Commit | Line | Data |
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cb1a09d0 | 1 | =head1 NAME |
4633a7c4 | 2 | |
19799a22 | 3 | perllol - Manipulating Arrays of Arrays in Perl |
4633a7c4 | 4 | |
cb1a09d0 AD |
5 | =head1 DESCRIPTION |
6 | ||
cea6626f | 7 | =head2 Declaration and Access of Arrays of Arrays |
4633a7c4 | 8 | |
21863e7e TC |
9 | The simplest two-level data structure to build in Perl is an array of |
10 | arrays, sometimes casually called a list of lists. It's reasonably easy to | |
11 | understand, and almost everything that applies here will also be applicable | |
12 | later on with the fancier data structures. | |
4633a7c4 | 13 | |
19799a22 GS |
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: | |
4633a7c4 | 17 | |
21863e7e TC |
18 | use 5.010; # so we can use say() |
19 | ||
19799a22 GS |
20 | # assign to our array, an array of array references |
21 | @AoA = ( | |
21863e7e TC |
22 | [ "fred", "barney", "pebbles", "bambam", "dino", ], |
23 | [ "george", "jane", "elroy", "judy", ], | |
24 | [ "homer", "bart", "marge", "maggie", ], | |
4633a7c4 | 25 | ); |
21863e7e | 26 | say $AoA[2][1]; |
4633a7c4 LW |
27 | bart |
28 | ||
29 | Now you should be very careful that the outer bracket type | |
5a964f20 | 30 | is a round one, that is, a parenthesis. That's because you're assigning to |
19799a22 | 31 | an @array, so you need parentheses. If you wanted there I<not> to be an @AoA, |
4633a7c4 LW |
32 | but rather just a reference to it, you could do something more like this: |
33 | ||
19799a22 GS |
34 | # assign a reference to array of array references |
35 | $ref_to_AoA = [ | |
4633a7c4 | 36 | [ "fred", "barney", "pebbles", "bambam", "dino", ], |
c2611fb3 | 37 | [ "george", "jane", "elroy", "judy", ], |
21863e7e | 38 | [ "homer", "bart", "marge", "maggie", ], |
4633a7c4 | 39 | ]; |
21863e7e TC |
40 | say $ref_to_AoA->[2][1]; |
41 | bart | |
4633a7c4 | 42 | |
54310121 | 43 | Notice that the outer bracket type has changed, and so our access syntax |
4633a7c4 | 44 | has also changed. That's because unlike C, in perl you can't freely |
19799a22 GS |
45 | interchange arrays and references thereto. $ref_to_AoA is a reference to an |
46 | array, whereas @AoA is an array proper. Likewise, C<$AoA[2]> is not an | |
4633a7c4 LW |
47 | array, but an array ref. So how come you can write these: |
48 | ||
19799a22 GS |
49 | $AoA[2][2] |
50 | $ref_to_AoA->[2][2] | |
4633a7c4 LW |
51 | |
52 | instead of having to write these: | |
53 | ||
19799a22 GS |
54 | $AoA[2]->[2] |
55 | $ref_to_AoA->[2]->[2] | |
4633a7c4 LW |
56 | |
57 | Well, that's because the rule is that on adjacent brackets only (whether | |
1fef88e7 | 58 | square or curly), you are free to omit the pointer dereferencing arrow. |
4d9142af | 59 | But you cannot do so for the very first one if it's a scalar containing |
19799a22 | 60 | a reference, which means that $ref_to_AoA always needs it. |
4633a7c4 | 61 | |
cea6626f | 62 | =head2 Growing Your Own |
4633a7c4 LW |
63 | |
64 | That's all well and good for declaration of a fixed data structure, | |
65 | but what if you wanted to add new elements on the fly, or build | |
66 | it up entirely from scratch? | |
67 | ||
68 | First, let's look at reading it in from a file. This is something like | |
69 | adding a row at a time. We'll assume that there's a flat file in which | |
70 | each line is a row and each word an element. If you're trying to develop an | |
19799a22 | 71 | @AoA array containing all these, here's the right way to do that: |
4633a7c4 LW |
72 | |
73 | while (<>) { | |
74 | @tmp = split; | |
19799a22 | 75 | push @AoA, [ @tmp ]; |
54310121 | 76 | } |
4633a7c4 LW |
77 | |
78 | You might also have loaded that from a function: | |
79 | ||
80 | for $i ( 1 .. 10 ) { | |
19799a22 | 81 | $AoA[$i] = [ somefunc($i) ]; |
4633a7c4 LW |
82 | } |
83 | ||
84 | Or you might have had a temporary variable sitting around with the | |
19799a22 | 85 | array in it. |
4633a7c4 LW |
86 | |
87 | for $i ( 1 .. 10 ) { | |
88 | @tmp = somefunc($i); | |
19799a22 | 89 | $AoA[$i] = [ @tmp ]; |
4633a7c4 LW |
90 | } |
91 | ||
21863e7e TC |
92 | It's important you make sure to use the C<[ ]> array reference |
93 | constructor. That's because this wouldn't work: | |
4633a7c4 | 94 | |
21863e7e | 95 | $AoA[$i] = @tmp; # WRONG! |
4633a7c4 | 96 | |
21863e7e TC |
97 | The reason that doesn't do what you want is because assigning a |
98 | named array like that to a scalar is taking an array in scalar | |
99 | context, which means just counts the number of elements in @tmp. | |
4633a7c4 | 100 | |
21863e7e TC |
101 | If you are running under C<use strict> (and if you aren't, why in |
102 | the world aren't you?), you'll have to add some declarations to | |
103 | make it happy: | |
4633a7c4 LW |
104 | |
105 | use strict; | |
19799a22 | 106 | my(@AoA, @tmp); |
4633a7c4 LW |
107 | while (<>) { |
108 | @tmp = split; | |
19799a22 | 109 | push @AoA, [ @tmp ]; |
54310121 | 110 | } |
4633a7c4 LW |
111 | |
112 | Of course, you don't need the temporary array to have a name at all: | |
113 | ||
114 | while (<>) { | |
19799a22 | 115 | push @AoA, [ split ]; |
54310121 | 116 | } |
4633a7c4 LW |
117 | |
118 | You also don't have to use push(). You could just make a direct assignment | |
119 | if you knew where you wanted to put it: | |
120 | ||
19799a22 | 121 | my (@AoA, $i, $line); |
1fef88e7 | 122 | for $i ( 0 .. 10 ) { |
4633a7c4 | 123 | $line = <>; |
21863e7e | 124 | $AoA[$i] = [ split " ", $line ]; |
54310121 | 125 | } |
4633a7c4 LW |
126 | |
127 | or even just | |
128 | ||
19799a22 | 129 | my (@AoA, $i); |
1fef88e7 | 130 | for $i ( 0 .. 10 ) { |
21863e7e | 131 | $AoA[$i] = [ split " ", <> ]; |
54310121 | 132 | } |
4633a7c4 | 133 | |
19799a22 GS |
134 | You should in general be leery of using functions that could |
135 | potentially return lists in scalar context without explicitly stating | |
136 | such. This would be clearer to the casual reader: | |
4633a7c4 | 137 | |
19799a22 | 138 | my (@AoA, $i); |
1fef88e7 | 139 | for $i ( 0 .. 10 ) { |
21863e7e | 140 | $AoA[$i] = [ split " ", scalar(<>) ]; |
54310121 | 141 | } |
4633a7c4 | 142 | |
19799a22 | 143 | If you wanted to have a $ref_to_AoA variable as a reference to an array, |
4633a7c4 LW |
144 | you'd have to do something like this: |
145 | ||
146 | while (<>) { | |
19799a22 | 147 | push @$ref_to_AoA, [ split ]; |
54310121 | 148 | } |
4633a7c4 | 149 | |
5a964f20 | 150 | Now you can add new rows. What about adding new columns? If you're |
5f05dabc | 151 | dealing with just matrices, it's often easiest to use simple assignment: |
4633a7c4 LW |
152 | |
153 | for $x (1 .. 10) { | |
154 | for $y (1 .. 10) { | |
19799a22 | 155 | $AoA[$x][$y] = func($x, $y); |
4633a7c4 LW |
156 | } |
157 | } | |
158 | ||
159 | for $x ( 3, 7, 9 ) { | |
19799a22 | 160 | $AoA[$x][20] += func2($x); |
54310121 | 161 | } |
4633a7c4 | 162 | |
54310121 | 163 | It doesn't matter whether those elements are already |
4633a7c4 LW |
164 | there or not: it'll gladly create them for you, setting |
165 | intervening elements to C<undef> as need be. | |
166 | ||
5f05dabc | 167 | If you wanted just to append to a row, you'd have |
4633a7c4 LW |
168 | to do something a bit funnier looking: |
169 | ||
170 | # add new columns to an existing row | |
21863e7e TC |
171 | push @{ $AoA[0] }, "wilma", "betty"; # explicit deref |
172 | ||
173 | Prior to Perl 5.14, this wouldn't even compile: | |
174 | ||
175 | push $AoA[0], "wilma", "betty"; # implicit deref | |
176 | ||
e001c712 | 177 | How come? Because once upon a time, the argument to push() had to be a |
21863e7e TC |
178 | real array, not just a reference to one. That's no longer true. In fact, |
179 | the line marked "implicit deref" above works just fine--in this | |
180 | instance--to do what the one that says explicit deref did. | |
181 | ||
182 | The reason I said "in this instance" is because that I<only> works | |
183 | because C<$AoA[0]> already held an array reference. If you try that on an | |
184 | undefined variable, you'll take an exception. That's because the implicit | |
185 | derefererence will never autovivify an undefined variable the way C<@{ }> | |
186 | always will: | |
4633a7c4 | 187 | |
21863e7e TC |
188 | my $aref = undef; |
189 | push $aref, qw(some more values); # WRONG! | |
190 | push @$aref, qw(a few more); # ok | |
4633a7c4 | 191 | |
21863e7e TC |
192 | If you want to take advantage of this new implicit dereferencing behavior, |
193 | go right ahead: it makes code easier on the eye and wrist. Just understand | |
194 | that older releases will choke on it during compilation. Whenever you make | |
195 | use of something that works only in some given release of Perl and later, | |
196 | but not earlier, you should place a prominent | |
4633a7c4 | 197 | |
21863e7e TC |
198 | use v5.14; # needed for implicit deref of array refs by array ops |
199 | ||
200 | directive at the top of the file that needs it. That way when somebody | |
201 | tries to run the new code under an old perl, rather than getting an error like | |
202 | ||
203 | Type of arg 1 to push must be array (not array element) at /tmp/a line 8, near ""betty";" | |
204 | Execution of /tmp/a aborted due to compilation errors. | |
205 | ||
206 | they'll be politely informed that | |
207 | ||
208 | Perl v5.14.0 required--this is only v5.12.3, stopped at /tmp/a line 1. | |
209 | BEGIN failed--compilation aborted at /tmp/a line 1. | |
4633a7c4 | 210 | |
cea6626f | 211 | =head2 Access and Printing |
4633a7c4 | 212 | |
54310121 | 213 | Now it's time to print your data structure out. How |
5f05dabc | 214 | are you going to do that? Well, if you want only one |
4633a7c4 LW |
215 | of the elements, it's trivial: |
216 | ||
19799a22 | 217 | print $AoA[0][0]; |
4633a7c4 LW |
218 | |
219 | If you want to print the whole thing, though, you can't | |
5f05dabc | 220 | say |
4633a7c4 | 221 | |
19799a22 | 222 | print @AoA; # WRONG |
4633a7c4 | 223 | |
5f05dabc | 224 | because you'll get just references listed, and perl will never |
54310121 | 225 | automatically dereference things for you. Instead, you have to |
4633a7c4 LW |
226 | roll yourself a loop or two. This prints the whole structure, |
227 | using the shell-style for() construct to loop across the outer | |
54310121 | 228 | set of subscripts. |
4633a7c4 | 229 | |
19799a22 | 230 | for $aref ( @AoA ) { |
21863e7e | 231 | say "\t [ @$aref ],"; |
4633a7c4 LW |
232 | } |
233 | ||
234 | If you wanted to keep track of subscripts, you might do this: | |
235 | ||
19799a22 | 236 | for $i ( 0 .. $#AoA ) { |
21863e7e | 237 | say "\t elt $i is [ @{$AoA[$i]} ],"; |
4633a7c4 LW |
238 | } |
239 | ||
240 | or maybe even this. Notice the inner loop. | |
241 | ||
19799a22 GS |
242 | for $i ( 0 .. $#AoA ) { |
243 | for $j ( 0 .. $#{$AoA[$i]} ) { | |
21863e7e | 244 | say "elt $i $j is $AoA[$i][$j]"; |
4633a7c4 LW |
245 | } |
246 | } | |
247 | ||
54310121 | 248 | As you can see, it's getting a bit complicated. That's why |
4633a7c4 LW |
249 | sometimes is easier to take a temporary on your way through: |
250 | ||
19799a22 GS |
251 | for $i ( 0 .. $#AoA ) { |
252 | $aref = $AoA[$i]; | |
4633a7c4 | 253 | for $j ( 0 .. $#{$aref} ) { |
21863e7e | 254 | say "elt $i $j is $AoA[$i][$j]"; |
4633a7c4 LW |
255 | } |
256 | } | |
257 | ||
5f05dabc | 258 | Hmm... that's still a bit ugly. How about this: |
4633a7c4 | 259 | |
19799a22 GS |
260 | for $i ( 0 .. $#AoA ) { |
261 | $aref = $AoA[$i]; | |
4633a7c4 LW |
262 | $n = @$aref - 1; |
263 | for $j ( 0 .. $n ) { | |
21863e7e | 264 | say "elt $i $j is $AoA[$i][$j]"; |
4633a7c4 LW |
265 | } |
266 | } | |
267 | ||
21863e7e TC |
268 | When you get tired of writing a custom print for your data structures, |
269 | you might look at the standard L<Dumpvalue> or L<Data::Dumper> modules. | |
270 | The former is what the Perl debugger uses, while the latter generates | |
271 | parsable Perl code. For example: | |
272 | ||
273 | use v5.14; # using the + prototype, new to v5.14 | |
274 | ||
275 | sub show(+) { | |
276 | require Dumpvalue; | |
277 | state $prettily = new Dumpvalue:: | |
278 | tick => q("), | |
279 | compactDump => 1, # comment these two lines out | |
280 | veryCompact => 1, # if you want a bigger dump | |
281 | ; | |
282 | dumpValue $prettily @_; | |
283 | } | |
284 | ||
285 | # Assign a list of array references to an array. | |
286 | my @AoA = ( | |
287 | [ "fred", "barney" ], | |
288 | [ "george", "jane", "elroy" ], | |
289 | [ "homer", "marge", "bart" ], | |
290 | ); | |
291 | push $AoA[0], "wilma", "betty"; | |
292 | show @AoA; | |
293 | ||
294 | will print out: | |
295 | ||
296 | 0 0..3 "fred" "barney" "wilma" "betty" | |
297 | 1 0..2 "george" "jane" "elroy" | |
298 | 2 0..2 "homer" "marge" "bart" | |
299 | ||
300 | Whereas if you comment out the two lines I said you might wish to, | |
301 | then it shows it to you this way instead: | |
302 | ||
303 | 0 ARRAY(0x8031d0) | |
304 | 0 "fred" | |
305 | 1 "barney" | |
306 | 2 "wilma" | |
307 | 3 "betty" | |
308 | 1 ARRAY(0x803d40) | |
309 | 0 "george" | |
310 | 1 "jane" | |
311 | 2 "elroy" | |
312 | 2 ARRAY(0x803e10) | |
313 | 0 "homer" | |
314 | 1 "marge" | |
315 | 2 "bart" | |
316 | ||
cea6626f | 317 | =head2 Slices |
4633a7c4 | 318 | |
4d9142af | 319 | If you want to get at a slice (part of a row) in a multidimensional |
4633a7c4 LW |
320 | array, you're going to have to do some fancy subscripting. That's |
321 | because while we have a nice synonym for single elements via the | |
322 | pointer arrow for dereferencing, no such convenience exists for slices. | |
4633a7c4 | 323 | |
19799a22 | 324 | Here's how to do one operation using a loop. We'll assume an @AoA |
4633a7c4 LW |
325 | variable as before. |
326 | ||
327 | @part = (); | |
54310121 | 328 | $x = 4; |
4633a7c4 | 329 | for ($y = 7; $y < 13; $y++) { |
19799a22 | 330 | push @part, $AoA[$x][$y]; |
54310121 | 331 | } |
4633a7c4 LW |
332 | |
333 | That same loop could be replaced with a slice operation: | |
334 | ||
21863e7e TC |
335 | @part = @{$AoA[4]}[7..12]; |
336 | ||
337 | or spaced out a bit: | |
338 | ||
19799a22 | 339 | @part = @{ $AoA[4] } [ 7..12 ]; |
4633a7c4 | 340 | |
21863e7e | 341 | But as you might well imagine, this can get pretty rough on the reader. |
4633a7c4 LW |
342 | |
343 | Ah, but what if you wanted a I<two-dimensional slice>, such as having | |
5f05dabc | 344 | $x run from 4..8 and $y run from 7 to 12? Hmm... here's the simple way: |
4633a7c4 | 345 | |
19799a22 | 346 | @newAoA = (); |
4633a7c4 | 347 | for ($startx = $x = 4; $x <= 8; $x++) { |
3e3baf6d | 348 | for ($starty = $y = 7; $y <= 12; $y++) { |
19799a22 | 349 | $newAoA[$x - $startx][$y - $starty] = $AoA[$x][$y]; |
4633a7c4 | 350 | } |
54310121 | 351 | } |
4633a7c4 | 352 | |
54310121 | 353 | We can reduce some of the looping through slices |
4633a7c4 LW |
354 | |
355 | for ($x = 4; $x <= 8; $x++) { | |
19799a22 | 356 | push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ]; |
4633a7c4 LW |
357 | } |
358 | ||
359 | If you were into Schwartzian Transforms, you would probably | |
360 | have selected map for that | |
361 | ||
19799a22 | 362 | @newAoA = map { [ @{ $AoA[$_] } [ 7..12 ] ] } 4 .. 8; |
4633a7c4 | 363 | |
384f06ae | 364 | Although if your manager accused you of seeking job security (or rapid |
4633a7c4 LW |
365 | insecurity) through inscrutable code, it would be hard to argue. :-) |
366 | If I were you, I'd put that in a function: | |
367 | ||
19799a22 | 368 | @newAoA = splice_2D( \@AoA, 4 => 8, 7 => 12 ); |
4633a7c4 | 369 | sub splice_2D { |
19799a22 | 370 | my $lrr = shift; # ref to array of array refs! |
54310121 | 371 | my ($x_lo, $x_hi, |
4633a7c4 LW |
372 | $y_lo, $y_hi) = @_; |
373 | ||
54310121 | 374 | return map { |
375 | [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ] | |
4633a7c4 | 376 | } $x_lo .. $x_hi; |
54310121 | 377 | } |
4633a7c4 LW |
378 | |
379 | ||
4633a7c4 LW |
380 | =head1 SEE ALSO |
381 | ||
ba555bf5 | 382 | L<perldata>, L<perlref>, L<perldsc> |
4633a7c4 LW |
383 | |
384 | =head1 AUTHOR | |
385 | ||
9607fc9c | 386 | Tom Christiansen <F<tchrist@perl.com>> |
4633a7c4 | 387 | |
21863e7e | 388 | Last update: Tue Apr 26 18:30:55 MDT 2011 |