
195 for $j ( 0 .. $n ) {
196 print
197
198
199 __Slices__
200
201
202 If you want to get at a slice (part of a row) in a
203 multidimensional array, you're going to have to do some
204 fancy subscripting. That's because while we have a nice
205 synonym for single elements via the pointer arrow for
206 dereferencing, no such convenience exists for slices.
207 (Remember, of course, that you can always write a loop to do
208 a slice operation.)
209
210
211 Here's how to do one operation using a loop. We'll assume an
212 @AoA variable as before.
213
214
215 @part = ();
216 $x = 4;
217 for ($y = 7; $y
218 That same loop could be replaced with a slice operation:
219
220
221 @part = @{ $AoA[[4] } [[ 7..12 ];
222 but as you might well imagine, this is pretty rough on the reader.
223
224
225 Ah, but what if you wanted a ''two-dimensional slice'',
226 such as having $x run from 4..8 and $y run
227 from 7 to 12? Hmm... here's the simple way:
228
229
230 @newAoA = ();
231 for ($startx = $x = 4; $x
232 We can reduce some of the looping through slices
233
234
235 for ($x = 4; $x
236 If you were into Schwartzian Transforms, you would probably have selected map for that
237
238
239 @newAoA = map { [[ @{ $AoA[[$_] } [[ 7..12 ] ] } 4 .. 8;
240 Although if your manager accused of seeking job security (or rapid insecurity) through inscrutable code, it would be hard to argue. :-) If I were you, I'd put that in a function:
241
242
243 @newAoA = splice_2D( @AoA, 4 =
244 return map {
245 [[ @{ $lrr-
246 !!SEE ALSO
247
248
249 perldata(1), perlref(1),
250 perldsc(1)
251 !!AUTHOR
252
253
254 Tom Christiansen
255 tchrist@perl.com''''
256
257
258 Last update: Thu Jun 4 16:16:23 MDT
259 1998
260 ----
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