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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLREFTUT 1" | |
132 | .TH PERLREFTUT 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlreftut \- Mark's very short tutorial about references | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | One of the most important new features in Perl 5 was the capability to | |
138 | manage complicated data structures like multidimensional arrays and | |
139 | nested hashes. To enable these, Perl 5 introduced a feature called | |
140 | `references', and using references is the key to managing complicated, | |
141 | structured data in Perl. Unfortunately, there's a lot of funny syntax | |
142 | to learn, and the main manual page can be hard to follow. The manual | |
143 | is quite complete, and sometimes people find that a problem, because | |
144 | it can be hard to tell what is important and what isn't. | |
145 | .PP | |
146 | Fortunately, you only need to know 10% of what's in the main page to get | |
147 | 90% of the benefit. This page will show you that 10%. | |
148 | .SH "Who Needs Complicated Data Structures?" | |
149 | .IX Header "Who Needs Complicated Data Structures?" | |
150 | One problem that came up all the time in Perl 4 was how to represent a | |
151 | hash whose values were lists. Perl 4 had hashes, of course, but the | |
152 | values had to be scalars; they couldn't be lists. | |
153 | .PP | |
154 | Why would you want a hash of lists? Let's take a simple example: You | |
155 | have a file of city and country names, like this: | |
156 | .PP | |
157 | .Vb 6 | |
158 | \& Chicago, USA | |
159 | \& Frankfurt, Germany | |
160 | \& Berlin, Germany | |
161 | \& Washington, USA | |
162 | \& Helsinki, Finland | |
163 | \& New York, USA | |
164 | .Ve | |
165 | .PP | |
166 | and you want to produce an output like this, with each country mentioned | |
167 | once, and then an alphabetical list of the cities in that country: | |
168 | .PP | |
169 | .Vb 3 | |
170 | \& Finland: Helsinki. | |
171 | \& Germany: Berlin, Frankfurt. | |
172 | \& USA: Chicago, New York, Washington. | |
173 | .Ve | |
174 | .PP | |
175 | The natural way to do this is to have a hash whose keys are country | |
176 | names. Associated with each country name key is a list of the cities in | |
177 | that country. Each time you read a line of input, split it into a country | |
178 | and a city, look up the list of cities already known to be in that | |
179 | country, and append the new city to the list. When you're done reading | |
180 | the input, iterate over the hash as usual, sorting each list of cities | |
181 | before you print it out. | |
182 | .PP | |
183 | If hash values can't be lists, you lose. In Perl 4, hash values can't | |
184 | be lists; they can only be strings. You lose. You'd probably have to | |
185 | combine all the cities into a single string somehow, and then when | |
186 | time came to write the output, you'd have to break the string into a | |
187 | list, sort the list, and turn it back into a string. This is messy | |
188 | and error\-prone. And it's frustrating, because Perl already has | |
189 | perfectly good lists that would solve the problem if only you could | |
190 | use them. | |
191 | .SH "The Solution" | |
192 | .IX Header "The Solution" | |
193 | By the time Perl 5 rolled around, we were already stuck with this | |
194 | design: Hash values must be scalars. The solution to this is | |
195 | references. | |
196 | .PP | |
197 | A reference is a scalar value that \fIrefers to\fR an entire array or an | |
198 | entire hash (or to just about anything else). Names are one kind of | |
199 | reference that you're already familiar with. Think of the President | |
200 | of the United States: a messy, inconvenient bag of blood and bones. | |
201 | But to talk about him, or to represent him in a computer program, all | |
202 | you need is the easy, convenient scalar string \*(L"George Bush\*(R". | |
203 | .PP | |
204 | References in Perl are like names for arrays and hashes. They're | |
205 | Perl's private, internal names, so you can be sure they're | |
206 | unambiguous. Unlike \*(L"George Bush\*(R", a reference only refers to one | |
207 | thing, and you always know what it refers to. If you have a reference | |
208 | to an array, you can recover the entire array from it. If you have a | |
209 | reference to a hash, you can recover the entire hash. But the | |
210 | reference is still an easy, compact scalar value. | |
211 | .PP | |
212 | You can't have a hash whose values are arrays; hash values can only be | |
213 | scalars. We're stuck with that. But a single reference can refer to | |
214 | an entire array, and references are scalars, so you can have a hash of | |
215 | references to arrays, and it'll act a lot like a hash of arrays, and | |
216 | it'll be just as useful as a hash of arrays. | |
217 | .PP | |
218 | We'll come back to this city-country problem later, after we've seen | |
219 | some syntax for managing references. | |
220 | .SH "Syntax" | |
221 | .IX Header "Syntax" | |
222 | There are just two ways to make a reference, and just two ways to use | |
223 | it once you have it. | |
224 | .Sh "Making References" | |
225 | .IX Subsection "Making References" | |
226 | \&\fBMake Rule 1\fR | |
227 | .PP | |
228 | If you put a \f(CW\*(C`\e\*(C'\fR in front of a variable, you get a | |
229 | reference to that variable. | |
230 | .PP | |
231 | .Vb 2 | |
232 | \& $aref = \e@array; # $aref now holds a reference to @array | |
233 | \& $href = \e%hash; # $href now holds a reference to %hash | |
234 | .Ve | |
235 | .PP | |
236 | Once the reference is stored in a variable like \f(CW$aref\fR or \f(CW$href\fR, you | |
237 | can copy it or store it just the same as any other scalar value: | |
238 | .PP | |
239 | .Vb 3 | |
240 | \& $xy = $aref; # $xy now holds a reference to @array | |
241 | \& $p[3] = $href; # $p[3] now holds a reference to %hash | |
242 | \& $z = $p[3]; # $z now holds a reference to %hash | |
243 | .Ve | |
244 | .PP | |
245 | These examples show how to make references to variables with names. | |
246 | Sometimes you want to make an array or a hash that doesn't have a | |
247 | name. This is analogous to the way you like to be able to use the | |
248 | string \f(CW"\en"\fR or the number 80 without having to store it in a named | |
249 | variable first. | |
250 | .PP | |
251 | \&\fBMake Rule 2\fR | |
252 | .PP | |
253 | \&\f(CW\*(C`[ ITEMS ]\*(C'\fR makes a new, anonymous array, and returns a reference to | |
254 | that array. \f(CW\*(C`{ ITEMS }\*(C'\fR makes a new, anonymous hash. and returns a | |
255 | reference to that hash. | |
256 | .PP | |
257 | .Vb 2 | |
258 | \& $aref = [ 1, "foo", undef, 13 ]; | |
259 | \& # $aref now holds a reference to an array | |
260 | .Ve | |
261 | .PP | |
262 | .Vb 2 | |
263 | \& $href = { APR => 4, AUG => 8 }; | |
264 | \& # $href now holds a reference to a hash | |
265 | .Ve | |
266 | .PP | |
267 | The references you get from rule 2 are the same kind of | |
268 | references that you get from rule 1: | |
269 | .PP | |
270 | .Vb 2 | |
271 | \& # This: | |
272 | \& $aref = [ 1, 2, 3 ]; | |
273 | .Ve | |
274 | .PP | |
275 | .Vb 3 | |
276 | \& # Does the same as this: | |
277 | \& @array = (1, 2, 3); | |
278 | \& $aref = \e@array; | |
279 | .Ve | |
280 | .PP | |
281 | The first line is an abbreviation for the following two lines, except | |
282 | that it doesn't create the superfluous array variable \f(CW@array\fR. | |
283 | .Sh "Using References" | |
284 | .IX Subsection "Using References" | |
285 | What can you do with a reference once you have it? It's a scalar | |
286 | value, and we've seen that you can store it as a scalar and get it back | |
287 | again just like any scalar. There are just two more ways to use it: | |
288 | .PP | |
289 | \&\fBUse Rule 1\fR | |
290 | .PP | |
291 | If \f(CW$aref\fR contains a reference to an array, then you | |
292 | can put \f(CW\*(C`{$aref}\*(C'\fR anywhere you would normally put the name of an | |
293 | array. For example, \f(CW\*(C`@{$aref}\*(C'\fR instead of \f(CW@array\fR. | |
294 | .PP | |
295 | Here are some examples of that: | |
296 | .PP | |
297 | Arrays: | |
298 | .PP | |
299 | .Vb 4 | |
300 | \& @a @{$aref} An array | |
301 | \& reverse @a reverse @{$aref} Reverse the array | |
302 | \& $a[3] ${$aref}[3] An element of the array | |
303 | \& $a[3] = 17; ${$aref}[3] = 17 Assigning an element | |
304 | .Ve | |
305 | .PP | |
306 | On each line are two expressions that do the same thing. The | |
307 | left-hand versions operate on the array \f(CW@a\fR, and the right-hand | |
308 | versions operate on the array that is referred to by \f(CW$aref\fR, but | |
309 | once they find the array they're operating on, they do the same things | |
310 | to the arrays. | |
311 | .PP | |
312 | Using a hash reference is \fIexactly\fR the same: | |
313 | .PP | |
314 | .Vb 4 | |
315 | \& %h %{$href} A hash | |
316 | \& keys %h keys %{$href} Get the keys from the hash | |
317 | \& $h{'red'} ${$href}{'red'} An element of the hash | |
318 | \& $h{'red'} = 17 ${$href}{'red'} = 17 Assigning an element | |
319 | .Ve | |
320 | .PP | |
321 | \&\fBUse Rule 2\fR | |
322 | .PP | |
323 | \&\f(CW\*(C`${$aref}[3]\*(C'\fR is too hard to read, so you can write \f(CW\*(C`$aref\->[3]\*(C'\fR | |
324 | instead. | |
325 | .PP | |
326 | \&\f(CW\*(C`${$href}{red}\*(C'\fR is too hard to read, so you can write | |
327 | \&\f(CW\*(C`$href\->{red}\*(C'\fR instead. | |
328 | .PP | |
329 | Most often, when you have an array or a hash, you want to get or set a | |
330 | single element from it. \f(CW\*(C`${$aref}[3]\*(C'\fR and \f(CW\*(C`${$href}{'red'}\*(C'\fR have | |
331 | too much punctuation, and Perl lets you abbreviate. | |
332 | .PP | |
333 | If \f(CW$aref\fR holds a reference to an array, then \f(CW\*(C`$aref\->[3]\*(C'\fR is | |
334 | the fourth element of the array. Don't confuse this with \f(CW$aref[3]\fR, | |
335 | which is the fourth element of a totally different array, one | |
336 | deceptively named \f(CW@aref\fR. \f(CW$aref\fR and \f(CW@aref\fR are unrelated the | |
337 | same way that \f(CW$item\fR and \f(CW@item\fR are. | |
338 | .PP | |
339 | Similarly, \f(CW\*(C`$href\->{'red'}\*(C'\fR is part of the hash referred to by | |
340 | the scalar variable \f(CW$href\fR, perhaps even one with no name. | |
341 | \&\f(CW$href{'red'}\fR is part of the deceptively named \f(CW%href\fR hash. It's | |
342 | easy to forget to leave out the \f(CW\*(C`\->\*(C'\fR, and if you do, you'll get | |
343 | bizarre results when your program gets array and hash elements out of | |
344 | totally unexpected hashes and arrays that weren't the ones you wanted | |
345 | to use. | |
346 | .SH "An Example" | |
347 | .IX Header "An Example" | |
348 | Let's see a quick example of how all this is useful. | |
349 | .PP | |
350 | First, remember that \f(CW\*(C`[1, 2, 3]\*(C'\fR makes an anonymous array containing | |
351 | \&\f(CW\*(C`(1, 2, 3)\*(C'\fR, and gives you a reference to that array. | |
352 | .PP | |
353 | Now think about | |
354 | .PP | |
355 | .Vb 4 | |
356 | \& @a = ( [1, 2, 3], | |
357 | \& [4, 5, 6], | |
358 | \& [7, 8, 9] | |
359 | \& ); | |
360 | .Ve | |
361 | .PP | |
362 | @a is an array with three elements, and each one is a reference to | |
363 | another array. | |
364 | .PP | |
365 | \&\f(CW$a[1]\fR is one of these references. It refers to an array, the array | |
366 | containing \f(CW\*(C`(4, 5, 6)\*(C'\fR, and because it is a reference to an array, | |
367 | \&\fB\s-1USE\s0 \s-1RULE\s0 2\fR says that we can write \f(CW$a[1]\->[2]\fR to get the | |
368 | third element from that array. \f(CW$a[1]\->[2]\fR is the 6. | |
369 | Similarly, \f(CW$a[0]\->[1]\fR is the 2. What we have here is like a | |
370 | two-dimensional array; you can write \f(CW$a[ROW]\->[COLUMN]\fR to get | |
371 | or set the element in any row and any column of the array. | |
372 | .PP | |
373 | The notation still looks a little cumbersome, so there's one more | |
374 | abbreviation: | |
375 | .SH "Arrow Rule" | |
376 | .IX Header "Arrow Rule" | |
377 | In between two \fBsubscripts\fR, the arrow is optional. | |
378 | .PP | |
379 | Instead of \f(CW$a[1]\->[2]\fR, we can write \f(CW$a[1][2]\fR; it means the | |
380 | same thing. Instead of \f(CW$a[0]\->[1]\fR, we can write \f(CW$a[0][1]\fR; | |
381 | it means the same thing. | |
382 | .PP | |
383 | Now it really looks like two-dimensional arrays! | |
384 | .PP | |
385 | You can see why the arrows are important. Without them, we would have | |
386 | had to write \f(CW\*(C`${$a[1]}[2]\*(C'\fR instead of \f(CW$a[1][2]\fR. For | |
387 | three-dimensional arrays, they let us write \f(CW$x[2][3][5]\fR instead of | |
388 | the unreadable \f(CW\*(C`${${$x[2]}[3]}[5]\*(C'\fR. | |
389 | .SH "Solution" | |
390 | .IX Header "Solution" | |
391 | Here's the answer to the problem I posed earlier, of reformatting a | |
392 | file of city and country names. | |
393 | .PP | |
394 | .Vb 12 | |
395 | \& 1 while (<>) { | |
396 | \& 2 chomp; | |
397 | \& 3 my ($city, $country) = split /, /; | |
398 | \& 4 push @{$table{$country}}, $city; | |
399 | \& 5 } | |
400 | \& 6 | |
401 | \& 7 foreach $country (sort keys %table) { | |
402 | \& 8 print "$country: "; | |
403 | \& 9 my @cities = @{$table{$country}}; | |
404 | \& 10 print join ', ', sort @cities; | |
405 | \& 11 print ".\en"; | |
406 | \& 12 } | |
407 | .Ve | |
408 | .PP | |
409 | The program has two pieces: Lines 1\-\-5 read the input and build a | |
410 | data structure, and lines 7\-\-12 analyze the data and print out the | |
411 | report. | |
412 | .PP | |
413 | In the first part, line 4 is the important one. We're going to have a | |
414 | hash, \f(CW%table\fR, whose keys are country names, and whose values are | |
415 | (references to) arrays of city names. After acquiring a city and | |
416 | country name, the program looks up \f(CW$table{$country}\fR, which holds (a | |
417 | reference to) the list of cities seen in that country so far. Line 4 is | |
418 | totally analogous to | |
419 | .PP | |
420 | .Vb 1 | |
421 | \& push @array, $city; | |
422 | .Ve | |
423 | .PP | |
424 | except that the name \f(CW\*(C`array\*(C'\fR has been replaced by the reference | |
425 | \&\f(CW\*(C`{$table{$country}}\*(C'\fR. The \f(CW\*(C`push\*(C'\fR adds a city name to the end of the | |
426 | referred-to array. | |
427 | .PP | |
428 | In the second part, line 9 is the important one. Again, | |
429 | \&\f(CW$table{$country}\fR is (a reference to) the list of cities in the country, so | |
430 | we can recover the original list, and copy it into the array \f(CW@cities\fR, | |
431 | by using \f(CW\*(C`@{$table{$country}}\*(C'\fR. Line 9 is totally analogous to | |
432 | .PP | |
433 | .Vb 1 | |
434 | \& @cities = @array; | |
435 | .Ve | |
436 | .PP | |
437 | except that the name \f(CW\*(C`array\*(C'\fR has been replaced by the reference | |
438 | \&\f(CW\*(C`{$table{$country}}\*(C'\fR. The \f(CW\*(C`@\*(C'\fR tells Perl to get the entire array. | |
439 | .PP | |
440 | The rest of the program is just familiar uses of \f(CW\*(C`chomp\*(C'\fR, \f(CW\*(C`split\*(C'\fR, \f(CW\*(C`sort\*(C'\fR, | |
441 | \&\f(CW\*(C`print\*(C'\fR, and doesn't involve references at all. | |
442 | .PP | |
443 | There's one fine point I skipped. Suppose the program has just read | |
444 | the first line in its input that happens to mention Greece. | |
445 | Control is at line 4, \f(CW$country\fR is \f(CW'Greece'\fR, and \f(CW$city\fR is | |
446 | \&\f(CW'Athens'\fR. Since this is the first city in Greece, | |
447 | \&\f(CW$table{$country}\fR is undefined\-\-\-in fact there isn't an \f(CW'Greece'\fR key | |
448 | in \f(CW%table\fR at all. What does line 4 do here? | |
449 | .PP | |
450 | .Vb 1 | |
451 | \& 4 push @{$table{$country}}, $city; | |
452 | .Ve | |
453 | .PP | |
454 | This is Perl, so it does the exact right thing. It sees that you want | |
455 | to push \f(CW\*(C`Athens\*(C'\fR onto an array that doesn't exist, so it helpfully | |
456 | makes a new, empty, anonymous array for you, installs it in the table, | |
457 | and then pushes \f(CW\*(C`Athens\*(C'\fR onto it. This is called `autovivification'. | |
458 | .SH "The Rest" | |
459 | .IX Header "The Rest" | |
460 | I promised to give you 90% of the benefit with 10% of the details, and | |
461 | that means I left out 90% of the details. Now that you have an | |
462 | overview of the important parts, it should be easier to read the | |
463 | perlref manual page, which discusses 100% of the details. | |
464 | .PP | |
465 | Some of the highlights of perlref: | |
466 | .IP "\(bu" 4 | |
467 | You can make references to anything, including scalars, functions, and | |
468 | other references. | |
469 | .IP "\(bu" 4 | |
470 | In \fB\s-1USE\s0 \s-1RULE\s0 1\fR, you can omit the curly brackets whenever the thing | |
471 | inside them is an atomic scalar variable like \f(CW$aref\fR. For example, | |
472 | \&\f(CW@$aref\fR is the same as \f(CW\*(C`@{$aref}\*(C'\fR, and \f(CW$$aref[1]\fR is the same as | |
473 | \&\f(CW\*(C`${$aref}[1]\*(C'\fR. If you're just starting out, you may want to adopt | |
474 | the habit of always including the curly brackets. | |
475 | .IP "\(bu" 4 | |
476 | To see if a variable contains a reference, use the `ref' function. | |
477 | It returns true if its argument is a reference. Actually it's a | |
478 | little better than that: It returns \s-1HASH\s0 for hash references and | |
479 | \&\s-1ARRAY\s0 for array references. | |
480 | .IP "\(bu" 4 | |
481 | If you try to use a reference like a string, you get strings like | |
482 | .Sp | |
483 | .Vb 1 | |
484 | \& ARRAY(0x80f5dec) or HASH(0x826afc0) | |
485 | .Ve | |
486 | .Sp | |
487 | If you ever see a string that looks like this, you'll know you | |
488 | printed out a reference by mistake. | |
489 | .Sp | |
490 | A side effect of this representation is that you can use \f(CW\*(C`eq\*(C'\fR to see | |
491 | if two references refer to the same thing. (But you should usually use | |
492 | \&\f(CW\*(C`==\*(C'\fR instead because it's much faster.) | |
493 | .IP "\(bu" 4 | |
494 | You can use a string as if it were a reference. If you use the string | |
495 | \&\f(CW"foo"\fR as an array reference, it's taken to be a reference to the | |
496 | array \f(CW@foo\fR. This is called a \fIsoft reference\fR or \fIsymbolic reference\fR. | |
497 | .PP | |
498 | You might prefer to go on to perllol instead of perlref; it | |
499 | discusses lists of lists and multidimensional arrays in detail. After | |
500 | that, you should move on to perldsc; it's a Data Structure Cookbook | |
501 | that shows recipes for using and printing out arrays of hashes, hashes | |
502 | of arrays, and other kinds of data. | |
503 | .SH "Summary" | |
504 | .IX Header "Summary" | |
505 | Everyone needs compound data structures, and in Perl the way you get | |
506 | them is with references. There are four important rules for managing | |
507 | references: Two for making references and two for using them. Once | |
508 | you know these rules you can do most of the important things you need | |
509 | to do with references. | |
510 | .SH "Credits" | |
511 | .IX Header "Credits" | |
512 | Author: Mark-Jason Dominus, Plover Systems (\f(CW\*(C`mjd\-perl\-ref+@plover.com\*(C'\fR) | |
513 | .PP | |
514 | This article originally appeared in \fIThe Perl Journal\fR | |
515 | ( http://www.tpj.com/ ) volume 3, #2. Reprinted with permission. | |
516 | .PP | |
517 | The original title was \fIUnderstand References Today\fR. | |
518 | .Sh "Distribution Conditions" | |
519 | .IX Subsection "Distribution Conditions" | |
520 | Copyright 1998 The Perl Journal. | |
521 | .PP | |
522 | When included as part of the Standard Version of Perl, or as part of | |
523 | its complete documentation whether printed or otherwise, this work may | |
524 | be distributed only under the terms of Perl's Artistic License. Any | |
525 | distribution of this file or derivatives thereof outside of that | |
526 | package require that special arrangements be made with copyright | |
527 | holder. | |
528 | .PP | |
529 | Irrespective of its distribution, all code examples in these files are | |
530 | hereby placed into the public domain. You are permitted and | |
531 | encouraged to use this code in your own programs for fun or for profit | |
532 | as you see fit. A simple comment in the code giving credit would be | |
533 | courteous but is not required. |