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 LW |
7 | |
8 | The single feature most sorely lacking in the Perl programming language | |
9 | prior to its 5.0 release was complex data structures. Even without direct | |
10 | language support, some valiant programmers did manage to emulate them, but | |
11 | it was hard work and not for the faint of heart. You could occasionally | |
19799a22 GS |
12 | get away with the C<$m{$AoA,$b}> notation borrowed from B<awk> in which the |
13 | keys are actually more like a single concatenated string C<"$AoA$b">, but | |
4633a7c4 LW |
14 | traversal and sorting were difficult. More desperate programmers even |
15 | hacked Perl's internal symbol table directly, a strategy that proved hard | |
16 | to develop and maintain--to put it mildly. | |
17 | ||
18 | The 5.0 release of Perl let us have complex data structures. You | |
d1be9408 | 19 | may now write something like this and all of a sudden, you'd have an array |
4633a7c4 LW |
20 | with three dimensions! |
21 | ||
84f709e7 JH |
22 | for $x (1 .. 10) { |
23 | for $y (1 .. 10) { | |
24 | for $z (1 .. 10) { | |
25 | $AoA[$x][$y][$z] = | |
26 | $x ** $y + $z; | |
4633a7c4 LW |
27 | } |
28 | } | |
29 | } | |
30 | ||
31 | Alas, however simple this may appear, underneath it's a much more | |
32 | elaborate construct than meets the eye! | |
33 | ||
19799a22 | 34 | How do you print it out? Why can't you say just C<print @AoA>? How do |
4633a7c4 | 35 | you sort it? How can you pass it to a function or get one of these back |
d1be9408 | 36 | from a function? Is it an object? Can you save it to disk to read |
4633a7c4 | 37 | back later? How do you access whole rows or columns of that matrix? Do |
4973169d | 38 | all the values have to be numeric? |
4633a7c4 LW |
39 | |
40 | As you see, it's quite easy to become confused. While some small portion | |
41 | of the blame for this can be attributed to the reference-based | |
42 | implementation, it's really more due to a lack of existing documentation with | |
43 | examples designed for the beginner. | |
44 | ||
5f05dabc | 45 | This document is meant to be a detailed but understandable treatment of the |
46 | many different sorts of data structures you might want to develop. It | |
47 | should also serve as a cookbook of examples. That way, when you need to | |
48 | create one of these complex data structures, you can just pinch, pilfer, or | |
49 | purloin a drop-in example from here. | |
4633a7c4 LW |
50 | |
51 | Let's look at each of these possible constructs in detail. There are separate | |
28757baa | 52 | sections on each of the following: |
4633a7c4 LW |
53 | |
54 | =over 5 | |
55 | ||
56 | =item * arrays of arrays | |
57 | ||
58 | =item * hashes of arrays | |
59 | ||
60 | =item * arrays of hashes | |
61 | ||
62 | =item * hashes of hashes | |
63 | ||
64 | =item * more elaborate constructs | |
65 | ||
4633a7c4 LW |
66 | =back |
67 | ||
5a964f20 TC |
68 | But for now, let's look at general issues common to all |
69 | these types of data structures. | |
4633a7c4 LW |
70 | |
71 | =head1 REFERENCES | |
d74e8afc | 72 | X<reference> X<dereference> X<dereferencing> X<pointer> |
4633a7c4 | 73 | |
1f025261 ML |
74 | The most important thing to understand about all data structures in |
75 | Perl--including multidimensional arrays--is that even though they might | |
4633a7c4 | 76 | appear otherwise, Perl C<@ARRAY>s and C<%HASH>es are all internally |
5f05dabc | 77 | one-dimensional. They can hold only scalar values (meaning a string, |
4633a7c4 LW |
78 | number, or a reference). They cannot directly contain other arrays or |
79 | hashes, but instead contain I<references> to other arrays or hashes. | |
d74e8afc | 80 | X<multidimensional array> X<array, multidimensional> |
4633a7c4 | 81 | |
d1be9408 | 82 | You can't use a reference to an array or hash in quite the same way that you |
5f05dabc | 83 | would a real array or hash. For C or C++ programmers unused to |
84 | distinguishing between arrays and pointers to the same, this can be | |
85 | confusing. If so, just think of it as the difference between a structure | |
86 | and a pointer to a structure. | |
4633a7c4 | 87 | |
ba555bf5 TH |
88 | You can (and should) read more about references in L<perlref>. |
89 | Briefly, references are rather like pointers that know what they | |
4633a7c4 | 90 | point to. (Objects are also a kind of reference, but we won't be needing |
4973169d | 91 | them right away--if ever.) This means that when you have something which |
92 | looks to you like an access to a two-or-more-dimensional array and/or hash, | |
93 | what's really going on is that the base type is | |
4633a7c4 LW |
94 | merely a one-dimensional entity that contains references to the next |
95 | level. It's just that you can I<use> it as though it were a | |
96 | two-dimensional one. This is actually the way almost all C | |
97 | multidimensional arrays work as well. | |
98 | ||
19799a22 GS |
99 | $array[7][12] # array of arrays |
100 | $array[7]{string} # array of hashes | |
4633a7c4 LW |
101 | $hash{string}[7] # hash of arrays |
102 | $hash{string}{'another string'} # hash of hashes | |
103 | ||
5f05dabc | 104 | Now, because the top level contains only references, if you try to print |
4633a7c4 LW |
105 | out your array in with a simple print() function, you'll get something |
106 | that doesn't look very nice, like this: | |
107 | ||
84f709e7 | 108 | @AoA = ( [2, 3], [4, 5, 7], [0] ); |
19799a22 | 109 | print $AoA[1][2]; |
4633a7c4 | 110 | 7 |
19799a22 | 111 | print @AoA; |
4633a7c4 LW |
112 | ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0) |
113 | ||
114 | ||
115 | That's because Perl doesn't (ever) implicitly dereference your variables. | |
116 | If you want to get at the thing a reference is referring to, then you have | |
117 | to do this yourself using either prefix typing indicators, like | |
118 | C<${$blah}>, C<@{$blah}>, C<@{$blah[$i]}>, or else postfix pointer arrows, | |
119 | like C<$a-E<gt>[3]>, C<$h-E<gt>{fred}>, or even C<$ob-E<gt>method()-E<gt>[3]>. | |
120 | ||
121 | =head1 COMMON MISTAKES | |
122 | ||
123 | The two most common mistakes made in constructing something like | |
124 | an array of arrays is either accidentally counting the number of | |
125 | elements or else taking a reference to the same memory location | |
126 | repeatedly. Here's the case where you just get the count instead | |
127 | of a nested array: | |
128 | ||
84f709e7 JH |
129 | for $i (1..10) { |
130 | @array = somefunc($i); | |
131 | $AoA[$i] = @array; # WRONG! | |
4973169d | 132 | } |
4633a7c4 | 133 | |
19799a22 | 134 | That's just the simple case of assigning an array to a scalar and getting |
4633a7c4 LW |
135 | its element count. If that's what you really and truly want, then you |
136 | might do well to consider being a tad more explicit about it, like this: | |
137 | ||
84f709e7 JH |
138 | for $i (1..10) { |
139 | @array = somefunc($i); | |
140 | $counts[$i] = scalar @array; | |
4973169d | 141 | } |
4633a7c4 | 142 | |
84f709e7 JH |
143 | Here's the case of taking a reference to the same memory location |
144 | again and again: | |
4633a7c4 | 145 | |
84f709e7 JH |
146 | for $i (1..10) { |
147 | @array = somefunc($i); | |
148 | $AoA[$i] = \@array; # WRONG! | |
149 | } | |
150 | ||
151 | So, what's the big problem with that? It looks right, doesn't it? | |
152 | After all, I just told you that you need an array of references, so by | |
153 | golly, you've made me one! | |
154 | ||
155 | Unfortunately, while this is true, it's still broken. All the references | |
156 | in @AoA refer to the I<very same place>, and they will therefore all hold | |
157 | whatever was last in @array! It's similar to the problem demonstrated in | |
158 | the following C program: | |
159 | ||
160 | #include <pwd.h> | |
161 | main() { | |
162 | struct passwd *getpwnam(), *rp, *dp; | |
163 | rp = getpwnam("root"); | |
164 | dp = getpwnam("daemon"); | |
165 | ||
166 | printf("daemon name is %s\nroot name is %s\n", | |
167 | dp->pw_name, rp->pw_name); | |
168 | } | |
169 | ||
170 | Which will print | |
171 | ||
172 | daemon name is daemon | |
173 | root name is daemon | |
174 | ||
175 | The problem is that both C<rp> and C<dp> are pointers to the same location | |
176 | in memory! In C, you'd have to remember to malloc() yourself some new | |
177 | memory. In Perl, you'll want to use the array constructor C<[]> or the | |
178 | hash constructor C<{}> instead. Here's the right way to do the preceding | |
179 | broken code fragments: | |
d74e8afc | 180 | X<[]> X<{}> |
84f709e7 JH |
181 | |
182 | for $i (1..10) { | |
183 | @array = somefunc($i); | |
184 | $AoA[$i] = [ @array ]; | |
4973169d | 185 | } |
4633a7c4 LW |
186 | |
187 | The square brackets make a reference to a new array with a I<copy> | |
84f709e7 JH |
188 | of what's in @array at the time of the assignment. This is what |
189 | you want. | |
4633a7c4 LW |
190 | |
191 | Note that this will produce something similar, but it's | |
192 | much harder to read: | |
193 | ||
84f709e7 JH |
194 | for $i (1..10) { |
195 | @array = 0 .. $i; | |
196 | @{$AoA[$i]} = @array; | |
4973169d | 197 | } |
4633a7c4 LW |
198 | |
199 | Is it the same? Well, maybe so--and maybe not. The subtle difference | |
200 | is that when you assign something in square brackets, you know for sure | |
201 | it's always a brand new reference with a new I<copy> of the data. | |
b5d81ce9 | 202 | Something else could be going on in this new case with the C<@{$AoA[$i]}> |
4633a7c4 | 203 | dereference on the left-hand-side of the assignment. It all depends on |
19799a22 GS |
204 | whether C<$AoA[$i]> had been undefined to start with, or whether it |
205 | already contained a reference. If you had already populated @AoA with | |
4633a7c4 LW |
206 | references, as in |
207 | ||
19799a22 | 208 | $AoA[3] = \@another_array; |
4633a7c4 LW |
209 | |
210 | Then the assignment with the indirection on the left-hand-side would | |
211 | use the existing reference that was already there: | |
212 | ||
84f709e7 | 213 | @{$AoA[3]} = @array; |
4633a7c4 LW |
214 | |
215 | Of course, this I<would> have the "interesting" effect of clobbering | |
19799a22 | 216 | @another_array. (Have you ever noticed how when a programmer says |
4633a7c4 LW |
217 | something is "interesting", that rather than meaning "intriguing", |
218 | they're disturbingly more apt to mean that it's "annoying", | |
219 | "difficult", or both? :-) | |
220 | ||
5f05dabc | 221 | So just remember always to use the array or hash constructors with C<[]> |
4633a7c4 | 222 | or C<{}>, and you'll be fine, although it's not always optimally |
4973169d | 223 | efficient. |
4633a7c4 LW |
224 | |
225 | Surprisingly, the following dangerous-looking construct will | |
226 | actually work out fine: | |
227 | ||
84f709e7 JH |
228 | for $i (1..10) { |
229 | my @array = somefunc($i); | |
230 | $AoA[$i] = \@array; | |
4973169d | 231 | } |
4633a7c4 LW |
232 | |
233 | That's because my() is more of a run-time statement than it is a | |
234 | compile-time declaration I<per se>. This means that the my() variable is | |
235 | remade afresh each time through the loop. So even though it I<looks> as | |
236 | though you stored the same variable reference each time, you actually did | |
237 | not! This is a subtle distinction that can produce more efficient code at | |
238 | the risk of misleading all but the most experienced of programmers. So I | |
239 | usually advise against teaching it to beginners. In fact, except for | |
240 | passing arguments to functions, I seldom like to see the gimme-a-reference | |
241 | operator (backslash) used much at all in code. Instead, I advise | |
242 | beginners that they (and most of the rest of us) should try to use the | |
243 | much more easily understood constructors C<[]> and C<{}> instead of | |
244 | relying upon lexical (or dynamic) scoping and hidden reference-counting to | |
245 | do the right thing behind the scenes. | |
246 | ||
247 | In summary: | |
248 | ||
84f709e7 JH |
249 | $AoA[$i] = [ @array ]; # usually best |
250 | $AoA[$i] = \@array; # perilous; just how my() was that array? | |
251 | @{ $AoA[$i] } = @array; # way too tricky for most programmers | |
4633a7c4 LW |
252 | |
253 | ||
4973169d | 254 | =head1 CAVEAT ON PRECEDENCE |
d74e8afc | 255 | X<dereference, precedence> X<dereferencing, precedence> |
4633a7c4 | 256 | |
84f709e7 | 257 | Speaking of things like C<@{$AoA[$i]}>, the following are actually the |
4633a7c4 | 258 | same thing: |
d74e8afc | 259 | X<< -> >> |
4633a7c4 | 260 | |
19799a22 GS |
261 | $aref->[2][2] # clear |
262 | $$aref[2][2] # confusing | |
4633a7c4 LW |
263 | |
264 | That's because Perl's precedence rules on its five prefix dereferencers | |
265 | (which look like someone swearing: C<$ @ * % &>) make them bind more | |
266 | tightly than the postfix subscripting brackets or braces! This will no | |
267 | doubt come as a great shock to the C or C++ programmer, who is quite | |
268 | accustomed to using C<*a[i]> to mean what's pointed to by the I<i'th> | |
269 | element of C<a>. That is, they first take the subscript, and only then | |
270 | dereference the thing at that subscript. That's fine in C, but this isn't C. | |
271 | ||
19799a22 GS |
272 | The seemingly equivalent construct in Perl, C<$$aref[$i]> first does |
273 | the deref of $aref, making it take $aref as a reference to an | |
4633a7c4 | 274 | array, and then dereference that, and finally tell you the I<i'th> value |
19799a22 GS |
275 | of the array pointed to by $AoA. If you wanted the C notion, you'd have to |
276 | write C<${$AoA[$i]}> to force the C<$AoA[$i]> to get evaluated first | |
4633a7c4 LW |
277 | before the leading C<$> dereferencer. |
278 | ||
279 | =head1 WHY YOU SHOULD ALWAYS C<use strict> | |
280 | ||
281 | If this is starting to sound scarier than it's worth, relax. Perl has | |
282 | some features to help you avoid its most common pitfalls. The best | |
283 | way to avoid getting confused is to start every program like this: | |
284 | ||
285 | #!/usr/bin/perl -w | |
286 | use strict; | |
287 | ||
288 | This way, you'll be forced to declare all your variables with my() and | |
289 | also disallow accidental "symbolic dereferencing". Therefore if you'd done | |
290 | this: | |
291 | ||
19799a22 | 292 | my $aref = [ |
84f709e7 JH |
293 | [ "fred", "barney", "pebbles", "bambam", "dino", ], |
294 | [ "homer", "bart", "marge", "maggie", ], | |
295 | [ "george", "jane", "elroy", "judy", ], | |
4633a7c4 LW |
296 | ]; |
297 | ||
19799a22 | 298 | print $aref[2][2]; |
4633a7c4 LW |
299 | |
300 | The compiler would immediately flag that as an error I<at compile time>, | |
19799a22 | 301 | because you were accidentally accessing C<@aref>, an undeclared |
5f05dabc | 302 | variable, and it would thereby remind you to write instead: |
4633a7c4 | 303 | |
19799a22 | 304 | print $aref->[2][2] |
4633a7c4 LW |
305 | |
306 | =head1 DEBUGGING | |
d74e8afc ITB |
307 | X<data structure, debugging> X<complex data structure, debugging> |
308 | X<AoA, debugging> X<HoA, debugging> X<AoH, debugging> X<HoH, debugging> | |
309 | X<array of arrays, debugging> X<hash of arrays, debugging> | |
310 | X<array of hashes, debugging> X<hash of hashes, debugging> | |
4633a7c4 | 311 | |
a6006777 | 312 | Before version 5.002, the standard Perl debugger didn't do a very nice job of |
313 | printing out complex data structures. With 5.002 or above, the | |
4973169d | 314 | debugger includes several new features, including command line editing as |
315 | well as the C<x> command to dump out complex data structures. For | |
19799a22 | 316 | example, given the assignment to $AoA above, here's the debugger output: |
4633a7c4 | 317 | |
19799a22 GS |
318 | DB<1> x $AoA |
319 | $AoA = ARRAY(0x13b5a0) | |
4633a7c4 LW |
320 | 0 ARRAY(0x1f0a24) |
321 | 0 'fred' | |
322 | 1 'barney' | |
323 | 2 'pebbles' | |
324 | 3 'bambam' | |
325 | 4 'dino' | |
326 | 1 ARRAY(0x13b558) | |
327 | 0 'homer' | |
328 | 1 'bart' | |
329 | 2 'marge' | |
330 | 3 'maggie' | |
331 | 2 ARRAY(0x13b540) | |
332 | 0 'george' | |
333 | 1 'jane' | |
5f05dabc | 334 | 2 'elroy' |
4633a7c4 LW |
335 | 3 'judy' |
336 | ||
cb1a09d0 AD |
337 | =head1 CODE EXAMPLES |
338 | ||
54310121 | 339 | Presented with little comment (these will get their own manpages someday) |
4973169d | 340 | here are short code examples illustrating access of various |
cb1a09d0 AD |
341 | types of data structures. |
342 | ||
19799a22 | 343 | =head1 ARRAYS OF ARRAYS |
d74e8afc | 344 | X<array of arrays> X<AoA> |
cb1a09d0 | 345 | |
d1be9408 | 346 | =head2 Declaration of an ARRAY OF ARRAYS |
cb1a09d0 | 347 | |
84f709e7 JH |
348 | @AoA = ( |
349 | [ "fred", "barney" ], | |
350 | [ "george", "jane", "elroy" ], | |
351 | [ "homer", "marge", "bart" ], | |
cb1a09d0 AD |
352 | ); |
353 | ||
d1be9408 | 354 | =head2 Generation of an ARRAY OF ARRAYS |
cb1a09d0 AD |
355 | |
356 | # reading from file | |
357 | while ( <> ) { | |
19799a22 | 358 | push @AoA, [ split ]; |
4973169d | 359 | } |
cb1a09d0 AD |
360 | |
361 | # calling a function | |
84f709e7 | 362 | for $i ( 1 .. 10 ) { |
19799a22 | 363 | $AoA[$i] = [ somefunc($i) ]; |
4973169d | 364 | } |
cb1a09d0 AD |
365 | |
366 | # using temp vars | |
84f709e7 JH |
367 | for $i ( 1 .. 10 ) { |
368 | @tmp = somefunc($i); | |
369 | $AoA[$i] = [ @tmp ]; | |
4973169d | 370 | } |
cb1a09d0 AD |
371 | |
372 | # add to an existing row | |
84f709e7 | 373 | push @{ $AoA[0] }, "wilma", "betty"; |
cb1a09d0 | 374 | |
d1be9408 | 375 | =head2 Access and Printing of an ARRAY OF ARRAYS |
cb1a09d0 AD |
376 | |
377 | # one element | |
84f709e7 | 378 | $AoA[0][0] = "Fred"; |
cb1a09d0 AD |
379 | |
380 | # another element | |
19799a22 | 381 | $AoA[1][1] =~ s/(\w)/\u$1/; |
cb1a09d0 AD |
382 | |
383 | # print the whole thing with refs | |
84f709e7 | 384 | for $aref ( @AoA ) { |
cb1a09d0 | 385 | print "\t [ @$aref ],\n"; |
4973169d | 386 | } |
cb1a09d0 AD |
387 | |
388 | # print the whole thing with indices | |
84f709e7 JH |
389 | for $i ( 0 .. $#AoA ) { |
390 | print "\t [ @{$AoA[$i]} ],\n"; | |
4973169d | 391 | } |
cb1a09d0 AD |
392 | |
393 | # print the whole thing one at a time | |
84f709e7 JH |
394 | for $i ( 0 .. $#AoA ) { |
395 | for $j ( 0 .. $#{ $AoA[$i] } ) { | |
396 | print "elt $i $j is $AoA[$i][$j]\n"; | |
cb1a09d0 | 397 | } |
4973169d | 398 | } |
cb1a09d0 | 399 | |
19799a22 | 400 | =head1 HASHES OF ARRAYS |
d74e8afc | 401 | X<hash of arrays> X<HoA> |
cb1a09d0 | 402 | |
19799a22 | 403 | =head2 Declaration of a HASH OF ARRAYS |
cb1a09d0 | 404 | |
84f709e7 JH |
405 | %HoA = ( |
406 | flintstones => [ "fred", "barney" ], | |
407 | jetsons => [ "george", "jane", "elroy" ], | |
408 | simpsons => [ "homer", "marge", "bart" ], | |
cb1a09d0 AD |
409 | ); |
410 | ||
19799a22 | 411 | =head2 Generation of a HASH OF ARRAYS |
cb1a09d0 AD |
412 | |
413 | # reading from file | |
414 | # flintstones: fred barney wilma dino | |
415 | while ( <> ) { | |
84f709e7 | 416 | next unless s/^(.*?):\s*//; |
19799a22 | 417 | $HoA{$1} = [ split ]; |
4973169d | 418 | } |
cb1a09d0 AD |
419 | |
420 | # reading from file; more temps | |
421 | # flintstones: fred barney wilma dino | |
84f709e7 JH |
422 | while ( $line = <> ) { |
423 | ($who, $rest) = split /:\s*/, $line, 2; | |
424 | @fields = split ' ', $rest; | |
425 | $HoA{$who} = [ @fields ]; | |
4973169d | 426 | } |
cb1a09d0 AD |
427 | |
428 | # calling a function that returns a list | |
84f709e7 | 429 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
19799a22 | 430 | $HoA{$group} = [ get_family($group) ]; |
4973169d | 431 | } |
cb1a09d0 AD |
432 | |
433 | # likewise, but using temps | |
84f709e7 JH |
434 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
435 | @members = get_family($group); | |
436 | $HoA{$group} = [ @members ]; | |
4973169d | 437 | } |
cb1a09d0 AD |
438 | |
439 | # append new members to an existing family | |
84f709e7 | 440 | push @{ $HoA{"flintstones"} }, "wilma", "betty"; |
cb1a09d0 | 441 | |
19799a22 | 442 | =head2 Access and Printing of a HASH OF ARRAYS |
cb1a09d0 AD |
443 | |
444 | # one element | |
84f709e7 | 445 | $HoA{flintstones}[0] = "Fred"; |
cb1a09d0 AD |
446 | |
447 | # another element | |
19799a22 | 448 | $HoA{simpsons}[1] =~ s/(\w)/\u$1/; |
cb1a09d0 AD |
449 | |
450 | # print the whole thing | |
84f709e7 JH |
451 | foreach $family ( keys %HoA ) { |
452 | print "$family: @{ $HoA{$family} }\n" | |
4973169d | 453 | } |
cb1a09d0 AD |
454 | |
455 | # print the whole thing with indices | |
84f709e7 JH |
456 | foreach $family ( keys %HoA ) { |
457 | print "family: "; | |
458 | foreach $i ( 0 .. $#{ $HoA{$family} } ) { | |
19799a22 | 459 | print " $i = $HoA{$family}[$i]"; |
cb1a09d0 AD |
460 | } |
461 | print "\n"; | |
4973169d | 462 | } |
cb1a09d0 AD |
463 | |
464 | # print the whole thing sorted by number of members | |
84f709e7 | 465 | foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) { |
19799a22 | 466 | print "$family: @{ $HoA{$family} }\n" |
4973169d | 467 | } |
cb1a09d0 AD |
468 | |
469 | # print the whole thing sorted by number of members and name | |
84f709e7 JH |
470 | foreach $family ( sort { |
471 | @{$HoA{$b}} <=> @{$HoA{$a}} | |
472 | || | |
473 | $a cmp $b | |
474 | } keys %HoA ) | |
475 | { | |
19799a22 | 476 | print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n"; |
4973169d | 477 | } |
cb1a09d0 | 478 | |
19799a22 | 479 | =head1 ARRAYS OF HASHES |
d74e8afc | 480 | X<array of hashes> X<AoH> |
cb1a09d0 | 481 | |
d1be9408 | 482 | =head2 Declaration of an ARRAY OF HASHES |
cb1a09d0 | 483 | |
84f709e7 | 484 | @AoH = ( |
cb1a09d0 | 485 | { |
84f709e7 JH |
486 | Lead => "fred", |
487 | Friend => "barney", | |
cb1a09d0 AD |
488 | }, |
489 | { | |
84f709e7 JH |
490 | Lead => "george", |
491 | Wife => "jane", | |
492 | Son => "elroy", | |
cb1a09d0 AD |
493 | }, |
494 | { | |
84f709e7 JH |
495 | Lead => "homer", |
496 | Wife => "marge", | |
497 | Son => "bart", | |
cb1a09d0 AD |
498 | } |
499 | ); | |
500 | ||
d1be9408 | 501 | =head2 Generation of an ARRAY OF HASHES |
cb1a09d0 AD |
502 | |
503 | # reading from file | |
504 | # format: LEAD=fred FRIEND=barney | |
505 | while ( <> ) { | |
84f709e7 JH |
506 | $rec = {}; |
507 | for $field ( split ) { | |
508 | ($key, $value) = split /=/, $field; | |
509 | $rec->{$key} = $value; | |
cb1a09d0 | 510 | } |
19799a22 | 511 | push @AoH, $rec; |
4973169d | 512 | } |
cb1a09d0 AD |
513 | |
514 | ||
515 | # reading from file | |
516 | # format: LEAD=fred FRIEND=barney | |
517 | # no temp | |
518 | while ( <> ) { | |
19799a22 | 519 | push @AoH, { split /[\s+=]/ }; |
4973169d | 520 | } |
cb1a09d0 | 521 | |
19799a22 | 522 | # calling a function that returns a key/value pair list, like |
84f709e7 JH |
523 | # "lead","fred","daughter","pebbles" |
524 | while ( %fields = getnextpairset() ) { | |
19799a22 | 525 | push @AoH, { %fields }; |
4973169d | 526 | } |
cb1a09d0 AD |
527 | |
528 | # likewise, but using no temp vars | |
529 | while (<>) { | |
19799a22 | 530 | push @AoH, { parsepairs($_) }; |
4973169d | 531 | } |
cb1a09d0 AD |
532 | |
533 | # add key/value to an element | |
84f709e7 | 534 | $AoH[0]{pet} = "dino"; |
19799a22 | 535 | $AoH[2]{pet} = "santa's little helper"; |
cb1a09d0 | 536 | |
d1be9408 | 537 | =head2 Access and Printing of an ARRAY OF HASHES |
cb1a09d0 AD |
538 | |
539 | # one element | |
84f709e7 | 540 | $AoH[0]{lead} = "fred"; |
cb1a09d0 AD |
541 | |
542 | # another element | |
19799a22 | 543 | $AoH[1]{lead} =~ s/(\w)/\u$1/; |
cb1a09d0 AD |
544 | |
545 | # print the whole thing with refs | |
84f709e7 JH |
546 | for $href ( @AoH ) { |
547 | print "{ "; | |
548 | for $role ( keys %$href ) { | |
549 | print "$role=$href->{$role} "; | |
cb1a09d0 AD |
550 | } |
551 | print "}\n"; | |
4973169d | 552 | } |
cb1a09d0 AD |
553 | |
554 | # print the whole thing with indices | |
84f709e7 | 555 | for $i ( 0 .. $#AoH ) { |
cb1a09d0 | 556 | print "$i is { "; |
84f709e7 JH |
557 | for $role ( keys %{ $AoH[$i] } ) { |
558 | print "$role=$AoH[$i]{$role} "; | |
cb1a09d0 AD |
559 | } |
560 | print "}\n"; | |
4973169d | 561 | } |
cb1a09d0 AD |
562 | |
563 | # print the whole thing one at a time | |
84f709e7 JH |
564 | for $i ( 0 .. $#AoH ) { |
565 | for $role ( keys %{ $AoH[$i] } ) { | |
566 | print "elt $i $role is $AoH[$i]{$role}\n"; | |
cb1a09d0 | 567 | } |
4973169d | 568 | } |
cb1a09d0 AD |
569 | |
570 | =head1 HASHES OF HASHES | |
8e0aa7ce | 571 | X<hash of hashes> X<HoH> |
cb1a09d0 AD |
572 | |
573 | =head2 Declaration of a HASH OF HASHES | |
574 | ||
84f709e7 | 575 | %HoH = ( |
28757baa | 576 | flintstones => { |
84f709e7 JH |
577 | lead => "fred", |
578 | pal => "barney", | |
cb1a09d0 | 579 | }, |
28757baa | 580 | jetsons => { |
84f709e7 JH |
581 | lead => "george", |
582 | wife => "jane", | |
583 | "his boy" => "elroy", | |
4973169d | 584 | }, |
28757baa | 585 | simpsons => { |
84f709e7 JH |
586 | lead => "homer", |
587 | wife => "marge", | |
588 | kid => "bart", | |
4973169d | 589 | }, |
590 | ); | |
cb1a09d0 AD |
591 | |
592 | =head2 Generation of a HASH OF HASHES | |
593 | ||
594 | # reading from file | |
595 | # flintstones: lead=fred pal=barney wife=wilma pet=dino | |
596 | while ( <> ) { | |
84f709e7 JH |
597 | next unless s/^(.*?):\s*//; |
598 | $who = $1; | |
599 | for $field ( split ) { | |
600 | ($key, $value) = split /=/, $field; | |
cb1a09d0 AD |
601 | $HoH{$who}{$key} = $value; |
602 | } | |
603 | ||
604 | ||
605 | # reading from file; more temps | |
606 | while ( <> ) { | |
84f709e7 JH |
607 | next unless s/^(.*?):\s*//; |
608 | $who = $1; | |
609 | $rec = {}; | |
cb1a09d0 | 610 | $HoH{$who} = $rec; |
84f709e7 JH |
611 | for $field ( split ) { |
612 | ($key, $value) = split /=/, $field; | |
613 | $rec->{$key} = $value; | |
cb1a09d0 | 614 | } |
4973169d | 615 | } |
cb1a09d0 | 616 | |
cb1a09d0 | 617 | # calling a function that returns a key,value hash |
84f709e7 | 618 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
cb1a09d0 | 619 | $HoH{$group} = { get_family($group) }; |
4973169d | 620 | } |
cb1a09d0 AD |
621 | |
622 | # likewise, but using temps | |
84f709e7 JH |
623 | for $group ( "simpsons", "jetsons", "flintstones" ) { |
624 | %members = get_family($group); | |
cb1a09d0 | 625 | $HoH{$group} = { %members }; |
4973169d | 626 | } |
cb1a09d0 AD |
627 | |
628 | # append new members to an existing family | |
84f709e7 JH |
629 | %new_folks = ( |
630 | wife => "wilma", | |
631 | pet => "dino", | |
cb1a09d0 | 632 | ); |
4973169d | 633 | |
84f709e7 | 634 | for $what (keys %new_folks) { |
cb1a09d0 | 635 | $HoH{flintstones}{$what} = $new_folks{$what}; |
4973169d | 636 | } |
cb1a09d0 AD |
637 | |
638 | =head2 Access and Printing of a HASH OF HASHES | |
639 | ||
640 | # one element | |
84f709e7 | 641 | $HoH{flintstones}{wife} = "wilma"; |
cb1a09d0 AD |
642 | |
643 | # another element | |
644 | $HoH{simpsons}{lead} =~ s/(\w)/\u$1/; | |
645 | ||
646 | # print the whole thing | |
84f709e7 | 647 | foreach $family ( keys %HoH ) { |
1fef88e7 | 648 | print "$family: { "; |
84f709e7 JH |
649 | for $role ( keys %{ $HoH{$family} } ) { |
650 | print "$role=$HoH{$family}{$role} "; | |
cb1a09d0 AD |
651 | } |
652 | print "}\n"; | |
4973169d | 653 | } |
cb1a09d0 AD |
654 | |
655 | # print the whole thing somewhat sorted | |
84f709e7 | 656 | foreach $family ( sort keys %HoH ) { |
1fef88e7 | 657 | print "$family: { "; |
84f709e7 JH |
658 | for $role ( sort keys %{ $HoH{$family} } ) { |
659 | print "$role=$HoH{$family}{$role} "; | |
cb1a09d0 AD |
660 | } |
661 | print "}\n"; | |
4973169d | 662 | } |
cb1a09d0 | 663 | |
84f709e7 | 664 | |
cb1a09d0 | 665 | # print the whole thing sorted by number of members |
84f709e7 | 666 | foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) { |
1fef88e7 | 667 | print "$family: { "; |
84f709e7 JH |
668 | for $role ( sort keys %{ $HoH{$family} } ) { |
669 | print "$role=$HoH{$family}{$role} "; | |
cb1a09d0 AD |
670 | } |
671 | print "}\n"; | |
4973169d | 672 | } |
cb1a09d0 AD |
673 | |
674 | # establish a sort order (rank) for each role | |
84f709e7 JH |
675 | $i = 0; |
676 | for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i } | |
cb1a09d0 AD |
677 | |
678 | # now print the whole thing sorted by number of members | |
84f709e7 | 679 | foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) { |
1fef88e7 | 680 | print "$family: { "; |
cb1a09d0 | 681 | # and print these according to rank order |
84f709e7 JH |
682 | for $role ( sort { $rank{$a} <=> $rank{$b} } keys %{ $HoH{$family} } ) { |
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 = { |
4973169d | 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>> |
4633a7c4 | 845 | |
84f709e7 | 846 | Last update: |
28757baa | 847 | Wed Oct 23 04:57:50 MET DST 1996 |