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1 | =head1 NAME |
2 | |
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3 | perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 1998/07/16 22:49:55 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | The section of the FAQ answers question related to the manipulation |
8 | of data as numbers, dates, strings, arrays, hashes, and miscellaneous |
9 | data issues. |
10 | |
11 | =head1 Data: Numbers |
12 | |
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13 | =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? |
14 | |
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15 | The infinite set that a mathematician thinks of as the real numbers can |
16 | only be approximate on a computer, since the computer only has a finite |
17 | number of bits to store an infinite number of, um, numbers. |
18 | |
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19 | Internally, your computer represents floating-point numbers in binary. |
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20 | Floating-point numbers read in from a file or appearing as literals |
21 | in your program are converted from their decimal floating-point |
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22 | representation (eg, 19.95) to the internal binary representation. |
23 | |
24 | However, 19.95 can't be precisely represented as a binary |
25 | floating-point number, just like 1/3 can't be exactly represented as a |
26 | decimal floating-point number. The computer's binary representation |
27 | of 19.95, therefore, isn't exactly 19.95. |
28 | |
29 | When a floating-point number gets printed, the binary floating-point |
30 | representation is converted back to decimal. These decimal numbers |
31 | are displayed in either the format you specify with printf(), or the |
32 | current output format for numbers (see L<perlvar/"$#"> if you use |
33 | print. C<$#> has a different default value in Perl5 than it did in |
34 | Perl4. Changing C<$#> yourself is deprecated. |
35 | |
36 | This affects B<all> computer languages that represent decimal |
37 | floating-point numbers in binary, not just Perl. Perl provides |
38 | arbitrary-precision decimal numbers with the Math::BigFloat module |
39 | (part of the standard Perl distribution), but mathematical operations |
40 | are consequently slower. |
41 | |
42 | To get rid of the superfluous digits, just use a format (eg, |
43 | C<printf("%.2f", 19.95)>) to get the required precision. |
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44 | See L<perlop/"Floating-point Arithmetic">. |
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45 | |
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46 | =head2 Why isn't my octal data interpreted correctly? |
47 | |
48 | Perl only understands octal and hex numbers as such when they occur |
49 | as literals in your program. If they are read in from somewhere and |
50 | assigned, no automatic conversion takes place. You must explicitly |
51 | use oct() or hex() if you want the values converted. oct() interprets |
52 | both hex ("0x350") numbers and octal ones ("0350" or even without the |
53 | leading "0", like "377"), while hex() only converts hexadecimal ones, |
54 | with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". |
55 | |
56 | This problem shows up most often when people try using chmod(), mkdir(), |
57 | umask(), or sysopen(), which all want permissions in octal. |
58 | |
59 | chmod(644, $file); # WRONG -- perl -w catches this |
60 | chmod(0644, $file); # right |
61 | |
5a964f20 |
62 | =head2 Does perl have a round function? What about ceil() and floor()? Trig functions? |
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63 | |
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64 | Remember that int() merely truncates toward 0. For rounding to a |
65 | certain number of digits, sprintf() or printf() is usually the easiest |
66 | route. |
67 | |
68 | printf("%.3f", 3.1415926535); # prints 3.142 |
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69 | |
70 | The POSIX module (part of the standard perl distribution) implements |
71 | ceil(), floor(), and a number of other mathematical and trigonometric |
72 | functions. |
73 | |
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74 | use POSIX; |
75 | $ceil = ceil(3.5); # 4 |
76 | $floor = floor(3.5); # 3 |
77 | |
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78 | In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex |
79 | module. With 5.004, the Math::Trig module (part of the standard perl |
80 | distribution) implements the trigonometric functions. Internally it |
81 | uses the Math::Complex module and some functions can break out from |
82 | the real axis into the complex plane, for example the inverse sine of |
83 | 2. |
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84 | |
85 | Rounding in financial applications can have serious implications, and |
86 | the rounding method used should be specified precisely. In these |
87 | cases, it probably pays not to trust whichever system rounding is |
88 | being used by Perl, but to instead implement the rounding function you |
89 | need yourself. |
90 | |
91 | =head2 How do I convert bits into ints? |
92 | |
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93 | To turn a string of 1s and 0s like C<10110110> into a scalar containing |
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94 | its binary value, use the pack() function (documented in |
95 | L<perlfunc/"pack">): |
96 | |
97 | $decimal = pack('B8', '10110110'); |
98 | |
99 | Here's an example of going the other way: |
100 | |
101 | $binary_string = join('', unpack('B*', "\x29")); |
102 | |
103 | =head2 How do I multiply matrices? |
104 | |
105 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) |
106 | or the PDL extension (also available from CPAN). |
107 | |
108 | =head2 How do I perform an operation on a series of integers? |
109 | |
110 | To call a function on each element in an array, and collect the |
111 | results, use: |
112 | |
113 | @results = map { my_func($_) } @array; |
114 | |
115 | For example: |
116 | |
117 | @triple = map { 3 * $_ } @single; |
118 | |
119 | To call a function on each element of an array, but ignore the |
120 | results: |
121 | |
122 | foreach $iterator (@array) { |
123 | &my_func($iterator); |
124 | } |
125 | |
126 | To call a function on each integer in a (small) range, you B<can> use: |
127 | |
128 | @results = map { &my_func($_) } (5 .. 25); |
129 | |
130 | but you should be aware that the C<..> operator creates an array of |
131 | all integers in the range. This can take a lot of memory for large |
132 | ranges. Instead use: |
133 | |
134 | @results = (); |
135 | for ($i=5; $i < 500_005; $i++) { |
136 | push(@results, &my_func($i)); |
137 | } |
138 | |
139 | =head2 How can I output Roman numerals? |
140 | |
141 | Get the http://www.perl.com/CPAN/modules/by-module/Roman module. |
142 | |
143 | =head2 Why aren't my random numbers random? |
144 | |
145 | The short explanation is that you're getting pseudorandom numbers, not |
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146 | random ones, because computers are good at being predictable and bad |
147 | at being random (despite appearances caused by bugs in your programs |
148 | :-). A longer explanation is available on |
149 | http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom |
150 | Phoenix. John von Neumann said, ``Anyone who attempts to generate |
151 | random numbers by deterministic means is, of course, living in a state |
152 | of sin.'' |
153 | |
154 | You should also check out the Math::TrulyRandom module from CPAN. It |
155 | uses the imperfections in your system's timer to generate random |
156 | numbers, but this takes quite a while. If you want a better |
157 | pseudorandom generator than comes with your operating system, look at |
158 | ``Numerical Recipes in C'' at http://nr.harvard.edu/nr/bookc.html . |
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159 | |
160 | =head1 Data: Dates |
161 | |
162 | =head2 How do I find the week-of-the-year/day-of-the-year? |
163 | |
164 | The day of the year is in the array returned by localtime() (see |
165 | L<perlfunc/"localtime">): |
166 | |
167 | $day_of_year = (localtime(time()))[7]; |
168 | |
169 | or more legibly (in 5.004 or higher): |
170 | |
171 | use Time::localtime; |
172 | $day_of_year = localtime(time())->yday; |
173 | |
174 | You can find the week of the year by dividing this by 7: |
175 | |
176 | $week_of_year = int($day_of_year / 7); |
177 | |
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178 | Of course, this believes that weeks start at zero. The Date::Calc |
179 | module from CPAN has a lot of date calculation functions, including |
180 | day of the year, week of the year, and so on. |
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181 | |
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182 | =head2 How can I compare two dates and find the difference? |
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183 | |
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184 | If you're storing your dates as epoch seconds then simply subtract one |
185 | from the other. If you've got a structured date (distinct year, day, |
186 | month, hour, minute, seconds values) then use one of the Date::Manip |
187 | and Date::Calc modules from CPAN. |
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188 | |
189 | =head2 How can I take a string and turn it into epoch seconds? |
190 | |
191 | If it's a regular enough string that it always has the same format, |
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192 | you can split it up and pass the parts to C<timelocal> in the standard |
193 | Time::Local module. Otherwise, you should look into the Date::Calc |
194 | and Date::Manip modules from CPAN. |
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195 | |
196 | =head2 How can I find the Julian Day? |
197 | |
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198 | Neither Date::Manip nor Date::Calc deal with Julian days. Instead, |
199 | there is an example of Julian date calculation that should help you in |
200 | http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz |
201 | . |
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202 | |
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203 | =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant? |
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204 | |
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205 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl |
206 | is Y2K compliant. |
207 | |
208 | Long answer: Perl is just as Y2K compliant as your pencil--no more, |
209 | and no less. The date and time functions supplied with perl (gmtime |
210 | and localtime) supply adequate information to determine the year well |
211 | beyond 2000 (2038 is when trouble strikes for 32-bit machines). The |
212 | year returned by these functions when used in an array context is the |
213 | year minus 1900. For years between 1910 and 1999 this I<happens> to |
214 | be a 2-digit decimal number. To avoid the year 2000 problem simply do |
215 | not treat the year as a 2-digit number. It isn't. |
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216 | |
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217 | When gmtime() and localtime() are used in scalar context they return |
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218 | a timestamp string that contains a fully-expanded year. For example, |
219 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 |
220 | 2001". There's no year 2000 problem here. |
221 | |
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222 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
223 | programs. It can. But so can your pencil. It's the fault of the user, |
224 | not the language. At the risk of inflaming the NRA: ``Perl doesn't |
225 | break Y2K, people do.'' See http://language.perl.com/news/y2k.html for |
226 | a longer exposition. |
227 | |
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228 | =head1 Data: Strings |
229 | |
230 | =head2 How do I validate input? |
231 | |
232 | The answer to this question is usually a regular expression, perhaps |
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233 | with auxiliary logic. See the more specific questions (numbers, mail |
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234 | addresses, etc.) for details. |
235 | |
236 | =head2 How do I unescape a string? |
237 | |
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238 | It depends just what you mean by ``escape''. URL escapes are dealt |
239 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
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240 | character are removed with: |
241 | |
242 | s/\\(.)/$1/g; |
243 | |
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244 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
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245 | |
246 | =head2 How do I remove consecutive pairs of characters? |
247 | |
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248 | To turn C<"abbcccd"> into C<"abccd">: |
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249 | |
250 | s/(.)\1/$1/g; |
251 | |
252 | =head2 How do I expand function calls in a string? |
253 | |
254 | This is documented in L<perlref>. In general, this is fraught with |
255 | quoting and readability problems, but it is possible. To interpolate |
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256 | a subroutine call (in list context) into a string: |
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257 | |
258 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
259 | |
260 | If you prefer scalar context, similar chicanery is also useful for |
261 | arbitrary expressions: |
262 | |
263 | print "That yields ${\($n + 5)} widgets\n"; |
264 | |
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265 | Version 5.004 of Perl had a bug that gave list context to the |
266 | expression in C<${...}>, but this is fixed in version 5.005. |
267 | |
268 | See also ``How can I expand variables in text strings?'' in this |
269 | section of the FAQ. |
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270 | |
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271 | =head2 How do I find matching/nesting anything? |
272 | |
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273 | This isn't something that can be done in one regular expression, no |
274 | matter how complicated. To find something between two single |
275 | characters, a pattern like C</x([^x]*)x/> will get the intervening |
276 | bits in $1. For multiple ones, then something more like |
277 | C</alpha(.*?)omega/> would be needed. But none of these deals with |
278 | nested patterns, nor can they. For that you'll have to write a |
279 | parser. |
280 | |
281 | If you are serious about writing a parser, there are a number of |
282 | modules or oddities that will make your life a lot easier. There is |
283 | the CPAN module Parse::RecDescent, the standard module Text::Balanced, |
284 | the byacc program, and Mark-Jason Dominus's excellent I<py> tool at |
285 | http://www.plover.com/~mjd/perl/py/ . |
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286 | |
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287 | One simple destructive, inside-out approach that you might try is to |
288 | pull out the smallest nesting parts one at a time: |
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289 | |
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290 | while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) { |
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291 | # do something with $1 |
292 | } |
293 | |
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294 | =head2 How do I reverse a string? |
295 | |
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296 | Use reverse() in scalar context, as documented in |
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297 | L<perlfunc/reverse>. |
298 | |
299 | $reversed = reverse $string; |
300 | |
301 | =head2 How do I expand tabs in a string? |
302 | |
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303 | You can do it yourself: |
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304 | |
305 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; |
306 | |
307 | Or you can just use the Text::Tabs module (part of the standard perl |
308 | distribution). |
309 | |
310 | use Text::Tabs; |
311 | @expanded_lines = expand(@lines_with_tabs); |
312 | |
313 | =head2 How do I reformat a paragraph? |
314 | |
315 | Use Text::Wrap (part of the standard perl distribution): |
316 | |
317 | use Text::Wrap; |
318 | print wrap("\t", ' ', @paragraphs); |
319 | |
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320 | The paragraphs you give to Text::Wrap should not contain embedded |
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321 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
322 | |
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323 | =head2 How can I access/change the first N letters of a string? |
324 | |
325 | There are many ways. If you just want to grab a copy, use |
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326 | substr(): |
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327 | |
328 | $first_byte = substr($a, 0, 1); |
329 | |
330 | If you want to modify part of a string, the simplest way is often to |
331 | use substr() as an lvalue: |
332 | |
333 | substr($a, 0, 3) = "Tom"; |
334 | |
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335 | Although those with a pattern matching kind of thought process will |
336 | likely prefer: |
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337 | |
338 | $a =~ s/^.../Tom/; |
339 | |
340 | =head2 How do I change the Nth occurrence of something? |
341 | |
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342 | You have to keep track of N yourself. For example, let's say you want |
343 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into |
344 | C<"whosoever"> or C<"whomsoever">, case insensitively. |
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345 | |
346 | $count = 0; |
347 | s{((whom?)ever)}{ |
348 | ++$count == 5 # is it the 5th? |
349 | ? "${2}soever" # yes, swap |
350 | : $1 # renege and leave it there |
351 | }igex; |
352 | |
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353 | In the more general case, you can use the C</g> modifier in a C<while> |
354 | loop, keeping count of matches. |
355 | |
356 | $WANT = 3; |
357 | $count = 0; |
358 | while (/(\w+)\s+fish\b/gi) { |
359 | if (++$count == $WANT) { |
360 | print "The third fish is a $1 one.\n"; |
361 | # Warning: don't `last' out of this loop |
362 | } |
363 | } |
364 | |
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365 | That prints out: C<"The third fish is a red one."> You can also use a |
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366 | repetition count and repeated pattern like this: |
367 | |
368 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; |
369 | |
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370 | =head2 How can I count the number of occurrences of a substring within a string? |
371 | |
372 | There are a number of ways, with varying efficiency: If you want a |
373 | count of a certain single character (X) within a string, you can use the |
374 | C<tr///> function like so: |
375 | |
376 | $string = "ThisXlineXhasXsomeXx'sXinXit": |
377 | $count = ($string =~ tr/X//); |
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378 | print "There are $count X charcters in the string"; |
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379 | |
380 | This is fine if you are just looking for a single character. However, |
381 | if you are trying to count multiple character substrings within a |
382 | larger string, C<tr///> won't work. What you can do is wrap a while() |
383 | loop around a global pattern match. For example, let's count negative |
384 | integers: |
385 | |
386 | $string = "-9 55 48 -2 23 -76 4 14 -44"; |
387 | while ($string =~ /-\d+/g) { $count++ } |
388 | print "There are $count negative numbers in the string"; |
389 | |
390 | =head2 How do I capitalize all the words on one line? |
391 | |
392 | To make the first letter of each word upper case: |
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393 | |
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394 | $line =~ s/\b(\w)/\U$1/g; |
395 | |
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396 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
397 | Do It>". Sometimes you might want this, instead (Suggested by Brian |
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398 | Foy): |
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399 | |
400 | $string =~ s/ ( |
401 | (^\w) #at the beginning of the line |
402 | | # or |
403 | (\s\w) #preceded by whitespace |
404 | ) |
405 | /\U$1/xg; |
406 | $string =~ /([\w']+)/\u\L$1/g; |
407 | |
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408 | To make the whole line upper case: |
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409 | |
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410 | $line = uc($line); |
411 | |
412 | To force each word to be lower case, with the first letter upper case: |
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413 | |
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414 | $line =~ s/(\w+)/\u\L$1/g; |
415 | |
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416 | You can (and probably should) enable locale awareness of those |
417 | characters by placing a C<use locale> pragma in your program. |
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418 | See L<perllocale> for endless details on locales. |
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419 | |
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420 | =head2 How can I split a [character] delimited string except when inside |
421 | [character]? (Comma-separated files) |
422 | |
423 | Take the example case of trying to split a string that is comma-separated |
424 | into its different fields. (We'll pretend you said comma-separated, not |
425 | comma-delimited, which is different and almost never what you mean.) You |
426 | can't use C<split(/,/)> because you shouldn't split if the comma is inside |
427 | quotes. For example, take a data line like this: |
428 | |
429 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" |
430 | |
431 | Due to the restriction of the quotes, this is a fairly complex |
432 | problem. Thankfully, we have Jeffrey Friedl, author of a highly |
433 | recommended book on regular expressions, to handle these for us. He |
434 | suggests (assuming your string is contained in $text): |
435 | |
436 | @new = (); |
437 | push(@new, $+) while $text =~ m{ |
438 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes |
439 | | ([^,]+),? |
440 | | , |
441 | }gx; |
442 | push(@new, undef) if substr($text,-1,1) eq ','; |
443 | |
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444 | If you want to represent quotation marks inside a |
445 | quotation-mark-delimited field, escape them with backslashes (eg, |
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446 | C<"like \"this\"">. Unescaping them is a task addressed earlier in |
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447 | this section. |
448 | |
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449 | Alternatively, the Text::ParseWords module (part of the standard perl |
450 | distribution) lets you say: |
451 | |
452 | use Text::ParseWords; |
453 | @new = quotewords(",", 0, $text); |
454 | |
455 | =head2 How do I strip blank space from the beginning/end of a string? |
456 | |
5a964f20 |
457 | Although the simplest approach would seem to be: |
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458 | |
459 | $string =~ s/^\s*(.*?)\s*$/$1/; |
460 | |
5a964f20 |
461 | This is unneccesarily slow, destructive, and fails with embedded newlines. |
462 | It is much better faster to do this in two steps: |
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463 | |
464 | $string =~ s/^\s+//; |
465 | $string =~ s/\s+$//; |
466 | |
467 | Or more nicely written as: |
468 | |
469 | for ($string) { |
470 | s/^\s+//; |
471 | s/\s+$//; |
472 | } |
473 | |
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474 | This idiom takes advantage of the C<for(each)> loop's aliasing |
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475 | behavior to factor out common code. You can do this |
476 | on several strings at once, or arrays, or even the |
477 | values of a hash if you use a slide: |
478 | |
479 | # trim whitespace in the scalar, the array, |
480 | # and all the values in the hash |
481 | foreach ($scalar, @array, @hash{keys %hash}) { |
482 | s/^\s+//; |
483 | s/\s+$//; |
484 | } |
485 | |
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486 | =head2 How do I extract selected columns from a string? |
487 | |
488 | Use substr() or unpack(), both documented in L<perlfunc>. |
5a964f20 |
489 | If you prefer thinking in terms of columns instead of widths, |
490 | you can use this kind of thing: |
491 | |
492 | # determine the unpack format needed to split Linux ps output |
493 | # arguments are cut columns |
494 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); |
495 | |
496 | sub cut2fmt { |
497 | my(@positions) = @_; |
498 | my $template = ''; |
499 | my $lastpos = 1; |
500 | for my $place (@positions) { |
501 | $template .= "A" . ($place - $lastpos) . " "; |
502 | $lastpos = $place; |
503 | } |
504 | $template .= "A*"; |
505 | return $template; |
506 | } |
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507 | |
508 | =head2 How do I find the soundex value of a string? |
509 | |
510 | Use the standard Text::Soundex module distributed with perl. |
511 | |
512 | =head2 How can I expand variables in text strings? |
513 | |
514 | Let's assume that you have a string like: |
515 | |
516 | $text = 'this has a $foo in it and a $bar'; |
5a964f20 |
517 | |
518 | If those were both global variables, then this would |
519 | suffice: |
520 | |
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521 | $text =~ s/\$(\w+)/${$1}/g; |
522 | |
5a964f20 |
523 | But since they are probably lexicals, or at least, they could |
524 | be, you'd have to do this: |
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525 | |
526 | $text =~ s/(\$\w+)/$1/eeg; |
5a964f20 |
527 | die if $@; # needed on /ee, not /e |
68dc0745 |
528 | |
5a964f20 |
529 | It's probably better in the general case to treat those |
530 | variables as entries in some special hash. For example: |
531 | |
532 | %user_defs = ( |
533 | foo => 23, |
534 | bar => 19, |
535 | ); |
536 | $text =~ s/\$(\w+)/$user_defs{$1}/g; |
68dc0745 |
537 | |
92c2ed05 |
538 | See also ``How do I expand function calls in a string?'' in this section |
46fc3d4c |
539 | of the FAQ. |
540 | |
68dc0745 |
541 | =head2 What's wrong with always quoting "$vars"? |
542 | |
543 | The problem is that those double-quotes force stringification, |
544 | coercing numbers and references into strings, even when you |
545 | don't want them to be. |
546 | |
547 | If you get used to writing odd things like these: |
548 | |
549 | print "$var"; # BAD |
550 | $new = "$old"; # BAD |
551 | somefunc("$var"); # BAD |
552 | |
553 | You'll be in trouble. Those should (in 99.8% of the cases) be |
554 | the simpler and more direct: |
555 | |
556 | print $var; |
557 | $new = $old; |
558 | somefunc($var); |
559 | |
560 | Otherwise, besides slowing you down, you're going to break code when |
561 | the thing in the scalar is actually neither a string nor a number, but |
562 | a reference: |
563 | |
564 | func(\@array); |
565 | sub func { |
566 | my $aref = shift; |
567 | my $oref = "$aref"; # WRONG |
568 | } |
569 | |
570 | You can also get into subtle problems on those few operations in Perl |
571 | that actually do care about the difference between a string and a |
572 | number, such as the magical C<++> autoincrement operator or the |
573 | syscall() function. |
574 | |
5a964f20 |
575 | Stringification also destroys arrays. |
576 | |
577 | @lines = `command`; |
578 | print "@lines"; # WRONG - extra blanks |
579 | print @lines; # right |
580 | |
68dc0745 |
581 | =head2 Why don't my <<HERE documents work? |
582 | |
583 | Check for these three things: |
584 | |
585 | =over 4 |
586 | |
587 | =item 1. There must be no space after the << part. |
588 | |
589 | =item 2. There (probably) should be a semicolon at the end. |
590 | |
591 | =item 3. You can't (easily) have any space in front of the tag. |
592 | |
593 | =back |
594 | |
5a964f20 |
595 | If you want to indent the text in the here document, you |
596 | can do this: |
597 | |
598 | # all in one |
599 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; |
600 | your text |
601 | goes here |
602 | HERE_TARGET |
603 | |
604 | But the HERE_TARGET must still be flush against the margin. |
605 | If you want that indented also, you'll have to quote |
606 | in the indentation. |
607 | |
608 | ($quote = <<' FINIS') =~ s/^\s+//gm; |
609 | ...we will have peace, when you and all your works have |
610 | perished--and the works of your dark master to whom you |
611 | would deliver us. You are a liar, Saruman, and a corrupter |
612 | of men's hearts. --Theoden in /usr/src/perl/taint.c |
613 | FINIS |
614 | $quote =~ s/\s*--/\n--/; |
615 | |
616 | A nice general-purpose fixer-upper function for indented here documents |
617 | follows. It expects to be called with a here document as its argument. |
618 | It looks to see whether each line begins with a common substring, and |
619 | if so, strips that off. Otherwise, it takes the amount of leading |
620 | white space found on the first line and removes that much off each |
621 | subsequent line. |
622 | |
623 | sub fix { |
624 | local $_ = shift; |
625 | my ($white, $leader); # common white space and common leading string |
626 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
627 | ($white, $leader) = ($2, quotemeta($1)); |
628 | } else { |
629 | ($white, $leader) = (/^(\s+)/, ''); |
630 | } |
631 | s/^\s*?$leader(?:$white)?//gm; |
632 | return $_; |
633 | } |
634 | |
c8db1d39 |
635 | This works with leading special strings, dynamically determined: |
5a964f20 |
636 | |
637 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; |
638 | @@@ int |
639 | @@@ runops() { |
640 | @@@ SAVEI32(runlevel); |
641 | @@@ runlevel++; |
642 | @@@ while ( op = (*op->op_ppaddr)() ) ; |
643 | @@@ TAINT_NOT; |
644 | @@@ return 0; |
645 | @@@ } |
646 | MAIN_INTERPRETER_LOOP |
647 | |
648 | Or with a fixed amount of leading white space, with remaining |
649 | indentation correctly preserved: |
650 | |
651 | $poem = fix<<EVER_ON_AND_ON; |
652 | Now far ahead the Road has gone, |
653 | And I must follow, if I can, |
654 | Pursuing it with eager feet, |
655 | Until it joins some larger way |
656 | Where many paths and errands meet. |
657 | And whither then? I cannot say. |
658 | --Bilbo in /usr/src/perl/pp_ctl.c |
659 | EVER_ON_AND_ON |
660 | |
68dc0745 |
661 | =head1 Data: Arrays |
662 | |
663 | =head2 What is the difference between $array[1] and @array[1]? |
664 | |
665 | The former is a scalar value, the latter an array slice, which makes |
666 | it a list with one (scalar) value. You should use $ when you want a |
667 | scalar value (most of the time) and @ when you want a list with one |
668 | scalar value in it (very, very rarely; nearly never, in fact). |
669 | |
670 | Sometimes it doesn't make a difference, but sometimes it does. |
671 | For example, compare: |
672 | |
673 | $good[0] = `some program that outputs several lines`; |
674 | |
675 | with |
676 | |
677 | @bad[0] = `same program that outputs several lines`; |
678 | |
679 | The B<-w> flag will warn you about these matters. |
680 | |
681 | =head2 How can I extract just the unique elements of an array? |
682 | |
683 | There are several possible ways, depending on whether the array is |
684 | ordered and whether you wish to preserve the ordering. |
685 | |
686 | =over 4 |
687 | |
688 | =item a) If @in is sorted, and you want @out to be sorted: |
5a964f20 |
689 | (this assumes all true values in the array) |
68dc0745 |
690 | |
691 | $prev = 'nonesuch'; |
692 | @out = grep($_ ne $prev && ($prev = $_), @in); |
693 | |
c8db1d39 |
694 | This is nice in that it doesn't use much extra memory, simulating |
695 | uniq(1)'s behavior of removing only adjacent duplicates. It's less |
696 | nice in that it won't work with false values like undef, 0, or ""; |
697 | "0 but true" is ok, though. |
68dc0745 |
698 | |
699 | =item b) If you don't know whether @in is sorted: |
700 | |
701 | undef %saw; |
702 | @out = grep(!$saw{$_}++, @in); |
703 | |
704 | =item c) Like (b), but @in contains only small integers: |
705 | |
706 | @out = grep(!$saw[$_]++, @in); |
707 | |
708 | =item d) A way to do (b) without any loops or greps: |
709 | |
710 | undef %saw; |
711 | @saw{@in} = (); |
712 | @out = sort keys %saw; # remove sort if undesired |
713 | |
714 | =item e) Like (d), but @in contains only small positive integers: |
715 | |
716 | undef @ary; |
717 | @ary[@in] = @in; |
718 | @out = @ary; |
719 | |
720 | =back |
721 | |
5a964f20 |
722 | =head2 How can I tell whether a list or array contains a certain element? |
723 | |
724 | Hearing the word "in" is an I<in>dication that you probably should have |
725 | used a hash, not a list or array, to store your data. Hashes are |
726 | designed to answer this question quickly and efficiently. Arrays aren't. |
68dc0745 |
727 | |
5a964f20 |
728 | That being said, there are several ways to approach this. If you |
729 | are going to make this query many times over arbitrary string values, |
730 | the fastest way is probably to invert the original array and keep an |
68dc0745 |
731 | associative array lying about whose keys are the first array's values. |
732 | |
733 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; |
734 | undef %is_blue; |
735 | for (@blues) { $is_blue{$_} = 1 } |
736 | |
737 | Now you can check whether $is_blue{$some_color}. It might have been a |
738 | good idea to keep the blues all in a hash in the first place. |
739 | |
740 | If the values are all small integers, you could use a simple indexed |
741 | array. This kind of an array will take up less space: |
742 | |
743 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); |
744 | undef @is_tiny_prime; |
745 | for (@primes) { $is_tiny_prime[$_] = 1; } |
746 | |
747 | Now you check whether $is_tiny_prime[$some_number]. |
748 | |
749 | If the values in question are integers instead of strings, you can save |
750 | quite a lot of space by using bit strings instead: |
751 | |
752 | @articles = ( 1..10, 150..2000, 2017 ); |
753 | undef $read; |
7b8d334a |
754 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 |
755 | |
756 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. |
757 | |
758 | Please do not use |
759 | |
760 | $is_there = grep $_ eq $whatever, @array; |
761 | |
762 | or worse yet |
763 | |
764 | $is_there = grep /$whatever/, @array; |
765 | |
766 | These are slow (checks every element even if the first matches), |
767 | inefficient (same reason), and potentially buggy (what if there are |
768 | regexp characters in $whatever?). |
769 | |
770 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
771 | |
772 | Use a hash. Here's code to do both and more. It assumes that |
773 | each element is unique in a given array: |
774 | |
775 | @union = @intersection = @difference = (); |
776 | %count = (); |
777 | foreach $element (@array1, @array2) { $count{$element}++ } |
778 | foreach $element (keys %count) { |
779 | push @union, $element; |
780 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; |
781 | } |
782 | |
783 | =head2 How do I find the first array element for which a condition is true? |
784 | |
785 | You can use this if you care about the index: |
786 | |
787 | for ($i=0; $i < @array; $i++) { |
788 | if ($array[$i] eq "Waldo") { |
789 | $found_index = $i; |
790 | last; |
791 | } |
792 | } |
793 | |
794 | Now C<$found_index> has what you want. |
795 | |
796 | =head2 How do I handle linked lists? |
797 | |
798 | In general, you usually don't need a linked list in Perl, since with |
799 | regular arrays, you can push and pop or shift and unshift at either end, |
5a964f20 |
800 | or you can use splice to add and/or remove arbitrary number of elements at |
801 | arbitrary points. Both pop and shift are both O(1) operations on perl's |
802 | dynamic arrays. In the absence of shifts and pops, push in general |
803 | needs to reallocate on the order every log(N) times, and unshift will |
804 | need to copy pointers each time. |
68dc0745 |
805 | |
806 | If you really, really wanted, you could use structures as described in |
807 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you |
808 | to do. |
809 | |
810 | =head2 How do I handle circular lists? |
811 | |
812 | Circular lists could be handled in the traditional fashion with linked |
813 | lists, or you could just do something like this with an array: |
814 | |
815 | unshift(@array, pop(@array)); # the last shall be first |
816 | push(@array, shift(@array)); # and vice versa |
817 | |
818 | =head2 How do I shuffle an array randomly? |
819 | |
5a964f20 |
820 | Use this: |
821 | |
822 | # fisher_yates_shuffle( \@array ) : |
823 | # generate a random permutation of @array in place |
824 | sub fisher_yates_shuffle { |
825 | my $array = shift; |
826 | my $i; |
827 | for ($i = @$array; --$i; ) { |
828 | my $j = int rand ($i+1); |
829 | next if $i == $j; |
830 | @$array[$i,$j] = @$array[$j,$i]; |
831 | } |
832 | } |
833 | |
834 | fisher_yates_shuffle( \@array ); # permutes @array in place |
835 | |
836 | You've probably seen shuffling algorithms that works using splice, |
68dc0745 |
837 | randomly picking another element to swap the current element with: |
838 | |
839 | srand; |
840 | @new = (); |
841 | @old = 1 .. 10; # just a demo |
842 | while (@old) { |
843 | push(@new, splice(@old, rand @old, 1)); |
844 | } |
845 | |
5a964f20 |
846 | This is bad because splice is already O(N), and since you do it N times, |
847 | you just invented a quadratic algorithm; that is, O(N**2). This does |
848 | not scale, although Perl is so efficient that you probably won't notice |
849 | this until you have rather largish arrays. |
68dc0745 |
850 | |
851 | =head2 How do I process/modify each element of an array? |
852 | |
853 | Use C<for>/C<foreach>: |
854 | |
855 | for (@lines) { |
5a964f20 |
856 | s/foo/bar/; # change that word |
857 | y/XZ/ZX/; # swap those letters |
68dc0745 |
858 | } |
859 | |
860 | Here's another; let's compute spherical volumes: |
861 | |
5a964f20 |
862 | for (@volumes = @radii) { # @volumes has changed parts |
68dc0745 |
863 | $_ **= 3; |
864 | $_ *= (4/3) * 3.14159; # this will be constant folded |
865 | } |
866 | |
5a964f20 |
867 | If you want to do the same thing to modify the values of the hash, |
868 | you may not use the C<values> function, oddly enough. You need a slice: |
869 | |
870 | for $orbit ( @orbits{keys %orbits} ) { |
871 | ($orbit **= 3) *= (4/3) * 3.14159; |
872 | } |
873 | |
68dc0745 |
874 | =head2 How do I select a random element from an array? |
875 | |
876 | Use the rand() function (see L<perlfunc/rand>): |
877 | |
5a964f20 |
878 | # at the top of the program: |
68dc0745 |
879 | srand; # not needed for 5.004 and later |
5a964f20 |
880 | |
881 | # then later on |
68dc0745 |
882 | $index = rand @array; |
883 | $element = $array[$index]; |
884 | |
5a964f20 |
885 | Make sure you I<only call srand once per program, if then>. |
886 | If you are calling it more than once (such as before each |
887 | call to rand), you're almost certainly doing something wrong. |
888 | |
68dc0745 |
889 | =head2 How do I permute N elements of a list? |
890 | |
891 | Here's a little program that generates all permutations |
892 | of all the words on each line of input. The algorithm embodied |
5a964f20 |
893 | in the permute() function should work on any list: |
68dc0745 |
894 | |
895 | #!/usr/bin/perl -n |
5a964f20 |
896 | # tsc-permute: permute each word of input |
897 | permute([split], []); |
898 | sub permute { |
899 | my @items = @{ $_[0] }; |
900 | my @perms = @{ $_[1] }; |
901 | unless (@items) { |
902 | print "@perms\n"; |
68dc0745 |
903 | } else { |
5a964f20 |
904 | my(@newitems,@newperms,$i); |
905 | foreach $i (0 .. $#items) { |
906 | @newitems = @items; |
907 | @newperms = @perms; |
908 | unshift(@newperms, splice(@newitems, $i, 1)); |
909 | permute([@newitems], [@newperms]); |
68dc0745 |
910 | } |
911 | } |
912 | } |
913 | |
914 | =head2 How do I sort an array by (anything)? |
915 | |
916 | Supply a comparison function to sort() (described in L<perlfunc/sort>): |
917 | |
918 | @list = sort { $a <=> $b } @list; |
919 | |
920 | The default sort function is cmp, string comparison, which would |
921 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is |
922 | the numerical comparison operator. |
923 | |
924 | If you have a complicated function needed to pull out the part you |
925 | want to sort on, then don't do it inside the sort function. Pull it |
926 | out first, because the sort BLOCK can be called many times for the |
927 | same element. Here's an example of how to pull out the first word |
928 | after the first number on each item, and then sort those words |
929 | case-insensitively. |
930 | |
931 | @idx = (); |
932 | for (@data) { |
933 | ($item) = /\d+\s*(\S+)/; |
934 | push @idx, uc($item); |
935 | } |
936 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; |
937 | |
938 | Which could also be written this way, using a trick |
939 | that's come to be known as the Schwartzian Transform: |
940 | |
941 | @sorted = map { $_->[0] } |
942 | sort { $a->[1] cmp $b->[1] } |
46fc3d4c |
943 | map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data; |
68dc0745 |
944 | |
945 | If you need to sort on several fields, the following paradigm is useful. |
946 | |
947 | @sorted = sort { field1($a) <=> field1($b) || |
948 | field2($a) cmp field2($b) || |
949 | field3($a) cmp field3($b) |
950 | } @data; |
951 | |
952 | This can be conveniently combined with precalculation of keys as given |
953 | above. |
954 | |
955 | See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about |
956 | this approach. |
957 | |
958 | See also the question below on sorting hashes. |
959 | |
960 | =head2 How do I manipulate arrays of bits? |
961 | |
962 | Use pack() and unpack(), or else vec() and the bitwise operations. |
963 | |
964 | For example, this sets $vec to have bit N set if $ints[N] was set: |
965 | |
966 | $vec = ''; |
967 | foreach(@ints) { vec($vec,$_,1) = 1 } |
968 | |
969 | And here's how, given a vector in $vec, you can |
970 | get those bits into your @ints array: |
971 | |
972 | sub bitvec_to_list { |
973 | my $vec = shift; |
974 | my @ints; |
975 | # Find null-byte density then select best algorithm |
976 | if ($vec =~ tr/\0// / length $vec > 0.95) { |
977 | use integer; |
978 | my $i; |
979 | # This method is faster with mostly null-bytes |
980 | while($vec =~ /[^\0]/g ) { |
981 | $i = -9 + 8 * pos $vec; |
982 | push @ints, $i if vec($vec, ++$i, 1); |
983 | push @ints, $i if vec($vec, ++$i, 1); |
984 | push @ints, $i if vec($vec, ++$i, 1); |
985 | push @ints, $i if vec($vec, ++$i, 1); |
986 | push @ints, $i if vec($vec, ++$i, 1); |
987 | push @ints, $i if vec($vec, ++$i, 1); |
988 | push @ints, $i if vec($vec, ++$i, 1); |
989 | push @ints, $i if vec($vec, ++$i, 1); |
990 | } |
991 | } else { |
992 | # This method is a fast general algorithm |
993 | use integer; |
994 | my $bits = unpack "b*", $vec; |
995 | push @ints, 0 if $bits =~ s/^(\d)// && $1; |
996 | push @ints, pos $bits while($bits =~ /1/g); |
997 | } |
998 | return \@ints; |
999 | } |
1000 | |
1001 | This method gets faster the more sparse the bit vector is. |
1002 | (Courtesy of Tim Bunce and Winfried Koenig.) |
1003 | |
1004 | =head2 Why does defined() return true on empty arrays and hashes? |
1005 | |
1006 | See L<perlfunc/defined> in the 5.004 release or later of Perl. |
1007 | |
1008 | =head1 Data: Hashes (Associative Arrays) |
1009 | |
1010 | =head2 How do I process an entire hash? |
1011 | |
1012 | Use the each() function (see L<perlfunc/each>) if you don't care |
1013 | whether it's sorted: |
1014 | |
5a964f20 |
1015 | while ( ($key, $value) = each %hash) { |
68dc0745 |
1016 | print "$key = $value\n"; |
1017 | } |
1018 | |
1019 | If you want it sorted, you'll have to use foreach() on the result of |
1020 | sorting the keys as shown in an earlier question. |
1021 | |
1022 | =head2 What happens if I add or remove keys from a hash while iterating over it? |
1023 | |
1024 | Don't do that. |
1025 | |
1026 | =head2 How do I look up a hash element by value? |
1027 | |
1028 | Create a reverse hash: |
1029 | |
1030 | %by_value = reverse %by_key; |
1031 | $key = $by_value{$value}; |
1032 | |
1033 | That's not particularly efficient. It would be more space-efficient |
1034 | to use: |
1035 | |
1036 | while (($key, $value) = each %by_key) { |
1037 | $by_value{$value} = $key; |
1038 | } |
1039 | |
1040 | If your hash could have repeated values, the methods above will only |
1041 | find one of the associated keys. This may or may not worry you. |
1042 | |
1043 | =head2 How can I know how many entries are in a hash? |
1044 | |
1045 | If you mean how many keys, then all you have to do is |
1046 | take the scalar sense of the keys() function: |
1047 | |
3fe9a6f1 |
1048 | $num_keys = scalar keys %hash; |
68dc0745 |
1049 | |
1050 | In void context it just resets the iterator, which is faster |
1051 | for tied hashes. |
1052 | |
1053 | =head2 How do I sort a hash (optionally by value instead of key)? |
1054 | |
1055 | Internally, hashes are stored in a way that prevents you from imposing |
1056 | an order on key-value pairs. Instead, you have to sort a list of the |
1057 | keys or values: |
1058 | |
1059 | @keys = sort keys %hash; # sorted by key |
1060 | @keys = sort { |
1061 | $hash{$a} cmp $hash{$b} |
1062 | } keys %hash; # and by value |
1063 | |
1064 | Here we'll do a reverse numeric sort by value, and if two keys are |
1065 | identical, sort by length of key, and if that fails, by straight ASCII |
1066 | comparison of the keys (well, possibly modified by your locale -- see |
1067 | L<perllocale>). |
1068 | |
1069 | @keys = sort { |
1070 | $hash{$b} <=> $hash{$a} |
1071 | || |
1072 | length($b) <=> length($a) |
1073 | || |
1074 | $a cmp $b |
1075 | } keys %hash; |
1076 | |
1077 | =head2 How can I always keep my hash sorted? |
1078 | |
1079 | You can look into using the DB_File module and tie() using the |
1080 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. |
5a964f20 |
1081 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 |
1082 | |
1083 | =head2 What's the difference between "delete" and "undef" with hashes? |
1084 | |
1085 | Hashes are pairs of scalars: the first is the key, the second is the |
1086 | value. The key will be coerced to a string, although the value can be |
1087 | any kind of scalar: string, number, or reference. If a key C<$key> is |
1088 | present in the array, C<exists($key)> will return true. The value for |
1089 | a given key can be C<undef>, in which case C<$array{$key}> will be |
1090 | C<undef> while C<$exists{$key}> will return true. This corresponds to |
1091 | (C<$key>, C<undef>) being in the hash. |
1092 | |
1093 | Pictures help... here's the C<%ary> table: |
1094 | |
1095 | keys values |
1096 | +------+------+ |
1097 | | a | 3 | |
1098 | | x | 7 | |
1099 | | d | 0 | |
1100 | | e | 2 | |
1101 | +------+------+ |
1102 | |
1103 | And these conditions hold |
1104 | |
1105 | $ary{'a'} is true |
1106 | $ary{'d'} is false |
1107 | defined $ary{'d'} is true |
1108 | defined $ary{'a'} is true |
1109 | exists $ary{'a'} is true (perl5 only) |
1110 | grep ($_ eq 'a', keys %ary) is true |
1111 | |
1112 | If you now say |
1113 | |
1114 | undef $ary{'a'} |
1115 | |
1116 | your table now reads: |
1117 | |
1118 | |
1119 | keys values |
1120 | +------+------+ |
1121 | | a | undef| |
1122 | | x | 7 | |
1123 | | d | 0 | |
1124 | | e | 2 | |
1125 | +------+------+ |
1126 | |
1127 | and these conditions now hold; changes in caps: |
1128 | |
1129 | $ary{'a'} is FALSE |
1130 | $ary{'d'} is false |
1131 | defined $ary{'d'} is true |
1132 | defined $ary{'a'} is FALSE |
1133 | exists $ary{'a'} is true (perl5 only) |
1134 | grep ($_ eq 'a', keys %ary) is true |
1135 | |
1136 | Notice the last two: you have an undef value, but a defined key! |
1137 | |
1138 | Now, consider this: |
1139 | |
1140 | delete $ary{'a'} |
1141 | |
1142 | your table now reads: |
1143 | |
1144 | keys values |
1145 | +------+------+ |
1146 | | x | 7 | |
1147 | | d | 0 | |
1148 | | e | 2 | |
1149 | +------+------+ |
1150 | |
1151 | and these conditions now hold; changes in caps: |
1152 | |
1153 | $ary{'a'} is false |
1154 | $ary{'d'} is false |
1155 | defined $ary{'d'} is true |
1156 | defined $ary{'a'} is false |
1157 | exists $ary{'a'} is FALSE (perl5 only) |
1158 | grep ($_ eq 'a', keys %ary) is FALSE |
1159 | |
1160 | See, the whole entry is gone! |
1161 | |
1162 | =head2 Why don't my tied hashes make the defined/exists distinction? |
1163 | |
1164 | They may or may not implement the EXISTS() and DEFINED() methods |
1165 | differently. For example, there isn't the concept of undef with hashes |
1166 | that are tied to DBM* files. This means the true/false tables above |
1167 | will give different results when used on such a hash. It also means |
1168 | that exists and defined do the same thing with a DBM* file, and what |
1169 | they end up doing is not what they do with ordinary hashes. |
1170 | |
1171 | =head2 How do I reset an each() operation part-way through? |
1172 | |
5a964f20 |
1173 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 |
1174 | the hash I<and> resets the iterator associated with the hash. You may |
1175 | need to do this if you use C<last> to exit a loop early so that when you |
46fc3d4c |
1176 | re-enter it, the hash iterator has been reset. |
68dc0745 |
1177 | |
1178 | =head2 How can I get the unique keys from two hashes? |
1179 | |
1180 | First you extract the keys from the hashes into arrays, and then solve |
1181 | the uniquifying the array problem described above. For example: |
1182 | |
1183 | %seen = (); |
1184 | for $element (keys(%foo), keys(%bar)) { |
1185 | $seen{$element}++; |
1186 | } |
1187 | @uniq = keys %seen; |
1188 | |
1189 | Or more succinctly: |
1190 | |
1191 | @uniq = keys %{{%foo,%bar}}; |
1192 | |
1193 | Or if you really want to save space: |
1194 | |
1195 | %seen = (); |
1196 | while (defined ($key = each %foo)) { |
1197 | $seen{$key}++; |
1198 | } |
1199 | while (defined ($key = each %bar)) { |
1200 | $seen{$key}++; |
1201 | } |
1202 | @uniq = keys %seen; |
1203 | |
1204 | =head2 How can I store a multidimensional array in a DBM file? |
1205 | |
1206 | Either stringify the structure yourself (no fun), or else |
1207 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer |
1208 | it on top of either DB_File or GDBM_File. |
1209 | |
1210 | =head2 How can I make my hash remember the order I put elements into it? |
1211 | |
1212 | Use the Tie::IxHash from CPAN. |
1213 | |
46fc3d4c |
1214 | use Tie::IxHash; |
1215 | tie(%myhash, Tie::IxHash); |
1216 | for ($i=0; $i<20; $i++) { |
1217 | $myhash{$i} = 2*$i; |
1218 | } |
1219 | @keys = keys %myhash; |
1220 | # @keys = (0,1,2,3,...) |
1221 | |
68dc0745 |
1222 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1223 | |
1224 | If you say something like: |
1225 | |
1226 | somefunc($hash{"nonesuch key here"}); |
1227 | |
1228 | Then that element "autovivifies"; that is, it springs into existence |
1229 | whether you store something there or not. That's because functions |
1230 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, |
1231 | it has to be ready to write it back into the caller's version. |
1232 | |
1233 | This has been fixed as of perl5.004. |
1234 | |
1235 | Normally, merely accessing a key's value for a nonexistent key does |
1236 | I<not> cause that key to be forever there. This is different than |
1237 | awk's behavior. |
1238 | |
fc36a67e |
1239 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 |
1240 | |
1241 | Use references (documented in L<perlref>). Examples of complex data |
1242 | structures are given in L<perldsc> and L<perllol>. Examples of |
1243 | structures and object-oriented classes are in L<perltoot>. |
1244 | |
1245 | =head2 How can I use a reference as a hash key? |
1246 | |
1247 | You can't do this directly, but you could use the standard Tie::Refhash |
1248 | module distributed with perl. |
1249 | |
1250 | =head1 Data: Misc |
1251 | |
1252 | =head2 How do I handle binary data correctly? |
1253 | |
1254 | Perl is binary clean, so this shouldn't be a problem. For example, |
1255 | this works fine (assuming the files are found): |
1256 | |
1257 | if (`cat /vmunix` =~ /gzip/) { |
1258 | print "Your kernel is GNU-zip enabled!\n"; |
1259 | } |
1260 | |
1261 | On some systems, however, you have to play tedious games with "text" |
1262 | versus "binary" files. See L<perlfunc/"binmode">. |
1263 | |
1264 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. |
1265 | |
54310121 |
1266 | If you want to deal with multibyte characters, however, there are |
68dc0745 |
1267 | some gotchas. See the section on Regular Expressions. |
1268 | |
1269 | =head2 How do I determine whether a scalar is a number/whole/integer/float? |
1270 | |
1271 | Assuming that you don't care about IEEE notations like "NaN" or |
1272 | "Infinity", you probably just want to use a regular expression. |
1273 | |
1274 | warn "has nondigits" if /\D/; |
5a964f20 |
1275 | warn "not a natural number" unless /^\d+$/; # rejects -3 |
1276 | warn "not an integer" unless /^-?\d+$/; # rejects +3 |
54310121 |
1277 | warn "not an integer" unless /^[+-]?\d+$/; |
68dc0745 |
1278 | warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 |
1279 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; |
1280 | warn "not a C float" |
1281 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
1282 | |
5a964f20 |
1283 | If you're on a POSIX system, Perl's supports the C<POSIX::strtod> |
1284 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> |
1285 | wrapper function for more convenient access. This function takes |
1286 | a string and returns the number it found, or C<undef> for input that |
1287 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> |
1288 | if you just want to say, ``Is this a float?'' |
1289 | |
1290 | sub getnum { |
1291 | use POSIX qw(strtod); |
1292 | my $str = shift; |
1293 | $str =~ s/^\s+//; |
1294 | $str =~ s/\s+$//; |
1295 | $! = 0; |
1296 | my($num, $unparsed) = strtod($str); |
1297 | if (($str eq '') || ($unparsed != 0) || $!) { |
1298 | return undef; |
1299 | } else { |
1300 | return $num; |
1301 | } |
1302 | } |
1303 | |
1304 | sub is_numeric { defined &getnum } |
1305 | |
68dc0745 |
1306 | Or you could check out |
1307 | http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz |
1308 | instead. The POSIX module (part of the standard Perl distribution) |
1309 | provides the C<strtol> and C<strtod> for converting strings to double |
1310 | and longs, respectively. |
1311 | |
1312 | =head2 How do I keep persistent data across program calls? |
1313 | |
1314 | For some specific applications, you can use one of the DBM modules. |
1315 | See L<AnyDBM_File>. More generically, you should consult the |
1316 | FreezeThaw, Storable, or Class::Eroot modules from CPAN. |
1317 | |
1318 | =head2 How do I print out or copy a recursive data structure? |
1319 | |
1320 | The Data::Dumper module on CPAN is nice for printing out |
1321 | data structures, and FreezeThaw for copying them. For example: |
1322 | |
1323 | use FreezeThaw qw(freeze thaw); |
1324 | $new = thaw freeze $old; |
1325 | |
1326 | Where $old can be (a reference to) any kind of data structure you'd like. |
1327 | It will be deeply copied. |
1328 | |
1329 | =head2 How do I define methods for every class/object? |
1330 | |
1331 | Use the UNIVERSAL class (see L<UNIVERSAL>). |
1332 | |
1333 | =head2 How do I verify a credit card checksum? |
1334 | |
1335 | Get the Business::CreditCard module from CPAN. |
1336 | |
1337 | =head1 AUTHOR AND COPYRIGHT |
1338 | |
5a964f20 |
1339 | Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington. |
1340 | All rights reserved. |
1341 | |
1342 | When included as part of the Standard Version of Perl, or as part of |
1343 | its complete documentation whether printed or otherwise, this work |
1344 | may be distributed only under the terms of Perl's Artistic License. |
1345 | Any distribution of this file or derivatives thereof I<outside> |
1346 | of that package require that special arrangements be made with |
1347 | copyright holder. |
1348 | |
1349 | Irrespective of its distribution, all code examples in this file |
1350 | are hereby placed into the public domain. You are permitted and |
1351 | encouraged to use this code in your own programs for fun |
1352 | or for profit as you see fit. A simple comment in the code giving |
1353 | credit would be courteous but is not required. |