Commit | Line | Data |
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cb1a09d0 | 1 | =head1 NAME |
d74e8afc | 2 | X<data structure> X<complex data structure> X<struct> |
4633a7c4 | 3 | |
cb1a09d0 | 4 | perldsc - Perl Data Structures Cookbook |
4633a7c4 | 5 | |
cb1a09d0 | 6 | =head1 DESCRIPTION |
4633a7c4 | 7 | |
cb1e035e BF |
8 | Perl lets us have complex data structures. You can write something like |
9 | this and all of a sudden, you'd have an array with three dimensions! | |
4633a7c4 | 10 | |
5939083a SF |
11 | for my $x (1 .. 10) { |
12 | for my $y (1 .. 10) { | |
13 | for my $z (1 .. 10) { | |
6a40a726 SF |
14 | $AoA[$x][$y][$z] = |
15 | $x ** $y + $z; | |
16 | } | |
17 | } | |
4633a7c4 LW |
18 | } |
19 | ||
20 | Alas, however simple this may appear, underneath it's a much more | |
21 | elaborate construct than meets the eye! | |
22 | ||
19799a22 | 23 | How do you print it out? Why can't you say just C<print @AoA>? How do |
4633a7c4 | 24 | you sort it? How can you pass it to a function or get one of these back |
d1be9408 | 25 | from a function? Is it an object? Can you save it to disk to read |
4633a7c4 | 26 | back later? How do you access whole rows or columns of that matrix? Do |
4973169d | 27 | all the values have to be numeric? |
4633a7c4 LW |
28 | |
29 | As you see, it's quite easy to become confused. While some small portion | |
30 | of the blame for this can be attributed to the reference-based | |
31 | implementation, it's really more due to a lack of existing documentation with | |
32 | examples designed for the beginner. | |
33 | ||
5f05dabc | 34 | This document is meant to be a detailed but understandable treatment of the |
35 | many different sorts of data structures you might want to develop. It | |
36 | should also serve as a cookbook of examples. That way, when you need to | |
37 | create one of these complex data structures, you can just pinch, pilfer, or | |
38 | purloin a drop-in example from here. | |
4633a7c4 LW |
39 | |
40 | Let's look at each of these possible constructs in detail. There are separate | |
28757baa | 41 | sections on each of the following: |
4633a7c4 LW |
42 | |
43 | =over 5 | |
44 | ||
45 | =item * arrays of arrays | |
46 | ||
47 | =item * hashes of arrays | |
48 | ||
49 | =item * arrays of hashes | |
50 | ||
51 | =item * hashes of hashes | |
52 | ||
53 | =item * more elaborate constructs | |
54 | ||
4633a7c4 LW |
55 | =back |
56 | ||
5a964f20 TC |
57 | But for now, let's look at general issues common to all |
58 | these types of data structures. | |
4633a7c4 LW |
59 | |
60 | =head1 REFERENCES | |
d74e8afc | 61 | X<reference> X<dereference> X<dereferencing> X<pointer> |
4633a7c4 | 62 | |
1f025261 ML |
63 | The most important thing to understand about all data structures in |
64 | Perl--including multidimensional arrays--is that even though they might | |
4633a7c4 | 65 | appear otherwise, Perl C<@ARRAY>s and C<%HASH>es are all internally |
5f05dabc | 66 | one-dimensional. They can hold only scalar values (meaning a string, |
4633a7c4 LW |
67 | number, or a reference). They cannot directly contain other arrays or |
68 | hashes, but instead contain I<references> to other arrays or hashes. | |
d74e8afc | 69 | X<multidimensional array> X<array, multidimensional> |
4633a7c4 | 70 | |
d1be9408 | 71 | You can't use a reference to an array or hash in quite the same way that you |
5f05dabc | 72 | would a real array or hash. For C or C++ programmers unused to |
73 | distinguishing between arrays and pointers to the same, this can be | |
74 | confusing. If so, just think of it as the difference between a structure | |
75 | and a pointer to a structure. | |
4633a7c4 | 76 | |
ba555bf5 TH |
77 | You can (and should) read more about references in L<perlref>. |
78 | Briefly, references are rather like pointers that know what they | |
4633a7c4 | 79 | point to. (Objects are also a kind of reference, but we won't be needing |
4973169d | 80 | them right away--if ever.) This means that when you have something which |
81 | looks to you like an access to a two-or-more-dimensional array and/or hash, | |
82 | what's really going on is that the base type is | |
4633a7c4 LW |
83 | merely a one-dimensional entity that contains references to the next |
84 | level. It's just that you can I<use> it as though it were a | |
85 | two-dimensional one. This is actually the way almost all C | |
86 | multidimensional arrays work as well. | |
87 | ||
6a40a726 SF |
88 | $array[7][12] # array of arrays |
89 | $array[7]{string} # array of hashes | |
90 | $hash{string}[7] # hash of arrays | |
91 | $hash{string}{'another string'} # hash of hashes | |
4633a7c4 | 92 | |
5f05dabc | 93 | Now, because the top level contains only references, if you try to print |
4633a7c4 LW |
94 | out your array in with a simple print() function, you'll get something |
95 | that doesn't look very nice, like this: | |
96 | ||
5939083a | 97 | my @AoA = ( [2, 3], [4, 5, 7], [0] ); |
19799a22 | 98 | print $AoA[1][2]; |
4633a7c4 | 99 | 7 |
19799a22 | 100 | print @AoA; |
4633a7c4 LW |
101 | ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0) |
102 | ||
103 | ||
104 | That's because Perl doesn't (ever) implicitly dereference your variables. | |
105 | If you want to get at the thing a reference is referring to, then you have | |
106 | to do this yourself using either prefix typing indicators, like | |
107 | C<${$blah}>, C<@{$blah}>, C<@{$blah[$i]}>, or else postfix pointer arrows, | |
108 | like C<$a-E<gt>[3]>, C<$h-E<gt>{fred}>, or even C<$ob-E<gt>method()-E<gt>[3]>. | |
109 | ||
110 | =head1 COMMON MISTAKES | |
111 | ||
112 | The two most common mistakes made in constructing something like | |
113 | an array of arrays is either accidentally counting the number of | |
114 | elements or else taking a reference to the same memory location | |
115 | repeatedly. Here's the case where you just get the count instead | |
116 | of a nested array: | |
117 | ||
5939083a SF |
118 | for my $i (1..10) { |
119 | my @array = somefunc($i); | |
6a40a726 | 120 | $AoA[$i] = @array; # WRONG! |
4973169d | 121 | } |
4633a7c4 | 122 | |
19799a22 | 123 | That's just the simple case of assigning an array to a scalar and getting |
4633a7c4 LW |
124 | its element count. If that's what you really and truly want, then you |
125 | might do well to consider being a tad more explicit about it, like this: | |
126 | ||
5939083a SF |
127 | for my $i (1..10) { |
128 | my @array = somefunc($i); | |
6a40a726 | 129 | $counts[$i] = scalar @array; |
4973169d | 130 | } |
4633a7c4 | 131 | |
84f709e7 JH |
132 | Here's the case of taking a reference to the same memory location |
133 | again and again: | |
4633a7c4 | 134 | |
bd45a9fb KW |
135 | # Either without strict or having an outer-scope my @array; |
136 | # declaration. | |
5939083a SF |
137 | |
138 | for my $i (1..10) { | |
6a40a726 SF |
139 | @array = somefunc($i); |
140 | $AoA[$i] = \@array; # WRONG! | |
84f709e7 JH |
141 | } |
142 | ||
143 | So, what's the big problem with that? It looks right, doesn't it? | |
144 | After all, I just told you that you need an array of references, so by | |
145 | golly, you've made me one! | |
146 | ||
147 | Unfortunately, while this is true, it's still broken. All the references | |
148 | in @AoA refer to the I<very same place>, and they will therefore all hold | |
149 | whatever was last in @array! It's similar to the problem demonstrated in | |
150 | the following C program: | |
151 | ||
152 | #include <pwd.h> | |
153 | main() { | |
6a40a726 SF |
154 | struct passwd *getpwnam(), *rp, *dp; |
155 | rp = getpwnam("root"); | |
156 | dp = getpwnam("daemon"); | |
84f709e7 | 157 | |
6a40a726 SF |
158 | printf("daemon name is %s\nroot name is %s\n", |
159 | dp->pw_name, rp->pw_name); | |
84f709e7 JH |
160 | } |
161 | ||
162 | Which will print | |
163 | ||
164 | daemon name is daemon | |
165 | root name is daemon | |
166 | ||
167 | The problem is that both C<rp> and C<dp> are pointers to the same location | |
168 | in memory! In C, you'd have to remember to malloc() yourself some new | |
169 | memory. In Perl, you'll want to use the array constructor C<[]> or the | |
170 | hash constructor C<{}> instead. Here's the right way to do the preceding | |
171 | broken code fragments: | |
d74e8afc | 172 | X<[]> X<{}> |
84f709e7 | 173 | |
bd45a9fb KW |
174 | # Either without strict or having an outer-scope my @array; |
175 | # declaration. | |
5939083a SF |
176 | |
177 | for my $i (1..10) { | |
6a40a726 SF |
178 | @array = somefunc($i); |
179 | $AoA[$i] = [ @array ]; | |
4973169d | 180 | } |
4633a7c4 LW |
181 | |
182 | The square brackets make a reference to a new array with a I<copy> | |
84f709e7 JH |
183 | of what's in @array at the time of the assignment. This is what |
184 | you want. | |
4633a7c4 LW |
185 | |
186 | Note that this will produce something similar, but it's | |
187 | much harder to read: | |
188 | ||
bd45a9fb KW |
189 | # Either without strict or having an outer-scope my @array; |
190 | # declaration. | |
5939083a | 191 | for my $i (1..10) { |
6a40a726 SF |
192 | @array = 0 .. $i; |
193 | @{$AoA[$i]} = @array; | |
4973169d | 194 | } |
4633a7c4 LW |
195 | |
196 | Is it the same? Well, maybe so--and maybe not. The subtle difference | |
197 | is that when you assign something in square brackets, you know for sure | |
198 | it's always a brand new reference with a new I<copy> of the data. | |
b5d81ce9 | 199 | Something else could be going on in this new case with the C<@{$AoA[$i]}> |
4633a7c4 | 200 | dereference on the left-hand-side of the assignment. It all depends on |
19799a22 GS |
201 | whether C<$AoA[$i]> had been undefined to start with, or whether it |
202 | already contained a reference. If you had already populated @AoA with | |
4633a7c4 LW |
203 | references, as in |
204 | ||
19799a22 | 205 | $AoA[3] = \@another_array; |
4633a7c4 LW |
206 | |
207 | Then the assignment with the indirection on the left-hand-side would | |
208 | use the existing reference that was already there: | |
209 | ||
84f709e7 | 210 | @{$AoA[3]} = @array; |
4633a7c4 LW |
211 | |
212 | Of course, this I<would> have the "interesting" effect of clobbering | |
19799a22 | 213 | @another_array. (Have you ever noticed how when a programmer says |
4633a7c4 LW |
214 | something is "interesting", that rather than meaning "intriguing", |
215 | they're disturbingly more apt to mean that it's "annoying", | |
216 | "difficult", or both? :-) | |
217 | ||
5f05dabc | 218 | So just remember always to use the array or hash constructors with C<[]> |
4633a7c4 | 219 | or C<{}>, and you'll be fine, although it's not always optimally |
4973169d | 220 | efficient. |
4633a7c4 LW |
221 | |
222 | Surprisingly, the following dangerous-looking construct will | |
223 | actually work out fine: | |
224 | ||
5939083a | 225 | for my $i (1..10) { |
84f709e7 JH |
226 | my @array = somefunc($i); |
227 | $AoA[$i] = \@array; | |
4973169d | 228 | } |
4633a7c4 LW |
229 | |
230 | That's because my() is more of a run-time statement than it is a | |
231 | compile-time declaration I<per se>. This means that the my() variable is | |
232 | remade afresh each time through the loop. So even though it I<looks> as | |
233 | though you stored the same variable reference each time, you actually did | |
234 | not! This is a subtle distinction that can produce more efficient code at | |
235 | the risk of misleading all but the most experienced of programmers. So I | |
236 | usually advise against teaching it to beginners. In fact, except for | |
237 | passing arguments to functions, I seldom like to see the gimme-a-reference | |
238 | operator (backslash) used much at all in code. Instead, I advise | |
239 | beginners that they (and most of the rest of us) should try to use the | |
240 | much more easily understood constructors C<[]> and C<{}> instead of | |
241 | relying upon lexical (or dynamic) scoping and hidden reference-counting to | |
242 | do the right thing behind the scenes. | |
243 | ||
244 | In summary: | |
245 | ||
bd45a9fb KW |
246 | $AoA[$i] = [ @array ]; # usually best |
247 | $AoA[$i] = \@array; # perilous; just how my() was that array? | |
248 | @{ $AoA[$i] } = @array; # way too tricky for most programmers | |
4633a7c4 LW |
249 | |
250 | ||
4973169d | 251 | =head1 CAVEAT ON PRECEDENCE |
d74e8afc | 252 | X<dereference, precedence> X<dereferencing, precedence> |
4633a7c4 | 253 | |
84f709e7 | 254 | Speaking of things like C<@{$AoA[$i]}>, the following are actually the |
4633a7c4 | 255 | same thing: |
d74e8afc | 256 | X<< -> >> |
4633a7c4 | 257 | |
6a40a726 SF |
258 | $aref->[2][2] # clear |
259 | $$aref[2][2] # confusing | |
4633a7c4 LW |
260 | |
261 | That's because Perl's precedence rules on its five prefix dereferencers | |
262 | (which look like someone swearing: C<$ @ * % &>) make them bind more | |
263 | tightly than the postfix subscripting brackets or braces! This will no | |
264 | doubt come as a great shock to the C or C++ programmer, who is quite | |
265 | accustomed to using C<*a[i]> to mean what's pointed to by the I<i'th> | |
266 | element of C<a>. That is, they first take the subscript, and only then | |
267 | dereference the thing at that subscript. That's fine in C, but this isn't C. | |
268 | ||
19799a22 GS |
269 | The seemingly equivalent construct in Perl, C<$$aref[$i]> first does |
270 | the deref of $aref, making it take $aref as a reference to an | |
4633a7c4 | 271 | array, and then dereference that, and finally tell you the I<i'th> value |
19799a22 GS |
272 | of the array pointed to by $AoA. If you wanted the C notion, you'd have to |
273 | write C<${$AoA[$i]}> to force the C<$AoA[$i]> to get evaluated first | |
4633a7c4 LW |
274 | before the leading C<$> dereferencer. |
275 | ||
276 | =head1 WHY YOU SHOULD ALWAYS C<use strict> | |
277 | ||
278 | If this is starting to sound scarier than it's worth, relax. Perl has | |
279 | some features to help you avoid its most common pitfalls. The best | |
280 | way to avoid getting confused is to start every program like this: | |
281 | ||
282 | #!/usr/bin/perl -w | |
283 | use strict; | |
284 | ||
285 | This way, you'll be forced to declare all your variables with my() and | |
286 | also disallow accidental "symbolic dereferencing". Therefore if you'd done | |
287 | this: | |
288 | ||
19799a22 | 289 | my $aref = [ |
6a40a726 SF |
290 | [ "fred", "barney", "pebbles", "bambam", "dino", ], |
291 | [ "homer", "bart", "marge", "maggie", ], | |
292 | [ "george", "jane", "elroy", "judy", ], | |
4633a7c4 LW |
293 | ]; |
294 | ||
19799a22 | 295 | print $aref[2][2]; |
4633a7c4 LW |
296 | |
297 | The compiler would immediately flag that as an error I<at compile time>, | |
19799a22 | 298 | because you were accidentally accessing C<@aref>, an undeclared |
5f05dabc | 299 | variable, and it would thereby remind you to write instead: |
4633a7c4 | 300 | |
19799a22 | 301 | print $aref->[2][2] |
4633a7c4 LW |
302 | |
303 | =head1 DEBUGGING | |
d74e8afc ITB |
304 | X<data structure, debugging> X<complex data structure, debugging> |
305 | X<AoA, debugging> X<HoA, debugging> X<AoH, debugging> X<HoH, debugging> | |
306 | X<array of arrays, debugging> X<hash of arrays, debugging> | |
307 | X<array of hashes, debugging> X<hash of hashes, debugging> | |
4633a7c4 | 308 | |
cb1e035e BF |
309 | You can use the debugger's C<x> command to dump out complex data structures. |
310 | For example, given the assignment to $AoA above, here's the debugger output: | |
4633a7c4 | 311 | |
19799a22 GS |
312 | DB<1> x $AoA |
313 | $AoA = ARRAY(0x13b5a0) | |
4633a7c4 | 314 | 0 ARRAY(0x1f0a24) |
6a40a726 SF |
315 | 0 'fred' |
316 | 1 'barney' | |
317 | 2 'pebbles' | |
318 | 3 'bambam' | |
319 | 4 'dino' | |
4633a7c4 | 320 | 1 ARRAY(0x13b558) |
6a40a726 SF |
321 | 0 'homer' |
322 | 1 'bart' | |
323 | 2 'marge' | |
324 | 3 'maggie' | |
4633a7c4 | 325 | 2 ARRAY(0x13b540) |
6a40a726 SF |
326 | 0 'george' |
327 | 1 'jane' | |
328 | 2 'elroy' | |
329 | 3 'judy' | |
4633a7c4 | 330 | |
cb1a09d0 AD |
331 | =head1 CODE EXAMPLES |
332 | ||
54310121 | 333 | Presented with little comment (these will get their own manpages someday) |
4973169d | 334 | here are short code examples illustrating access of various |
cb1a09d0 AD |
335 | types of data structures. |
336 | ||
19799a22 | 337 | =head1 ARRAYS OF ARRAYS |
d74e8afc | 338 | X<array of arrays> X<AoA> |
cb1a09d0 | 339 | |
d1be9408 | 340 | =head2 Declaration of an ARRAY OF ARRAYS |
cb1a09d0 | 341 | |
84f709e7 JH |
342 | @AoA = ( |
343 | [ "fred", "barney" ], | |
344 | [ "george", "jane", "elroy" ], | |
345 | [ "homer", "marge", "bart" ], | |
cb1a09d0 AD |
346 | ); |
347 | ||
d1be9408 | 348 | =head2 Generation of an ARRAY OF ARRAYS |
cb1a09d0 AD |
349 | |
350 | # reading from file | |
351 | while ( <> ) { | |
19799a22 | 352 | push @AoA, [ split ]; |
4973169d | 353 | } |
cb1a09d0 AD |
354 | |
355 | # calling a function | |
84f709e7 | 356 | for $i ( 1 .. 10 ) { |
19799a22 | 357 | $AoA[$i] = [ somefunc($i) ]; |
4973169d | 358 | } |
cb1a09d0 AD |
359 | |
360 | # using temp vars | |
84f709e7 JH |
361 | for $i ( 1 .. 10 ) { |
362 | @tmp = somefunc($i); | |
363 | $AoA[$i] = [ @tmp ]; | |
4973169d | 364 | } |
cb1a09d0 AD |
365 | |
366 | # add to an existing row | |
84f709e7 | 367 | push @{ $AoA[0] }, "wilma", "betty"; |
cb1a09d0 | 368 | |
d1be9408 | 369 | =head2 Access and Printing of an ARRAY OF ARRAYS |
cb1a09d0 AD |
370 | |
371 | # one element | |
84f709e7 | 372 | $AoA[0][0] = "Fred"; |
cb1a09d0 AD |
373 | |
374 | # another element | |
19799a22 | 375 | $AoA[1][1] =~ s/(\w)/\u$1/; |
cb1a09d0 AD |
376 | |
377 | # print the whole thing with refs | |
84f709e7 | 378 | for $aref ( @AoA ) { |
cb1a09d0 | 379 | print "\t [ @$aref ],\n"; |
4973169d | 380 | } |
cb1a09d0 AD |
381 | |
382 | # print the whole thing with indices | |
84f709e7 JH |
383 | for $i ( 0 .. $#AoA ) { |
384 | print "\t [ @{$AoA[$i]} ],\n"; | |
4973169d | 385 | } |
cb1a09d0 AD |
386 | |
387 | # print the whole thing one at a time | |
84f709e7 JH |
388 | for $i ( 0 .. $#AoA ) { |
389 | for $j ( 0 .. $#{ $AoA[$i] } ) { | |
390 | print "elt $i $j is $AoA[$i][$j]\n"; | |
cb1a09d0 | 391 | } |
4973169d | 392 | } |
cb1a09d0 | 393 | |
19799a22 | 394 | =head1 HASHES OF ARRAYS |
d74e8afc | 395 | X<hash of arrays> X<HoA> |
cb1a09d0 | 396 | |
19799a22 | 397 | =head2 Declaration of a HASH OF ARRAYS |
cb1a09d0 | 398 | |
84f709e7 JH |
399 | %HoA = ( |
400 | flintstones => [ "fred", "barney" ], | |
401 | jetsons => [ "george", "jane", "elroy" ], | |
402 | simpsons => [ "homer", "marge", "bart" ], | |
cb1a09d0 AD |
403 | ); |
404 | ||
19799a22 | 405 | =head2 Generation of a HASH OF ARRAYS |
cb1a09d0 AD |
406 | |
407 | # reading from file | |
408 | # flintstones: fred barney wilma dino | |
409 | while ( <> ) { | |
84f709e7 | 410 | next unless s/^(.*?):\s*//; |
19799a22 | 411 | $HoA{$1} = [ split ]; |
4973169d | 412 | } |
cb1a09d0 AD |
413 | |
414 | # reading from file; more temps | |
415 | # flintstones: fred barney wilma dino | |
84f709e7 JH |
416 | while ( $line = <> ) { |
417 | ($who, $rest) = split /:\s*/, $line, 2; | |
418 | @fields = split ' ', $rest; | |
419 | $HoA{$who} = [ @fields ]; | |
4973169d | 420 | } |
cb1a09d0 AD |
421 | |
422 | # calling a function that returns a list | |
84f709e7 | 423 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
19799a22 | 424 | $HoA{$group} = [ get_family($group) ]; |
4973169d | 425 | } |
cb1a09d0 AD |
426 | |
427 | # likewise, but using temps | |
84f709e7 JH |
428 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
429 | @members = get_family($group); | |
430 | $HoA{$group} = [ @members ]; | |
4973169d | 431 | } |
cb1a09d0 AD |
432 | |
433 | # append new members to an existing family | |
84f709e7 | 434 | push @{ $HoA{"flintstones"} }, "wilma", "betty"; |
cb1a09d0 | 435 | |
19799a22 | 436 | =head2 Access and Printing of a HASH OF ARRAYS |
cb1a09d0 AD |
437 | |
438 | # one element | |
84f709e7 | 439 | $HoA{flintstones}[0] = "Fred"; |
cb1a09d0 AD |
440 | |
441 | # another element | |
19799a22 | 442 | $HoA{simpsons}[1] =~ s/(\w)/\u$1/; |
cb1a09d0 AD |
443 | |
444 | # print the whole thing | |
84f709e7 JH |
445 | foreach $family ( keys %HoA ) { |
446 | print "$family: @{ $HoA{$family} }\n" | |
4973169d | 447 | } |
cb1a09d0 AD |
448 | |
449 | # print the whole thing with indices | |
84f709e7 JH |
450 | foreach $family ( keys %HoA ) { |
451 | print "family: "; | |
452 | foreach $i ( 0 .. $#{ $HoA{$family} } ) { | |
19799a22 | 453 | print " $i = $HoA{$family}[$i]"; |
cb1a09d0 AD |
454 | } |
455 | print "\n"; | |
4973169d | 456 | } |
cb1a09d0 AD |
457 | |
458 | # print the whole thing sorted by number of members | |
84f709e7 | 459 | foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) { |
19799a22 | 460 | print "$family: @{ $HoA{$family} }\n" |
4973169d | 461 | } |
cb1a09d0 AD |
462 | |
463 | # print the whole thing sorted by number of members and name | |
84f709e7 | 464 | foreach $family ( sort { |
6a40a726 SF |
465 | @{$HoA{$b}} <=> @{$HoA{$a}} |
466 | || | |
467 | $a cmp $b | |
468 | } keys %HoA ) | |
84f709e7 | 469 | { |
19799a22 | 470 | print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n"; |
4973169d | 471 | } |
cb1a09d0 | 472 | |
19799a22 | 473 | =head1 ARRAYS OF HASHES |
d74e8afc | 474 | X<array of hashes> X<AoH> |
cb1a09d0 | 475 | |
d1be9408 | 476 | =head2 Declaration of an ARRAY OF HASHES |
cb1a09d0 | 477 | |
84f709e7 | 478 | @AoH = ( |
cb1a09d0 | 479 | { |
84f709e7 JH |
480 | Lead => "fred", |
481 | Friend => "barney", | |
cb1a09d0 AD |
482 | }, |
483 | { | |
84f709e7 JH |
484 | Lead => "george", |
485 | Wife => "jane", | |
486 | Son => "elroy", | |
cb1a09d0 AD |
487 | }, |
488 | { | |
84f709e7 JH |
489 | Lead => "homer", |
490 | Wife => "marge", | |
491 | Son => "bart", | |
cb1a09d0 AD |
492 | } |
493 | ); | |
494 | ||
d1be9408 | 495 | =head2 Generation of an ARRAY OF HASHES |
cb1a09d0 AD |
496 | |
497 | # reading from file | |
498 | # format: LEAD=fred FRIEND=barney | |
499 | while ( <> ) { | |
84f709e7 JH |
500 | $rec = {}; |
501 | for $field ( split ) { | |
502 | ($key, $value) = split /=/, $field; | |
503 | $rec->{$key} = $value; | |
cb1a09d0 | 504 | } |
19799a22 | 505 | push @AoH, $rec; |
4973169d | 506 | } |
cb1a09d0 AD |
507 | |
508 | ||
509 | # reading from file | |
510 | # format: LEAD=fred FRIEND=barney | |
511 | # no temp | |
512 | while ( <> ) { | |
19799a22 | 513 | push @AoH, { split /[\s+=]/ }; |
4973169d | 514 | } |
cb1a09d0 | 515 | |
19799a22 | 516 | # calling a function that returns a key/value pair list, like |
84f709e7 JH |
517 | # "lead","fred","daughter","pebbles" |
518 | while ( %fields = getnextpairset() ) { | |
19799a22 | 519 | push @AoH, { %fields }; |
4973169d | 520 | } |
cb1a09d0 AD |
521 | |
522 | # likewise, but using no temp vars | |
523 | while (<>) { | |
19799a22 | 524 | push @AoH, { parsepairs($_) }; |
4973169d | 525 | } |
cb1a09d0 AD |
526 | |
527 | # add key/value to an element | |
84f709e7 | 528 | $AoH[0]{pet} = "dino"; |
19799a22 | 529 | $AoH[2]{pet} = "santa's little helper"; |
cb1a09d0 | 530 | |
d1be9408 | 531 | =head2 Access and Printing of an ARRAY OF HASHES |
cb1a09d0 AD |
532 | |
533 | # one element | |
84f709e7 | 534 | $AoH[0]{lead} = "fred"; |
cb1a09d0 AD |
535 | |
536 | # another element | |
19799a22 | 537 | $AoH[1]{lead} =~ s/(\w)/\u$1/; |
cb1a09d0 AD |
538 | |
539 | # print the whole thing with refs | |
84f709e7 JH |
540 | for $href ( @AoH ) { |
541 | print "{ "; | |
542 | for $role ( keys %$href ) { | |
543 | print "$role=$href->{$role} "; | |
cb1a09d0 AD |
544 | } |
545 | print "}\n"; | |
4973169d | 546 | } |
cb1a09d0 AD |
547 | |
548 | # print the whole thing with indices | |
84f709e7 | 549 | for $i ( 0 .. $#AoH ) { |
cb1a09d0 | 550 | print "$i is { "; |
84f709e7 JH |
551 | for $role ( keys %{ $AoH[$i] } ) { |
552 | print "$role=$AoH[$i]{$role} "; | |
cb1a09d0 AD |
553 | } |
554 | print "}\n"; | |
4973169d | 555 | } |
cb1a09d0 AD |
556 | |
557 | # print the whole thing one at a time | |
84f709e7 JH |
558 | for $i ( 0 .. $#AoH ) { |
559 | for $role ( keys %{ $AoH[$i] } ) { | |
560 | print "elt $i $role is $AoH[$i]{$role}\n"; | |
cb1a09d0 | 561 | } |
4973169d | 562 | } |
cb1a09d0 AD |
563 | |
564 | =head1 HASHES OF HASHES | |
8e0aa7ce | 565 | X<hash of hashes> X<HoH> |
cb1a09d0 AD |
566 | |
567 | =head2 Declaration of a HASH OF HASHES | |
568 | ||
84f709e7 | 569 | %HoH = ( |
28757baa | 570 | flintstones => { |
6a40a726 SF |
571 | lead => "fred", |
572 | pal => "barney", | |
cb1a09d0 | 573 | }, |
28757baa | 574 | jetsons => { |
6a40a726 SF |
575 | lead => "george", |
576 | wife => "jane", | |
577 | "his boy" => "elroy", | |
4973169d | 578 | }, |
28757baa | 579 | simpsons => { |
6a40a726 SF |
580 | lead => "homer", |
581 | wife => "marge", | |
582 | kid => "bart", | |
583 | }, | |
4973169d | 584 | ); |
cb1a09d0 AD |
585 | |
586 | =head2 Generation of a HASH OF HASHES | |
587 | ||
588 | # reading from file | |
589 | # flintstones: lead=fred pal=barney wife=wilma pet=dino | |
590 | while ( <> ) { | |
84f709e7 JH |
591 | next unless s/^(.*?):\s*//; |
592 | $who = $1; | |
593 | for $field ( split ) { | |
594 | ($key, $value) = split /=/, $field; | |
cb1a09d0 AD |
595 | $HoH{$who}{$key} = $value; |
596 | } | |
597 | ||
598 | ||
599 | # reading from file; more temps | |
600 | while ( <> ) { | |
84f709e7 JH |
601 | next unless s/^(.*?):\s*//; |
602 | $who = $1; | |
603 | $rec = {}; | |
cb1a09d0 | 604 | $HoH{$who} = $rec; |
84f709e7 JH |
605 | for $field ( split ) { |
606 | ($key, $value) = split /=/, $field; | |
607 | $rec->{$key} = $value; | |
cb1a09d0 | 608 | } |
4973169d | 609 | } |
cb1a09d0 | 610 | |
cb1a09d0 | 611 | # calling a function that returns a key,value hash |
84f709e7 | 612 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
cb1a09d0 | 613 | $HoH{$group} = { get_family($group) }; |
4973169d | 614 | } |
cb1a09d0 AD |
615 | |
616 | # likewise, but using temps | |
84f709e7 JH |
617 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
618 | %members = get_family($group); | |
cb1a09d0 | 619 | $HoH{$group} = { %members }; |
4973169d | 620 | } |
cb1a09d0 AD |
621 | |
622 | # append new members to an existing family | |
84f709e7 JH |
623 | %new_folks = ( |
624 | wife => "wilma", | |
625 | pet => "dino", | |
cb1a09d0 | 626 | ); |
4973169d | 627 | |
84f709e7 | 628 | for $what (keys %new_folks) { |
cb1a09d0 | 629 | $HoH{flintstones}{$what} = $new_folks{$what}; |
4973169d | 630 | } |
cb1a09d0 AD |
631 | |
632 | =head2 Access and Printing of a HASH OF HASHES | |
633 | ||
634 | # one element | |
84f709e7 | 635 | $HoH{flintstones}{wife} = "wilma"; |
cb1a09d0 AD |
636 | |
637 | # another element | |
638 | $HoH{simpsons}{lead} =~ s/(\w)/\u$1/; | |
639 | ||
640 | # print the whole thing | |
84f709e7 | 641 | foreach $family ( keys %HoH ) { |
1fef88e7 | 642 | print "$family: { "; |
84f709e7 JH |
643 | for $role ( keys %{ $HoH{$family} } ) { |
644 | print "$role=$HoH{$family}{$role} "; | |
cb1a09d0 AD |
645 | } |
646 | print "}\n"; | |
4973169d | 647 | } |
cb1a09d0 AD |
648 | |
649 | # print the whole thing somewhat sorted | |
84f709e7 | 650 | foreach $family ( sort keys %HoH ) { |
1fef88e7 | 651 | print "$family: { "; |
84f709e7 JH |
652 | for $role ( sort keys %{ $HoH{$family} } ) { |
653 | print "$role=$HoH{$family}{$role} "; | |
cb1a09d0 AD |
654 | } |
655 | print "}\n"; | |
4973169d | 656 | } |
cb1a09d0 | 657 | |
84f709e7 | 658 | |
cb1a09d0 | 659 | # print the whole thing sorted by number of members |
bd45a9fb KW |
660 | foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } |
661 | keys %HoH ) | |
662 | { | |
1fef88e7 | 663 | print "$family: { "; |
84f709e7 JH |
664 | for $role ( sort keys %{ $HoH{$family} } ) { |
665 | print "$role=$HoH{$family}{$role} "; | |
cb1a09d0 AD |
666 | } |
667 | print "}\n"; | |
4973169d | 668 | } |
cb1a09d0 AD |
669 | |
670 | # establish a sort order (rank) for each role | |
84f709e7 JH |
671 | $i = 0; |
672 | for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i } | |
cb1a09d0 AD |
673 | |
674 | # now print the whole thing sorted by number of members | |
bd45a9fb KW |
675 | foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } |
676 | keys %HoH ) | |
677 | { | |
1fef88e7 | 678 | print "$family: { "; |
cb1a09d0 | 679 | # and print these according to rank order |
bd45a9fb KW |
680 | for $role ( sort { $rank{$a} <=> $rank{$b} } |
681 | keys %{ $HoH{$family} } ) | |
682 | { | |
84f709e7 | 683 | print "$role=$HoH{$family}{$role} "; |
cb1a09d0 AD |
684 | } |
685 | print "}\n"; | |
4973169d | 686 | } |
cb1a09d0 AD |
687 | |
688 | ||
689 | =head1 MORE ELABORATE RECORDS | |
d74e8afc | 690 | X<record> X<structure> X<struct> |
cb1a09d0 AD |
691 | |
692 | =head2 Declaration of MORE ELABORATE RECORDS | |
693 | ||
694 | Here's a sample showing how to create and use a record whose fields are of | |
695 | many different sorts: | |
696 | ||
84f709e7 | 697 | $rec = { |
6a40a726 SF |
698 | TEXT => $string, |
699 | SEQUENCE => [ @old_values ], | |
700 | LOOKUP => { %some_table }, | |
701 | THATCODE => \&some_function, | |
702 | THISCODE => sub { $_[0] ** $_[1] }, | |
703 | HANDLE => \*STDOUT, | |
cb1a09d0 AD |
704 | }; |
705 | ||
4973169d | 706 | print $rec->{TEXT}; |
cb1a09d0 | 707 | |
84f709e7 JH |
708 | print $rec->{SEQUENCE}[0]; |
709 | $last = pop @ { $rec->{SEQUENCE} }; | |
cb1a09d0 | 710 | |
84f709e7 JH |
711 | print $rec->{LOOKUP}{"key"}; |
712 | ($first_k, $first_v) = each %{ $rec->{LOOKUP} }; | |
cb1a09d0 | 713 | |
84f709e7 JH |
714 | $answer = $rec->{THATCODE}->($arg); |
715 | $answer = $rec->{THISCODE}->($arg1, $arg2); | |
cb1a09d0 AD |
716 | |
717 | # careful of extra block braces on fh ref | |
4973169d | 718 | print { $rec->{HANDLE} } "a string\n"; |
cb1a09d0 AD |
719 | |
720 | use FileHandle; | |
4973169d | 721 | $rec->{HANDLE}->autoflush(1); |
722 | $rec->{HANDLE}->print(" a string\n"); | |
cb1a09d0 AD |
723 | |
724 | =head2 Declaration of a HASH OF COMPLEX RECORDS | |
725 | ||
84f709e7 | 726 | %TV = ( |
28757baa | 727 | flintstones => { |
84f709e7 | 728 | series => "flintstones", |
4973169d | 729 | nights => [ qw(monday thursday friday) ], |
cb1a09d0 | 730 | members => [ |
84f709e7 JH |
731 | { name => "fred", role => "lead", age => 36, }, |
732 | { name => "wilma", role => "wife", age => 31, }, | |
733 | { name => "pebbles", role => "kid", age => 4, }, | |
cb1a09d0 AD |
734 | ], |
735 | }, | |
736 | ||
28757baa | 737 | jetsons => { |
84f709e7 | 738 | series => "jetsons", |
4973169d | 739 | nights => [ qw(wednesday saturday) ], |
cb1a09d0 | 740 | members => [ |
84f709e7 JH |
741 | { name => "george", role => "lead", age => 41, }, |
742 | { name => "jane", role => "wife", age => 39, }, | |
743 | { name => "elroy", role => "kid", age => 9, }, | |
cb1a09d0 AD |
744 | ], |
745 | }, | |
746 | ||
28757baa | 747 | simpsons => { |
84f709e7 | 748 | series => "simpsons", |
4973169d | 749 | nights => [ qw(monday) ], |
cb1a09d0 | 750 | members => [ |
84f709e7 JH |
751 | { name => "homer", role => "lead", age => 34, }, |
752 | { name => "marge", role => "wife", age => 37, }, | |
753 | { name => "bart", role => "kid", age => 11, }, | |
cb1a09d0 AD |
754 | ], |
755 | }, | |
756 | ); | |
757 | ||
758 | =head2 Generation of a HASH OF COMPLEX RECORDS | |
759 | ||
84f709e7 JH |
760 | # reading from file |
761 | # this is most easily done by having the file itself be | |
762 | # in the raw data format as shown above. perl is happy | |
763 | # to parse complex data structures if declared as data, so | |
764 | # sometimes it's easiest to do that | |
cb1a09d0 | 765 | |
84f709e7 JH |
766 | # here's a piece by piece build up |
767 | $rec = {}; | |
768 | $rec->{series} = "flintstones"; | |
cb1a09d0 AD |
769 | $rec->{nights} = [ find_days() ]; |
770 | ||
84f709e7 | 771 | @members = (); |
cb1a09d0 | 772 | # assume this file in field=value syntax |
84f709e7 JH |
773 | while (<>) { |
774 | %fields = split /[\s=]+/; | |
cb1a09d0 AD |
775 | push @members, { %fields }; |
776 | } | |
777 | $rec->{members} = [ @members ]; | |
778 | ||
779 | # now remember the whole thing | |
780 | $TV{ $rec->{series} } = $rec; | |
781 | ||
84f709e7 JH |
782 | ########################################################### |
783 | # now, you might want to make interesting extra fields that | |
784 | # include pointers back into the same data structure so if | |
785 | # change one piece, it changes everywhere, like for example | |
786 | # if you wanted a {kids} field that was a reference | |
787 | # to an array of the kids' records without having duplicate | |
788 | # records and thus update problems. | |
789 | ########################################################### | |
790 | foreach $family (keys %TV) { | |
791 | $rec = $TV{$family}; # temp pointer | |
792 | @kids = (); | |
793 | for $person ( @{ $rec->{members} } ) { | |
794 | if ($person->{role} =~ /kid|son|daughter/) { | |
cb1a09d0 AD |
795 | push @kids, $person; |
796 | } | |
797 | } | |
798 | # REMEMBER: $rec and $TV{$family} point to same data!! | |
799 | $rec->{kids} = [ @kids ]; | |
800 | } | |
801 | ||
84f709e7 JH |
802 | # you copied the array, but the array itself contains pointers |
803 | # to uncopied objects. this means that if you make bart get | |
804 | # older via | |
cb1a09d0 AD |
805 | |
806 | $TV{simpsons}{kids}[0]{age}++; | |
807 | ||
84f709e7 JH |
808 | # then this would also change in |
809 | print $TV{simpsons}{members}[2]{age}; | |
cb1a09d0 | 810 | |
84f709e7 JH |
811 | # because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2] |
812 | # both point to the same underlying anonymous hash table | |
6ba6f0ec | 813 | |
84f709e7 JH |
814 | # print the whole thing |
815 | foreach $family ( keys %TV ) { | |
816 | print "the $family"; | |
817 | print " is on during @{ $TV{$family}{nights} }\n"; | |
818 | print "its members are:\n"; | |
819 | for $who ( @{ $TV{$family}{members} } ) { | |
cb1a09d0 AD |
820 | print " $who->{name} ($who->{role}), age $who->{age}\n"; |
821 | } | |
84f709e7 JH |
822 | print "it turns out that $TV{$family}{lead} has "; |
823 | print scalar ( @{ $TV{$family}{kids} } ), " kids named "; | |
824 | print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } ); | |
825 | print "\n"; | |
cb1a09d0 AD |
826 | } |
827 | ||
c07a80fd | 828 | =head1 Database Ties |
829 | ||
830 | You cannot easily tie a multilevel data structure (such as a hash of | |
831 | hashes) to a dbm file. The first problem is that all but GDBM and | |
832 | Berkeley DB have size limitations, but beyond that, you also have problems | |
833 | with how references are to be represented on disk. One experimental | |
5f05dabc | 834 | module that does partially attempt to address this need is the MLDBM |
f102b883 | 835 | module. Check your nearest CPAN site as described in L<perlmodlib> for |
c07a80fd | 836 | source code to MLDBM. |
837 | ||
4633a7c4 LW |
838 | =head1 SEE ALSO |
839 | ||
ba555bf5 | 840 | L<perlref>, L<perllol>, L<perldata>, L<perlobj> |
4633a7c4 LW |
841 | |
842 | =head1 AUTHOR | |
843 | ||
9607fc9c | 844 | Tom Christiansen <F<tchrist@perl.com>> |