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1 | =head1 NAME |
2 | ||
3 | perllol - Manipulating Arrays of Arrays in Perl | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Declaration and Access of Arrays of Arrays | |
8 | ||
9 | The simplest thing to build is an array of arrays (sometimes imprecisely | |
10 | called a list of lists). It's reasonably easy to understand, and | |
11 | almost everything that applies here will also be applicable later | |
12 | on with the fancier data structures. | |
13 | ||
14 | An array of an array is just a regular old array @AoA that you can | |
15 | get at with two subscripts, like C<$AoA[3][2]>. Here's a declaration | |
16 | of the array: | |
17 | ||
18 | # assign to our array, an array of array references | |
19 | @AoA = ( | |
20 | [ "fred", "barney" ], | |
21 | [ "george", "jane", "elroy" ], | |
22 | [ "homer", "marge", "bart" ], | |
23 | ); | |
24 | ||
25 | print $AoA[2][2]; | |
26 | bart | |
27 | ||
28 | Now you should be very careful that the outer bracket type | |
29 | is a round one, that is, a parenthesis. That's because you're assigning to | |
30 | an @array, so you need parentheses. If you wanted there I<not> to be an @AoA, | |
31 | but rather just a reference to it, you could do something more like this: | |
32 | ||
33 | # assign a reference to array of array references | |
34 | $ref_to_AoA = [ | |
35 | [ "fred", "barney", "pebbles", "bambam", "dino", ], | |
36 | [ "homer", "bart", "marge", "maggie", ], | |
37 | [ "george", "jane", "elroy", "judy", ], | |
38 | ]; | |
39 | ||
40 | print $ref_to_AoA->[2][2]; | |
41 | ||
42 | Notice that the outer bracket type has changed, and so our access syntax | |
43 | has also changed. That's because unlike C, in perl you can't freely | |
44 | interchange arrays and references thereto. $ref_to_AoA is a reference to an | |
45 | array, whereas @AoA is an array proper. Likewise, C<$AoA[2]> is not an | |
46 | array, but an array ref. So how come you can write these: | |
47 | ||
48 | $AoA[2][2] | |
49 | $ref_to_AoA->[2][2] | |
50 | ||
51 | instead of having to write these: | |
52 | ||
53 | $AoA[2]->[2] | |
54 | $ref_to_AoA->[2]->[2] | |
55 | ||
56 | Well, that's because the rule is that on adjacent brackets only (whether | |
57 | square or curly), you are free to omit the pointer dereferencing arrow. | |
58 | But you cannot do so for the very first one if it's a scalar containing | |
59 | a reference, which means that $ref_to_AoA always needs it. | |
60 | ||
61 | =head2 Growing Your Own | |
62 | ||
63 | That's all well and good for declaration of a fixed data structure, | |
64 | but what if you wanted to add new elements on the fly, or build | |
65 | it up entirely from scratch? | |
66 | ||
67 | First, let's look at reading it in from a file. This is something like | |
68 | adding a row at a time. We'll assume that there's a flat file in which | |
69 | each line is a row and each word an element. If you're trying to develop an | |
70 | @AoA array containing all these, here's the right way to do that: | |
71 | ||
72 | while (<>) { | |
73 | @tmp = split; | |
74 | push @AoA, [ @tmp ]; | |
75 | } | |
76 | ||
77 | You might also have loaded that from a function: | |
78 | ||
79 | for $i ( 1 .. 10 ) { | |
80 | $AoA[$i] = [ somefunc($i) ]; | |
81 | } | |
82 | ||
83 | Or you might have had a temporary variable sitting around with the | |
84 | array in it. | |
85 | ||
86 | for $i ( 1 .. 10 ) { | |
87 | @tmp = somefunc($i); | |
88 | $AoA[$i] = [ @tmp ]; | |
89 | } | |
90 | ||
91 | It's very important that you make sure to use the C<[]> array reference | |
92 | constructor. That's because this will be very wrong: | |
93 | ||
94 | $AoA[$i] = @tmp; | |
95 | ||
96 | You see, assigning a named array like that to a scalar just counts the | |
97 | number of elements in @tmp, which probably isn't what you want. | |
98 | ||
99 | If you are running under C<use strict>, you'll have to add some | |
100 | declarations to make it happy: | |
101 | ||
102 | use strict; | |
103 | my(@AoA, @tmp); | |
104 | while (<>) { | |
105 | @tmp = split; | |
106 | push @AoA, [ @tmp ]; | |
107 | } | |
108 | ||
109 | Of course, you don't need the temporary array to have a name at all: | |
110 | ||
111 | while (<>) { | |
112 | push @AoA, [ split ]; | |
113 | } | |
114 | ||
115 | You also don't have to use push(). You could just make a direct assignment | |
116 | if you knew where you wanted to put it: | |
117 | ||
118 | my (@AoA, $i, $line); | |
119 | for $i ( 0 .. 10 ) { | |
120 | $line = <>; | |
121 | $AoA[$i] = [ split ' ', $line ]; | |
122 | } | |
123 | ||
124 | or even just | |
125 | ||
126 | my (@AoA, $i); | |
127 | for $i ( 0 .. 10 ) { | |
128 | $AoA[$i] = [ split ' ', <> ]; | |
129 | } | |
130 | ||
131 | You should in general be leery of using functions that could | |
132 | potentially return lists in scalar context without explicitly stating | |
133 | such. This would be clearer to the casual reader: | |
134 | ||
135 | my (@AoA, $i); | |
136 | for $i ( 0 .. 10 ) { | |
137 | $AoA[$i] = [ split ' ', scalar(<>) ]; | |
138 | } | |
139 | ||
140 | If you wanted to have a $ref_to_AoA variable as a reference to an array, | |
141 | you'd have to do something like this: | |
142 | ||
143 | while (<>) { | |
144 | push @$ref_to_AoA, [ split ]; | |
145 | } | |
146 | ||
147 | Now you can add new rows. What about adding new columns? If you're | |
148 | dealing with just matrices, it's often easiest to use simple assignment: | |
149 | ||
150 | for $x (1 .. 10) { | |
151 | for $y (1 .. 10) { | |
152 | $AoA[$x][$y] = func($x, $y); | |
153 | } | |
154 | } | |
155 | ||
156 | for $x ( 3, 7, 9 ) { | |
157 | $AoA[$x][20] += func2($x); | |
158 | } | |
159 | ||
160 | It doesn't matter whether those elements are already | |
161 | there or not: it'll gladly create them for you, setting | |
162 | intervening elements to C<undef> as need be. | |
163 | ||
164 | If you wanted just to append to a row, you'd have | |
165 | to do something a bit funnier looking: | |
166 | ||
167 | # add new columns to an existing row | |
168 | push @{ $AoA[0] }, "wilma", "betty"; | |
169 | ||
170 | Notice that I I<couldn't> say just: | |
171 | ||
172 | push $AoA[0], "wilma", "betty"; # WRONG! | |
173 | ||
174 | In fact, that wouldn't even compile. How come? Because the argument | |
175 | to push() must be a real array, not just a reference to such. | |
176 | ||
177 | =head2 Access and Printing | |
178 | ||
179 | Now it's time to print your data structure out. How | |
180 | are you going to do that? Well, if you want only one | |
181 | of the elements, it's trivial: | |
182 | ||
183 | print $AoA[0][0]; | |
184 | ||
185 | If you want to print the whole thing, though, you can't | |
186 | say | |
187 | ||
188 | print @AoA; # WRONG | |
189 | ||
190 | because you'll get just references listed, and perl will never | |
191 | automatically dereference things for you. Instead, you have to | |
192 | roll yourself a loop or two. This prints the whole structure, | |
193 | using the shell-style for() construct to loop across the outer | |
194 | set of subscripts. | |
195 | ||
196 | for $aref ( @AoA ) { | |
197 | print "\t [ @$aref ],\n"; | |
198 | } | |
199 | ||
200 | If you wanted to keep track of subscripts, you might do this: | |
201 | ||
202 | for $i ( 0 .. $#AoA ) { | |
203 | print "\t elt $i is [ @{$AoA[$i]} ],\n"; | |
204 | } | |
205 | ||
206 | or maybe even this. Notice the inner loop. | |
207 | ||
208 | for $i ( 0 .. $#AoA ) { | |
209 | for $j ( 0 .. $#{$AoA[$i]} ) { | |
210 | print "elt $i $j is $AoA[$i][$j]\n"; | |
211 | } | |
212 | } | |
213 | ||
214 | As you can see, it's getting a bit complicated. That's why | |
215 | sometimes is easier to take a temporary on your way through: | |
216 | ||
217 | for $i ( 0 .. $#AoA ) { | |
218 | $aref = $AoA[$i]; | |
219 | for $j ( 0 .. $#{$aref} ) { | |
220 | print "elt $i $j is $AoA[$i][$j]\n"; | |
221 | } | |
222 | } | |
223 | ||
224 | Hmm... that's still a bit ugly. How about this: | |
225 | ||
226 | for $i ( 0 .. $#AoA ) { | |
227 | $aref = $AoA[$i]; | |
228 | $n = @$aref - 1; | |
229 | for $j ( 0 .. $n ) { | |
230 | print "elt $i $j is $AoA[$i][$j]\n"; | |
231 | } | |
232 | } | |
233 | ||
234 | =head2 Slices | |
235 | ||
236 | If you want to get at a slice (part of a row) in a multidimensional | |
237 | array, you're going to have to do some fancy subscripting. That's | |
238 | because while we have a nice synonym for single elements via the | |
239 | pointer arrow for dereferencing, no such convenience exists for slices. | |
240 | (Remember, of course, that you can always write a loop to do a slice | |
241 | operation.) | |
242 | ||
243 | Here's how to do one operation using a loop. We'll assume an @AoA | |
244 | variable as before. | |
245 | ||
246 | @part = (); | |
247 | $x = 4; | |
248 | for ($y = 7; $y < 13; $y++) { | |
249 | push @part, $AoA[$x][$y]; | |
250 | } | |
251 | ||
252 | That same loop could be replaced with a slice operation: | |
253 | ||
254 | @part = @{ $AoA[4] } [ 7..12 ]; | |
255 | ||
256 | but as you might well imagine, this is pretty rough on the reader. | |
257 | ||
258 | Ah, but what if you wanted a I<two-dimensional slice>, such as having | |
259 | $x run from 4..8 and $y run from 7 to 12? Hmm... here's the simple way: | |
260 | ||
261 | @newAoA = (); | |
262 | for ($startx = $x = 4; $x <= 8; $x++) { | |
263 | for ($starty = $y = 7; $y <= 12; $y++) { | |
264 | $newAoA[$x - $startx][$y - $starty] = $AoA[$x][$y]; | |
265 | } | |
266 | } | |
267 | ||
268 | We can reduce some of the looping through slices | |
269 | ||
270 | for ($x = 4; $x <= 8; $x++) { | |
271 | push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ]; | |
272 | } | |
273 | ||
274 | If you were into Schwartzian Transforms, you would probably | |
275 | have selected map for that | |
276 | ||
277 | @newAoA = map { [ @{ $AoA[$_] } [ 7..12 ] ] } 4 .. 8; | |
278 | ||
279 | Although if your manager accused of seeking job security (or rapid | |
280 | insecurity) through inscrutable code, it would be hard to argue. :-) | |
281 | If I were you, I'd put that in a function: | |
282 | ||
283 | @newAoA = splice_2D( \@AoA, 4 => 8, 7 => 12 ); | |
284 | sub splice_2D { | |
285 | my $lrr = shift; # ref to array of array refs! | |
286 | my ($x_lo, $x_hi, | |
287 | $y_lo, $y_hi) = @_; | |
288 | ||
289 | return map { | |
290 | [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ] | |
291 | } $x_lo .. $x_hi; | |
292 | } | |
293 | ||
294 | ||
295 | =head1 SEE ALSO | |
296 | ||
297 | perldata(1), perlref(1), perldsc(1) | |
298 | ||
299 | =head1 AUTHOR | |
300 | ||
301 | Tom Christiansen <F<tchrist@perl.com>> | |
302 | ||
303 | Last update: Thu Jun 4 16:16:23 MDT 1998 |