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1 | =head1 NAME |
2 | |
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3 | perlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date: 1999/01/08 04:26:39 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | The section of the FAQ answers question related to the manipulation |
8 | of data as numbers, dates, strings, arrays, hashes, and miscellaneous |
9 | data issues. |
10 | |
11 | =head1 Data: Numbers |
12 | |
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13 | =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? |
14 | |
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15 | The infinite set that a mathematician thinks of as the real numbers can |
16 | only be approximate on a computer, since the computer only has a finite |
17 | number of bits to store an infinite number of, um, numbers. |
18 | |
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19 | Internally, your computer represents floating-point numbers in binary. |
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20 | Floating-point numbers read in from a file or appearing as literals |
21 | in your program are converted from their decimal floating-point |
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22 | representation (eg, 19.95) to the internal binary representation. |
23 | |
24 | However, 19.95 can't be precisely represented as a binary |
25 | floating-point number, just like 1/3 can't be exactly represented as a |
26 | decimal floating-point number. The computer's binary representation |
27 | of 19.95, therefore, isn't exactly 19.95. |
28 | |
29 | When a floating-point number gets printed, the binary floating-point |
30 | representation is converted back to decimal. These decimal numbers |
31 | are displayed in either the format you specify with printf(), or the |
32 | current output format for numbers (see L<perlvar/"$#"> 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 | |
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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 | $floor = floor(3.5); # 3 |
77 | |
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78 | In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex |
79 | module. With 5.004, the Math::Trig module (part of the standard perl |
80 | distribution) implements the trigonometric functions. Internally it |
81 | uses the Math::Complex module and some functions can break out from |
82 | the real axis into the complex plane, for example the inverse sine of |
83 | 2. |
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84 | |
85 | Rounding in financial applications can have serious implications, and |
86 | the rounding method used should be specified precisely. In these |
87 | cases, it probably pays not to trust whichever system rounding is |
88 | being used by Perl, but to instead implement the rounding function you |
89 | need yourself. |
90 | |
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91 | To see why, notice how you'll still have an issue on half-way-point |
92 | alternation: |
93 | |
94 | for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} |
95 | |
96 | 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7 |
97 | 0.8 0.8 0.9 0.9 1.0 1.0 |
98 | |
99 | Don't blame Perl. It's the same as in C. IEEE says we have to do this. |
100 | Perl numbers whose absolute values are integers under 2**31 (on 32 bit |
101 | machines) will work pretty much like mathematical integers. Other numbers |
102 | are not guaranteed. |
103 | |
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104 | =head2 How do I convert bits into ints? |
105 | |
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106 | To turn a string of 1s and 0s like C<10110110> into a scalar containing |
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107 | its binary value, use the pack() function (documented in |
108 | L<perlfunc/"pack">): |
109 | |
110 | $decimal = pack('B8', '10110110'); |
111 | |
112 | Here's an example of going the other way: |
113 | |
114 | $binary_string = join('', unpack('B*', "\x29")); |
115 | |
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116 | =head2 Why doesn't & work the way I want it to? |
117 | |
118 | The behavior of binary arithmetic operators depends on whether they're |
119 | used on numbers or strings. The operators treat a string as a series |
120 | of bits and work with that (the string C<"3"> is the bit pattern |
121 | C<00110011>). The operators work with the binary form of a number |
122 | (the number C<3> is treated as the bit pattern C<00000011>). |
123 | |
124 | So, saying C<11 & 3> performs the "and" operation on numbers (yielding |
125 | C<1>). Saying C<"11" & "3"> performs the "and" operation on strings |
126 | (yielding C<"1">). |
127 | |
128 | Most problems with C<&> and C<|> arise because the programmer thinks |
129 | they have a number but really it's a string. The rest arise because |
130 | the programmer says: |
131 | |
132 | if ("\020\020" & "\101\101") { |
133 | # ... |
134 | } |
135 | |
136 | but a string consisting of two null bytes (the result of C<"\020\020" |
137 | & "\101\101">) is not a false value in Perl. You need: |
138 | |
139 | if ( ("\020\020" & "\101\101") !~ /[^\000]/) { |
140 | # ... |
141 | } |
142 | |
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143 | =head2 How do I multiply matrices? |
144 | |
145 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) |
146 | or the PDL extension (also available from CPAN). |
147 | |
148 | =head2 How do I perform an operation on a series of integers? |
149 | |
150 | To call a function on each element in an array, and collect the |
151 | results, use: |
152 | |
153 | @results = map { my_func($_) } @array; |
154 | |
155 | For example: |
156 | |
157 | @triple = map { 3 * $_ } @single; |
158 | |
159 | To call a function on each element of an array, but ignore the |
160 | results: |
161 | |
162 | foreach $iterator (@array) { |
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163 | some_func($iterator); |
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164 | } |
165 | |
166 | To call a function on each integer in a (small) range, you B<can> use: |
167 | |
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168 | @results = map { some_func($_) } (5 .. 25); |
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169 | |
170 | but you should be aware that the C<..> operator creates an array of |
171 | all integers in the range. This can take a lot of memory for large |
172 | ranges. Instead use: |
173 | |
174 | @results = (); |
175 | for ($i=5; $i < 500_005; $i++) { |
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176 | push(@results, some_func($i)); |
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177 | } |
178 | |
179 | =head2 How can I output Roman numerals? |
180 | |
181 | Get the http://www.perl.com/CPAN/modules/by-module/Roman module. |
182 | |
183 | =head2 Why aren't my random numbers random? |
184 | |
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185 | If you're using a version of Perl before 5.004, you must call C<srand> |
186 | once at the start of your program to seed the random number generator. |
187 | 5.004 and later automatically call C<srand> at the beginning. Don't |
188 | call C<srand> more than once--you make your numbers less random, rather |
189 | than more. |
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190 | |
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191 | Computers are good at being predictable and bad at being random |
192 | (despite appearances caused by bugs in your programs :-). |
193 | http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom |
194 | Phoenix, talks more about this.. John von Neumann said, ``Anyone who |
195 | attempts to generate random numbers by deterministic means is, of |
196 | course, living in a state of sin.'' |
197 | |
198 | If you want numbers that are more random than C<rand> with C<srand> |
199 | provides, you should also check out the Math::TrulyRandom module from |
200 | CPAN. It uses the imperfections in your system's timer to generate |
201 | random numbers, but this takes quite a while. If you want a better |
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202 | pseudorandom generator than comes with your operating system, look at |
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203 | ``Numerical Recipes in C'' at http://www.nr.com/ . |
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204 | |
205 | =head1 Data: Dates |
206 | |
207 | =head2 How do I find the week-of-the-year/day-of-the-year? |
208 | |
209 | The day of the year is in the array returned by localtime() (see |
210 | L<perlfunc/"localtime">): |
211 | |
212 | $day_of_year = (localtime(time()))[7]; |
213 | |
214 | or more legibly (in 5.004 or higher): |
215 | |
216 | use Time::localtime; |
217 | $day_of_year = localtime(time())->yday; |
218 | |
219 | You can find the week of the year by dividing this by 7: |
220 | |
221 | $week_of_year = int($day_of_year / 7); |
222 | |
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223 | Of course, this believes that weeks start at zero. The Date::Calc |
224 | module from CPAN has a lot of date calculation functions, including |
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225 | day of the year, week of the year, and so on. Note that not |
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226 | all businesses consider ``week 1'' to be the same; for example, |
227 | American businesses often consider the first week with a Monday |
228 | in it to be Work Week #1, despite ISO 8601, which considers |
229 | WW1 to be the first week with a Thursday in it. |
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230 | |
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231 | =head2 How can I compare two dates and find the difference? |
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232 | |
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233 | If you're storing your dates as epoch seconds then simply subtract one |
234 | from the other. If you've got a structured date (distinct year, day, |
235 | month, hour, minute, seconds values) then use one of the Date::Manip |
236 | and Date::Calc modules from CPAN. |
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237 | |
238 | =head2 How can I take a string and turn it into epoch seconds? |
239 | |
240 | If it's a regular enough string that it always has the same format, |
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241 | you can split it up and pass the parts to C<timelocal> in the standard |
242 | Time::Local module. Otherwise, you should look into the Date::Calc |
243 | and Date::Manip modules from CPAN. |
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244 | |
245 | =head2 How can I find the Julian Day? |
246 | |
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247 | Neither Date::Manip nor Date::Calc deal with Julian days. Instead, |
248 | there is an example of Julian date calculation that should help you in |
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249 | Time::JulianDay (part of the Time-modules bundle) which can be found at |
250 | http://www.perl.com/CPAN/modules/by-module/Time/. |
251 | |
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252 | |
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253 | =head2 How do I find yesterday's date? |
254 | |
255 | The C<time()> function returns the current time in seconds since the |
256 | epoch. Take one day off that: |
257 | |
258 | $yesterday = time() - ( 24 * 60 * 60 ); |
259 | |
260 | Then you can pass this to C<localtime()> and get the individual year, |
261 | month, day, hour, minute, seconds values. |
262 | |
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263 | =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant? |
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264 | |
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265 | Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is |
266 | Y2K compliant (whatever that means). The programmers you've hired to |
267 | use it, however, probably are not. |
268 | |
269 | Long answer: The question belies a true understanding of the issue. |
270 | Perl is just as Y2K compliant as your pencil--no more, and no less. |
271 | Can you use your pencil to write a non-Y2K-compliant memo? Of course |
272 | you can. Is that the pencil's fault? Of course it isn't. |
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273 | |
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274 | The date and time functions supplied with perl (gmtime and localtime) |
275 | supply adequate information to determine the year well beyond 2000 |
276 | (2038 is when trouble strikes for 32-bit machines). The year returned |
277 | by these functions when used in an array context is the year minus 1900. |
278 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
279 | number. To avoid the year 2000 problem simply do not treat the year as |
280 | a 2-digit number. It isn't. |
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281 | |
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282 | When gmtime() and localtime() are used in scalar context they return |
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283 | a timestamp string that contains a fully-expanded year. For example, |
284 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 |
285 | 2001". There's no year 2000 problem here. |
286 | |
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287 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
288 | programs. It can. But so can your pencil. It's the fault of the user, |
289 | not the language. At the risk of inflaming the NRA: ``Perl doesn't |
290 | break Y2K, people do.'' See http://language.perl.com/news/y2k.html for |
291 | a longer exposition. |
292 | |
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293 | =head1 Data: Strings |
294 | |
295 | =head2 How do I validate input? |
296 | |
297 | The answer to this question is usually a regular expression, perhaps |
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298 | with auxiliary logic. See the more specific questions (numbers, mail |
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299 | addresses, etc.) for details. |
300 | |
301 | =head2 How do I unescape a string? |
302 | |
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303 | It depends just what you mean by ``escape''. URL escapes are dealt |
304 | with in L<perlfaq9>. Shell escapes with the backslash (C<\>) |
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305 | character are removed with: |
306 | |
307 | s/\\(.)/$1/g; |
308 | |
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309 | This won't expand C<"\n"> or C<"\t"> or any other special escapes. |
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310 | |
311 | =head2 How do I remove consecutive pairs of characters? |
312 | |
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313 | To turn C<"abbcccd"> into C<"abccd">: |
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314 | |
315 | s/(.)\1/$1/g; |
316 | |
317 | =head2 How do I expand function calls in a string? |
318 | |
319 | This is documented in L<perlref>. In general, this is fraught with |
320 | quoting and readability problems, but it is possible. To interpolate |
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321 | a subroutine call (in list context) into a string: |
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322 | |
323 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
324 | |
325 | If you prefer scalar context, similar chicanery is also useful for |
326 | arbitrary expressions: |
327 | |
328 | print "That yields ${\($n + 5)} widgets\n"; |
329 | |
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330 | Version 5.004 of Perl had a bug that gave list context to the |
331 | expression in C<${...}>, but this is fixed in version 5.005. |
332 | |
333 | See also ``How can I expand variables in text strings?'' in this |
334 | section of the FAQ. |
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335 | |
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336 | =head2 How do I find matching/nesting anything? |
337 | |
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338 | This isn't something that can be done in one regular expression, no |
339 | matter how complicated. To find something between two single |
340 | characters, a pattern like C</x([^x]*)x/> will get the intervening |
341 | bits in $1. For multiple ones, then something more like |
342 | C</alpha(.*?)omega/> would be needed. But none of these deals with |
343 | nested patterns, nor can they. For that you'll have to write a |
344 | parser. |
345 | |
346 | If you are serious about writing a parser, there are a number of |
347 | modules or oddities that will make your life a lot easier. There is |
348 | the CPAN module Parse::RecDescent, the standard module Text::Balanced, |
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349 | the byacc program, the CPAN module Parse::Yapp, and Mark-Jason |
350 | Dominus's excellent I<py> tool at http://www.plover.com/~mjd/perl/py/ |
351 | . |
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352 | |
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353 | One simple destructive, inside-out approach that you might try is to |
354 | pull out the smallest nesting parts one at a time: |
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355 | |
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356 | while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) { |
5a964f20 |
357 | # do something with $1 |
358 | } |
359 | |
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360 | A more complicated and sneaky approach is to make Perl's regular |
361 | expression engine do it for you. This is courtesy Dean Inada, and |
362 | rather has the nature of an Obfuscated Perl Contest entry, but it |
363 | really does work: |
364 | |
365 | # $_ contains the string to parse |
366 | # BEGIN and END are the opening and closing markers for the |
367 | # nested text. |
368 | |
369 | @( = ('(',''); |
370 | @) = (')',''); |
371 | ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; |
372 | @$ = (eval{/$re/},$@!~/unmatched/); |
373 | print join("\n",@$[0..$#$]) if( $$[-1] ); |
374 | |
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375 | =head2 How do I reverse a string? |
376 | |
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377 | Use reverse() in scalar context, as documented in |
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378 | L<perlfunc/reverse>. |
379 | |
380 | $reversed = reverse $string; |
381 | |
382 | =head2 How do I expand tabs in a string? |
383 | |
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384 | You can do it yourself: |
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385 | |
386 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; |
387 | |
388 | Or you can just use the Text::Tabs module (part of the standard perl |
389 | distribution). |
390 | |
391 | use Text::Tabs; |
392 | @expanded_lines = expand(@lines_with_tabs); |
393 | |
394 | =head2 How do I reformat a paragraph? |
395 | |
396 | Use Text::Wrap (part of the standard perl distribution): |
397 | |
398 | use Text::Wrap; |
399 | print wrap("\t", ' ', @paragraphs); |
400 | |
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401 | The paragraphs you give to Text::Wrap should not contain embedded |
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402 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
403 | |
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404 | =head2 How can I access/change the first N letters of a string? |
405 | |
406 | There are many ways. If you just want to grab a copy, use |
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407 | substr(): |
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408 | |
409 | $first_byte = substr($a, 0, 1); |
410 | |
411 | If you want to modify part of a string, the simplest way is often to |
412 | use substr() as an lvalue: |
413 | |
414 | substr($a, 0, 3) = "Tom"; |
415 | |
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416 | Although those with a pattern matching kind of thought process will |
417 | likely prefer: |
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418 | |
419 | $a =~ s/^.../Tom/; |
420 | |
421 | =head2 How do I change the Nth occurrence of something? |
422 | |
92c2ed05 |
423 | You have to keep track of N yourself. For example, let's say you want |
424 | to change the fifth occurrence of C<"whoever"> or C<"whomever"> into |
425 | C<"whosoever"> or C<"whomsoever">, case insensitively. |
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426 | |
427 | $count = 0; |
428 | s{((whom?)ever)}{ |
429 | ++$count == 5 # is it the 5th? |
430 | ? "${2}soever" # yes, swap |
431 | : $1 # renege and leave it there |
432 | }igex; |
433 | |
5a964f20 |
434 | In the more general case, you can use the C</g> modifier in a C<while> |
435 | loop, keeping count of matches. |
436 | |
437 | $WANT = 3; |
438 | $count = 0; |
439 | while (/(\w+)\s+fish\b/gi) { |
440 | if (++$count == $WANT) { |
441 | print "The third fish is a $1 one.\n"; |
442 | # Warning: don't `last' out of this loop |
443 | } |
444 | } |
445 | |
92c2ed05 |
446 | That prints out: C<"The third fish is a red one."> You can also use a |
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447 | repetition count and repeated pattern like this: |
448 | |
449 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; |
450 | |
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451 | =head2 How can I count the number of occurrences of a substring within a string? |
452 | |
453 | There are a number of ways, with varying efficiency: If you want a |
454 | count of a certain single character (X) within a string, you can use the |
455 | C<tr///> function like so: |
456 | |
368c9434 |
457 | $string = "ThisXlineXhasXsomeXx'sXinXit"; |
68dc0745 |
458 | $count = ($string =~ tr/X//); |
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459 | print "There are $count X charcters in the string"; |
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460 | |
461 | This is fine if you are just looking for a single character. However, |
462 | if you are trying to count multiple character substrings within a |
463 | larger string, C<tr///> won't work. What you can do is wrap a while() |
464 | loop around a global pattern match. For example, let's count negative |
465 | integers: |
466 | |
467 | $string = "-9 55 48 -2 23 -76 4 14 -44"; |
468 | while ($string =~ /-\d+/g) { $count++ } |
469 | print "There are $count negative numbers in the string"; |
470 | |
471 | =head2 How do I capitalize all the words on one line? |
472 | |
473 | To make the first letter of each word upper case: |
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474 | |
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475 | $line =~ s/\b(\w)/\U$1/g; |
476 | |
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477 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
478 | Do It>". Sometimes you might want this, instead (Suggested by Brian |
92c2ed05 |
479 | Foy): |
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480 | |
481 | $string =~ s/ ( |
482 | (^\w) #at the beginning of the line |
483 | | # or |
484 | (\s\w) #preceded by whitespace |
485 | ) |
486 | /\U$1/xg; |
487 | $string =~ /([\w']+)/\u\L$1/g; |
488 | |
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489 | To make the whole line upper case: |
3fe9a6f1 |
490 | |
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491 | $line = uc($line); |
492 | |
493 | To force each word to be lower case, with the first letter upper case: |
3fe9a6f1 |
494 | |
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495 | $line =~ s/(\w+)/\u\L$1/g; |
496 | |
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497 | You can (and probably should) enable locale awareness of those |
498 | characters by placing a C<use locale> pragma in your program. |
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499 | See L<perllocale> for endless details on locales. |
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500 | |
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501 | This is sometimes referred to as putting something into "title |
502 | case", but that's not quite accurate. Consdier the proper |
503 | capitalization of the movie I<Dr. Strangelove or: How I Learned to |
504 | Stop Worrying and Love the Bomb>, for example. |
505 | |
68dc0745 |
506 | =head2 How can I split a [character] delimited string except when inside |
507 | [character]? (Comma-separated files) |
508 | |
509 | Take the example case of trying to split a string that is comma-separated |
510 | into its different fields. (We'll pretend you said comma-separated, not |
511 | comma-delimited, which is different and almost never what you mean.) You |
512 | can't use C<split(/,/)> because you shouldn't split if the comma is inside |
513 | quotes. For example, take a data line like this: |
514 | |
515 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" |
516 | |
517 | Due to the restriction of the quotes, this is a fairly complex |
518 | problem. Thankfully, we have Jeffrey Friedl, author of a highly |
519 | recommended book on regular expressions, to handle these for us. He |
520 | suggests (assuming your string is contained in $text): |
521 | |
522 | @new = (); |
523 | push(@new, $+) while $text =~ m{ |
524 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes |
525 | | ([^,]+),? |
526 | | , |
527 | }gx; |
528 | push(@new, undef) if substr($text,-1,1) eq ','; |
529 | |
46fc3d4c |
530 | If you want to represent quotation marks inside a |
531 | quotation-mark-delimited field, escape them with backslashes (eg, |
2ceaccd7 |
532 | C<"like \"this\"">. Unescaping them is a task addressed earlier in |
46fc3d4c |
533 | this section. |
534 | |
68dc0745 |
535 | Alternatively, the Text::ParseWords module (part of the standard perl |
536 | distribution) lets you say: |
537 | |
538 | use Text::ParseWords; |
539 | @new = quotewords(",", 0, $text); |
540 | |
65acb1b1 |
541 | There's also a Text::CSV module on CPAN. |
542 | |
68dc0745 |
543 | =head2 How do I strip blank space from the beginning/end of a string? |
544 | |
5a964f20 |
545 | Although the simplest approach would seem to be: |
68dc0745 |
546 | |
547 | $string =~ s/^\s*(.*?)\s*$/$1/; |
548 | |
65acb1b1 |
549 | This is unnecessarily slow, destructive, and fails with embedded newlines. |
5a964f20 |
550 | It is much better faster to do this in two steps: |
68dc0745 |
551 | |
552 | $string =~ s/^\s+//; |
553 | $string =~ s/\s+$//; |
554 | |
555 | Or more nicely written as: |
556 | |
557 | for ($string) { |
558 | s/^\s+//; |
559 | s/\s+$//; |
560 | } |
561 | |
5e3006a4 |
562 | This idiom takes advantage of the C<foreach> loop's aliasing |
5a964f20 |
563 | behavior to factor out common code. You can do this |
564 | on several strings at once, or arrays, or even the |
565 | values of a hash if you use a slide: |
566 | |
567 | # trim whitespace in the scalar, the array, |
568 | # and all the values in the hash |
569 | foreach ($scalar, @array, @hash{keys %hash}) { |
570 | s/^\s+//; |
571 | s/\s+$//; |
572 | } |
573 | |
65acb1b1 |
574 | =head2 How do I pad a string with blanks or pad a number with zeroes? |
575 | |
576 | (This answer contributed by Uri Guttman) |
577 | |
578 | In the following examples, C<$pad_len> is the length to which you wish |
579 | to pad the string, C<$text> or C<$num> contains the string to be |
580 | padded, and C<$pad_char> contains the padding character. You can use a |
581 | single character string constant instead of the C<$pad_char> variable |
582 | if you know what it is in advance. |
583 | |
584 | The simplest method use the C<sprintf> function. It can pad on the |
585 | left or right with blanks and on the left with zeroes. |
586 | |
587 | # Left padding with blank: |
588 | $padded = sprintf( "%${pad_len}s", $text ) ; |
589 | |
590 | # Right padding with blank: |
591 | $padded = sprintf( "%${pad_len}s", $text ) ; |
592 | |
593 | # Left padding with 0: |
594 | $padded = sprintf( "%0${pad_len}d", $num ) ; |
595 | |
596 | If you need to pad with a character other than blank or zero you can use |
597 | one of the following methods. |
598 | |
599 | These methods generate a pad string with the C<x> operator and |
600 | concatenate that with the original text. |
601 | |
602 | Left and right padding with any character: |
603 | |
604 | $padded = $pad_char x ( $pad_len - length( $text ) ) . $text ; |
605 | $padded = $text . $pad_char x ( $pad_len - length( $text ) ) ; |
606 | |
607 | Or you can left or right pad $text directly: |
608 | |
609 | $text .= $pad_char x ( $pad_len - length( $text ) ) ; |
610 | substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ) ; |
611 | |
68dc0745 |
612 | =head2 How do I extract selected columns from a string? |
613 | |
614 | Use substr() or unpack(), both documented in L<perlfunc>. |
5a964f20 |
615 | If you prefer thinking in terms of columns instead of widths, |
616 | you can use this kind of thing: |
617 | |
618 | # determine the unpack format needed to split Linux ps output |
619 | # arguments are cut columns |
620 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); |
621 | |
622 | sub cut2fmt { |
623 | my(@positions) = @_; |
624 | my $template = ''; |
625 | my $lastpos = 1; |
626 | for my $place (@positions) { |
627 | $template .= "A" . ($place - $lastpos) . " "; |
628 | $lastpos = $place; |
629 | } |
630 | $template .= "A*"; |
631 | return $template; |
632 | } |
68dc0745 |
633 | |
634 | =head2 How do I find the soundex value of a string? |
635 | |
636 | Use the standard Text::Soundex module distributed with perl. |
637 | |
638 | =head2 How can I expand variables in text strings? |
639 | |
640 | Let's assume that you have a string like: |
641 | |
642 | $text = 'this has a $foo in it and a $bar'; |
5a964f20 |
643 | |
644 | If those were both global variables, then this would |
645 | suffice: |
646 | |
65acb1b1 |
647 | $text =~ s/\$(\w+)/${$1}/g; # no /e needed |
68dc0745 |
648 | |
5a964f20 |
649 | But since they are probably lexicals, or at least, they could |
650 | be, you'd have to do this: |
68dc0745 |
651 | |
652 | $text =~ s/(\$\w+)/$1/eeg; |
65acb1b1 |
653 | die if $@; # needed /ee, not /e |
68dc0745 |
654 | |
5a964f20 |
655 | It's probably better in the general case to treat those |
656 | variables as entries in some special hash. For example: |
657 | |
658 | %user_defs = ( |
659 | foo => 23, |
660 | bar => 19, |
661 | ); |
662 | $text =~ s/\$(\w+)/$user_defs{$1}/g; |
68dc0745 |
663 | |
92c2ed05 |
664 | See also ``How do I expand function calls in a string?'' in this section |
46fc3d4c |
665 | of the FAQ. |
666 | |
68dc0745 |
667 | =head2 What's wrong with always quoting "$vars"? |
668 | |
669 | The problem is that those double-quotes force stringification, |
670 | coercing numbers and references into strings, even when you |
65acb1b1 |
671 | don't want them to be. Think of it this way: double-quote |
672 | expansion is used to produce new strings. If you already |
673 | have a string, why do you need more? |
68dc0745 |
674 | |
675 | If you get used to writing odd things like these: |
676 | |
677 | print "$var"; # BAD |
678 | $new = "$old"; # BAD |
679 | somefunc("$var"); # BAD |
680 | |
681 | You'll be in trouble. Those should (in 99.8% of the cases) be |
682 | the simpler and more direct: |
683 | |
684 | print $var; |
685 | $new = $old; |
686 | somefunc($var); |
687 | |
688 | Otherwise, besides slowing you down, you're going to break code when |
689 | the thing in the scalar is actually neither a string nor a number, but |
690 | a reference: |
691 | |
692 | func(\@array); |
693 | sub func { |
694 | my $aref = shift; |
695 | my $oref = "$aref"; # WRONG |
696 | } |
697 | |
698 | You can also get into subtle problems on those few operations in Perl |
699 | that actually do care about the difference between a string and a |
700 | number, such as the magical C<++> autoincrement operator or the |
701 | syscall() function. |
702 | |
5a964f20 |
703 | Stringification also destroys arrays. |
704 | |
705 | @lines = `command`; |
706 | print "@lines"; # WRONG - extra blanks |
707 | print @lines; # right |
708 | |
65acb1b1 |
709 | =head2 Why don't my E<lt>E<lt>HERE documents work? |
68dc0745 |
710 | |
711 | Check for these three things: |
712 | |
713 | =over 4 |
714 | |
715 | =item 1. There must be no space after the << part. |
716 | |
717 | =item 2. There (probably) should be a semicolon at the end. |
718 | |
719 | =item 3. You can't (easily) have any space in front of the tag. |
720 | |
721 | =back |
722 | |
5a964f20 |
723 | If you want to indent the text in the here document, you |
724 | can do this: |
725 | |
726 | # all in one |
727 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; |
728 | your text |
729 | goes here |
730 | HERE_TARGET |
731 | |
732 | But the HERE_TARGET must still be flush against the margin. |
733 | If you want that indented also, you'll have to quote |
734 | in the indentation. |
735 | |
736 | ($quote = <<' FINIS') =~ s/^\s+//gm; |
737 | ...we will have peace, when you and all your works have |
738 | perished--and the works of your dark master to whom you |
739 | would deliver us. You are a liar, Saruman, and a corrupter |
740 | of men's hearts. --Theoden in /usr/src/perl/taint.c |
741 | FINIS |
742 | $quote =~ s/\s*--/\n--/; |
743 | |
744 | A nice general-purpose fixer-upper function for indented here documents |
745 | follows. It expects to be called with a here document as its argument. |
746 | It looks to see whether each line begins with a common substring, and |
747 | if so, strips that off. Otherwise, it takes the amount of leading |
748 | white space found on the first line and removes that much off each |
749 | subsequent line. |
750 | |
751 | sub fix { |
752 | local $_ = shift; |
753 | my ($white, $leader); # common white space and common leading string |
754 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
755 | ($white, $leader) = ($2, quotemeta($1)); |
756 | } else { |
757 | ($white, $leader) = (/^(\s+)/, ''); |
758 | } |
759 | s/^\s*?$leader(?:$white)?//gm; |
760 | return $_; |
761 | } |
762 | |
c8db1d39 |
763 | This works with leading special strings, dynamically determined: |
5a964f20 |
764 | |
765 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; |
766 | @@@ int |
767 | @@@ runops() { |
768 | @@@ SAVEI32(runlevel); |
769 | @@@ runlevel++; |
770 | @@@ while ( op = (*op->op_ppaddr)() ) ; |
771 | @@@ TAINT_NOT; |
772 | @@@ return 0; |
773 | @@@ } |
774 | MAIN_INTERPRETER_LOOP |
775 | |
776 | Or with a fixed amount of leading white space, with remaining |
777 | indentation correctly preserved: |
778 | |
779 | $poem = fix<<EVER_ON_AND_ON; |
780 | Now far ahead the Road has gone, |
781 | And I must follow, if I can, |
782 | Pursuing it with eager feet, |
783 | Until it joins some larger way |
784 | Where many paths and errands meet. |
785 | And whither then? I cannot say. |
786 | --Bilbo in /usr/src/perl/pp_ctl.c |
787 | EVER_ON_AND_ON |
788 | |
68dc0745 |
789 | =head1 Data: Arrays |
790 | |
65acb1b1 |
791 | =head2 What is the difference between a list and an array? |
792 | |
793 | An array has a changeable length. A list does not. An array is something |
794 | you can push or pop, while a list is a set of values. Some people make |
795 | the distinction that a list is a value while an array is a variable. |
796 | Subroutines are passed and return lists, you put things into list |
797 | context, you initialize arrays with lists, and you foreach() across |
798 | a list. C<@> variables are arrays, anonymous arrays are arrays, arrays |
799 | in scalar context behave like the number of elements in them, subroutines |
800 | access their arguments through the array C<@_>, push/pop/shift only work |
801 | on arrays. |
802 | |
803 | As a side note, there's no such thing as a list in scalar context. |
804 | When you say |
805 | |
806 | $scalar = (2, 5, 7, 9); |
807 | |
808 | you're using the comma operator in scalar context, so it evaluates the |
809 | left hand side, then evaluates and returns the left hand side. This |
810 | causes the last value to be returned: 9. |
811 | |
68dc0745 |
812 | =head2 What is the difference between $array[1] and @array[1]? |
813 | |
814 | The former is a scalar value, the latter an array slice, which makes |
815 | it a list with one (scalar) value. You should use $ when you want a |
816 | scalar value (most of the time) and @ when you want a list with one |
817 | scalar value in it (very, very rarely; nearly never, in fact). |
818 | |
819 | Sometimes it doesn't make a difference, but sometimes it does. |
820 | For example, compare: |
821 | |
822 | $good[0] = `some program that outputs several lines`; |
823 | |
824 | with |
825 | |
826 | @bad[0] = `same program that outputs several lines`; |
827 | |
828 | The B<-w> flag will warn you about these matters. |
829 | |
830 | =head2 How can I extract just the unique elements of an array? |
831 | |
832 | There are several possible ways, depending on whether the array is |
833 | ordered and whether you wish to preserve the ordering. |
834 | |
835 | =over 4 |
836 | |
837 | =item a) If @in is sorted, and you want @out to be sorted: |
5a964f20 |
838 | (this assumes all true values in the array) |
68dc0745 |
839 | |
840 | $prev = 'nonesuch'; |
841 | @out = grep($_ ne $prev && ($prev = $_), @in); |
842 | |
c8db1d39 |
843 | This is nice in that it doesn't use much extra memory, simulating |
844 | uniq(1)'s behavior of removing only adjacent duplicates. It's less |
845 | nice in that it won't work with false values like undef, 0, or ""; |
846 | "0 but true" is ok, though. |
68dc0745 |
847 | |
848 | =item b) If you don't know whether @in is sorted: |
849 | |
850 | undef %saw; |
851 | @out = grep(!$saw{$_}++, @in); |
852 | |
853 | =item c) Like (b), but @in contains only small integers: |
854 | |
855 | @out = grep(!$saw[$_]++, @in); |
856 | |
857 | =item d) A way to do (b) without any loops or greps: |
858 | |
859 | undef %saw; |
860 | @saw{@in} = (); |
861 | @out = sort keys %saw; # remove sort if undesired |
862 | |
863 | =item e) Like (d), but @in contains only small positive integers: |
864 | |
865 | undef @ary; |
866 | @ary[@in] = @in; |
867 | @out = @ary; |
868 | |
869 | =back |
870 | |
65acb1b1 |
871 | But perhaps you should have been using a hash all along, eh? |
872 | |
5a964f20 |
873 | =head2 How can I tell whether a list or array contains a certain element? |
874 | |
875 | Hearing the word "in" is an I<in>dication that you probably should have |
876 | used a hash, not a list or array, to store your data. Hashes are |
877 | designed to answer this question quickly and efficiently. Arrays aren't. |
68dc0745 |
878 | |
5a964f20 |
879 | That being said, there are several ways to approach this. If you |
880 | are going to make this query many times over arbitrary string values, |
881 | the fastest way is probably to invert the original array and keep an |
68dc0745 |
882 | associative array lying about whose keys are the first array's values. |
883 | |
884 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; |
885 | undef %is_blue; |
886 | for (@blues) { $is_blue{$_} = 1 } |
887 | |
888 | Now you can check whether $is_blue{$some_color}. It might have been a |
889 | good idea to keep the blues all in a hash in the first place. |
890 | |
891 | If the values are all small integers, you could use a simple indexed |
892 | array. This kind of an array will take up less space: |
893 | |
894 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); |
895 | undef @is_tiny_prime; |
896 | for (@primes) { $is_tiny_prime[$_] = 1; } |
897 | |
898 | Now you check whether $is_tiny_prime[$some_number]. |
899 | |
900 | If the values in question are integers instead of strings, you can save |
901 | quite a lot of space by using bit strings instead: |
902 | |
903 | @articles = ( 1..10, 150..2000, 2017 ); |
904 | undef $read; |
7b8d334a |
905 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 |
906 | |
907 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. |
908 | |
909 | Please do not use |
910 | |
911 | $is_there = grep $_ eq $whatever, @array; |
912 | |
913 | or worse yet |
914 | |
915 | $is_there = grep /$whatever/, @array; |
916 | |
917 | These are slow (checks every element even if the first matches), |
918 | inefficient (same reason), and potentially buggy (what if there are |
65acb1b1 |
919 | regexp characters in $whatever?). If you're only testing once, then |
920 | use: |
921 | |
922 | $is_there = 0; |
923 | foreach $elt (@array) { |
924 | if ($elt eq $elt_to_find) { |
925 | $is_there = 1; |
926 | last; |
927 | } |
928 | } |
929 | if ($is_there) { ... } |
68dc0745 |
930 | |
931 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
932 | |
933 | Use a hash. Here's code to do both and more. It assumes that |
934 | each element is unique in a given array: |
935 | |
936 | @union = @intersection = @difference = (); |
937 | %count = (); |
938 | foreach $element (@array1, @array2) { $count{$element}++ } |
939 | foreach $element (keys %count) { |
940 | push @union, $element; |
941 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; |
942 | } |
943 | |
65acb1b1 |
944 | =head2 How do I test whether two arrays or hashes are equal? |
945 | |
946 | The following code works for single-level arrays. It uses a stringwise |
947 | comparison, and does not distinguish defined versus undefined empty |
948 | strings. Modify if you have other needs. |
949 | |
950 | $are_equal = compare_arrays(\@frogs, \@toads); |
951 | |
952 | sub compare_arrays { |
953 | my ($first, $second) = @_; |
954 | local $^W = 0; # silence spurious -w undef complaints |
955 | return 0 unless @$first == @$second; |
956 | for (my $i = 0; $i < @$first; $i++) { |
957 | return 0 if $first->[$i] ne $second->[$i]; |
958 | } |
959 | return 1; |
960 | } |
961 | |
962 | For multilevel structures, you may wish to use an approach more |
963 | like this one. It uses the CPAN module FreezeThaw: |
964 | |
965 | use FreezeThaw qw(cmpStr); |
966 | @a = @b = ( "this", "that", [ "more", "stuff" ] ); |
967 | |
968 | printf "a and b contain %s arrays\n", |
969 | cmpStr(\@a, \@b) == 0 |
970 | ? "the same" |
971 | : "different"; |
972 | |
973 | This approach also works for comparing hashes. Here |
974 | we'll demonstrate two different answers: |
975 | |
976 | use FreezeThaw qw(cmpStr cmpStrHard); |
977 | |
978 | %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); |
979 | $a{EXTRA} = \%b; |
980 | $b{EXTRA} = \%a; |
981 | |
982 | printf "a and b contain %s hashes\n", |
983 | cmpStr(\%a, \%b) == 0 ? "the same" : "different"; |
984 | |
985 | printf "a and b contain %s hashes\n", |
986 | cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; |
987 | |
988 | |
989 | The first reports that both those the hashes contain the same data, |
990 | while the second reports that they do not. Which you prefer is left as |
991 | an exercise to the reader. |
992 | |
68dc0745 |
993 | =head2 How do I find the first array element for which a condition is true? |
994 | |
995 | You can use this if you care about the index: |
996 | |
65acb1b1 |
997 | for ($i= 0; $i < @array; $i++) { |
68dc0745 |
998 | if ($array[$i] eq "Waldo") { |
999 | $found_index = $i; |
1000 | last; |
1001 | } |
1002 | } |
1003 | |
1004 | Now C<$found_index> has what you want. |
1005 | |
1006 | =head2 How do I handle linked lists? |
1007 | |
1008 | In general, you usually don't need a linked list in Perl, since with |
1009 | regular arrays, you can push and pop or shift and unshift at either end, |
5a964f20 |
1010 | or you can use splice to add and/or remove arbitrary number of elements at |
1011 | arbitrary points. Both pop and shift are both O(1) operations on perl's |
1012 | dynamic arrays. In the absence of shifts and pops, push in general |
1013 | needs to reallocate on the order every log(N) times, and unshift will |
1014 | need to copy pointers each time. |
68dc0745 |
1015 | |
1016 | If you really, really wanted, you could use structures as described in |
1017 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you |
65acb1b1 |
1018 | to do. For example, imagine a list node like this: |
1019 | |
1020 | $node = { |
1021 | VALUE => 42, |
1022 | LINK => undef, |
1023 | }; |
1024 | |
1025 | You could walk the list this way: |
1026 | |
1027 | print "List: "; |
1028 | for ($node = $head; $node; $node = $node->{LINK}) { |
1029 | print $node->{VALUE}, " "; |
1030 | } |
1031 | print "\n"; |
1032 | |
1033 | You could grow the list this way: |
1034 | |
1035 | my ($head, $tail); |
1036 | $tail = append($head, 1); # grow a new head |
1037 | for $value ( 2 .. 10 ) { |
1038 | $tail = append($tail, $value); |
1039 | } |
1040 | |
1041 | sub append { |
1042 | my($list, $value) = @_; |
1043 | my $node = { VALUE => $value }; |
1044 | if ($list) { |
1045 | $node->{LINK} = $list->{LINK}; |
1046 | $list->{LINK} = $node; |
1047 | } else { |
1048 | $_[0] = $node; # replace caller's version |
1049 | } |
1050 | return $node; |
1051 | } |
1052 | |
1053 | But again, Perl's built-in are virtually always good enough. |
68dc0745 |
1054 | |
1055 | =head2 How do I handle circular lists? |
1056 | |
1057 | Circular lists could be handled in the traditional fashion with linked |
1058 | lists, or you could just do something like this with an array: |
1059 | |
1060 | unshift(@array, pop(@array)); # the last shall be first |
1061 | push(@array, shift(@array)); # and vice versa |
1062 | |
1063 | =head2 How do I shuffle an array randomly? |
1064 | |
5a964f20 |
1065 | Use this: |
1066 | |
1067 | # fisher_yates_shuffle( \@array ) : |
1068 | # generate a random permutation of @array in place |
1069 | sub fisher_yates_shuffle { |
1070 | my $array = shift; |
1071 | my $i; |
1072 | for ($i = @$array; --$i; ) { |
1073 | my $j = int rand ($i+1); |
1074 | next if $i == $j; |
1075 | @$array[$i,$j] = @$array[$j,$i]; |
1076 | } |
1077 | } |
1078 | |
1079 | fisher_yates_shuffle( \@array ); # permutes @array in place |
1080 | |
1081 | You've probably seen shuffling algorithms that works using splice, |
68dc0745 |
1082 | randomly picking another element to swap the current element with: |
1083 | |
1084 | srand; |
1085 | @new = (); |
1086 | @old = 1 .. 10; # just a demo |
1087 | while (@old) { |
1088 | push(@new, splice(@old, rand @old, 1)); |
1089 | } |
1090 | |
5a964f20 |
1091 | This is bad because splice is already O(N), and since you do it N times, |
1092 | you just invented a quadratic algorithm; that is, O(N**2). This does |
1093 | not scale, although Perl is so efficient that you probably won't notice |
1094 | this until you have rather largish arrays. |
68dc0745 |
1095 | |
1096 | =head2 How do I process/modify each element of an array? |
1097 | |
1098 | Use C<for>/C<foreach>: |
1099 | |
1100 | for (@lines) { |
5a964f20 |
1101 | s/foo/bar/; # change that word |
1102 | y/XZ/ZX/; # swap those letters |
68dc0745 |
1103 | } |
1104 | |
1105 | Here's another; let's compute spherical volumes: |
1106 | |
5a964f20 |
1107 | for (@volumes = @radii) { # @volumes has changed parts |
68dc0745 |
1108 | $_ **= 3; |
1109 | $_ *= (4/3) * 3.14159; # this will be constant folded |
1110 | } |
1111 | |
5a964f20 |
1112 | If you want to do the same thing to modify the values of the hash, |
1113 | you may not use the C<values> function, oddly enough. You need a slice: |
1114 | |
1115 | for $orbit ( @orbits{keys %orbits} ) { |
1116 | ($orbit **= 3) *= (4/3) * 3.14159; |
1117 | } |
1118 | |
68dc0745 |
1119 | =head2 How do I select a random element from an array? |
1120 | |
1121 | Use the rand() function (see L<perlfunc/rand>): |
1122 | |
5a964f20 |
1123 | # at the top of the program: |
68dc0745 |
1124 | srand; # not needed for 5.004 and later |
5a964f20 |
1125 | |
1126 | # then later on |
68dc0745 |
1127 | $index = rand @array; |
1128 | $element = $array[$index]; |
1129 | |
5a964f20 |
1130 | Make sure you I<only call srand once per program, if then>. |
1131 | If you are calling it more than once (such as before each |
1132 | call to rand), you're almost certainly doing something wrong. |
1133 | |
68dc0745 |
1134 | =head2 How do I permute N elements of a list? |
1135 | |
1136 | Here's a little program that generates all permutations |
1137 | of all the words on each line of input. The algorithm embodied |
5a964f20 |
1138 | in the permute() function should work on any list: |
68dc0745 |
1139 | |
1140 | #!/usr/bin/perl -n |
5a964f20 |
1141 | # tsc-permute: permute each word of input |
1142 | permute([split], []); |
1143 | sub permute { |
1144 | my @items = @{ $_[0] }; |
1145 | my @perms = @{ $_[1] }; |
1146 | unless (@items) { |
1147 | print "@perms\n"; |
68dc0745 |
1148 | } else { |
5a964f20 |
1149 | my(@newitems,@newperms,$i); |
1150 | foreach $i (0 .. $#items) { |
1151 | @newitems = @items; |
1152 | @newperms = @perms; |
1153 | unshift(@newperms, splice(@newitems, $i, 1)); |
1154 | permute([@newitems], [@newperms]); |
68dc0745 |
1155 | } |
1156 | } |
1157 | } |
1158 | |
1159 | =head2 How do I sort an array by (anything)? |
1160 | |
1161 | Supply a comparison function to sort() (described in L<perlfunc/sort>): |
1162 | |
1163 | @list = sort { $a <=> $b } @list; |
1164 | |
1165 | The default sort function is cmp, string comparison, which would |
1166 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is |
1167 | the numerical comparison operator. |
1168 | |
1169 | If you have a complicated function needed to pull out the part you |
1170 | want to sort on, then don't do it inside the sort function. Pull it |
1171 | out first, because the sort BLOCK can be called many times for the |
1172 | same element. Here's an example of how to pull out the first word |
1173 | after the first number on each item, and then sort those words |
1174 | case-insensitively. |
1175 | |
1176 | @idx = (); |
1177 | for (@data) { |
1178 | ($item) = /\d+\s*(\S+)/; |
1179 | push @idx, uc($item); |
1180 | } |
1181 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; |
1182 | |
1183 | Which could also be written this way, using a trick |
1184 | that's come to be known as the Schwartzian Transform: |
1185 | |
1186 | @sorted = map { $_->[0] } |
1187 | sort { $a->[1] cmp $b->[1] } |
46fc3d4c |
1188 | map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data; |
68dc0745 |
1189 | |
1190 | If you need to sort on several fields, the following paradigm is useful. |
1191 | |
1192 | @sorted = sort { field1($a) <=> field1($b) || |
1193 | field2($a) cmp field2($b) || |
1194 | field3($a) cmp field3($b) |
1195 | } @data; |
1196 | |
1197 | This can be conveniently combined with precalculation of keys as given |
1198 | above. |
1199 | |
1200 | See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about |
1201 | this approach. |
1202 | |
1203 | See also the question below on sorting hashes. |
1204 | |
1205 | =head2 How do I manipulate arrays of bits? |
1206 | |
1207 | Use pack() and unpack(), or else vec() and the bitwise operations. |
1208 | |
1209 | For example, this sets $vec to have bit N set if $ints[N] was set: |
1210 | |
1211 | $vec = ''; |
1212 | foreach(@ints) { vec($vec,$_,1) = 1 } |
1213 | |
1214 | And here's how, given a vector in $vec, you can |
1215 | get those bits into your @ints array: |
1216 | |
1217 | sub bitvec_to_list { |
1218 | my $vec = shift; |
1219 | my @ints; |
1220 | # Find null-byte density then select best algorithm |
1221 | if ($vec =~ tr/\0// / length $vec > 0.95) { |
1222 | use integer; |
1223 | my $i; |
1224 | # This method is faster with mostly null-bytes |
1225 | while($vec =~ /[^\0]/g ) { |
1226 | $i = -9 + 8 * pos $vec; |
1227 | push @ints, $i if vec($vec, ++$i, 1); |
1228 | push @ints, $i if vec($vec, ++$i, 1); |
1229 | push @ints, $i if vec($vec, ++$i, 1); |
1230 | push @ints, $i if vec($vec, ++$i, 1); |
1231 | push @ints, $i if vec($vec, ++$i, 1); |
1232 | push @ints, $i if vec($vec, ++$i, 1); |
1233 | push @ints, $i if vec($vec, ++$i, 1); |
1234 | push @ints, $i if vec($vec, ++$i, 1); |
1235 | } |
1236 | } else { |
1237 | # This method is a fast general algorithm |
1238 | use integer; |
1239 | my $bits = unpack "b*", $vec; |
1240 | push @ints, 0 if $bits =~ s/^(\d)// && $1; |
1241 | push @ints, pos $bits while($bits =~ /1/g); |
1242 | } |
1243 | return \@ints; |
1244 | } |
1245 | |
1246 | This method gets faster the more sparse the bit vector is. |
1247 | (Courtesy of Tim Bunce and Winfried Koenig.) |
1248 | |
65acb1b1 |
1249 | Here's a demo on how to use vec(): |
1250 | |
1251 | # vec demo |
1252 | $vector = "\xff\x0f\xef\xfe"; |
1253 | print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", |
1254 | unpack("N", $vector), "\n"; |
1255 | $is_set = vec($vector, 23, 1); |
1256 | print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; |
1257 | pvec($vector); |
1258 | |
1259 | set_vec(1,1,1); |
1260 | set_vec(3,1,1); |
1261 | set_vec(23,1,1); |
1262 | |
1263 | set_vec(3,1,3); |
1264 | set_vec(3,2,3); |
1265 | set_vec(3,4,3); |
1266 | set_vec(3,4,7); |
1267 | set_vec(3,8,3); |
1268 | set_vec(3,8,7); |
1269 | |
1270 | set_vec(0,32,17); |
1271 | set_vec(1,32,17); |
1272 | |
1273 | sub set_vec { |
1274 | my ($offset, $width, $value) = @_; |
1275 | my $vector = ''; |
1276 | vec($vector, $offset, $width) = $value; |
1277 | print "offset=$offset width=$width value=$value\n"; |
1278 | pvec($vector); |
1279 | } |
1280 | |
1281 | sub pvec { |
1282 | my $vector = shift; |
1283 | my $bits = unpack("b*", $vector); |
1284 | my $i = 0; |
1285 | my $BASE = 8; |
1286 | |
1287 | print "vector length in bytes: ", length($vector), "\n"; |
1288 | @bytes = unpack("A8" x length($vector), $bits); |
1289 | print "bits are: @bytes\n\n"; |
1290 | } |
1291 | |
68dc0745 |
1292 | =head2 Why does defined() return true on empty arrays and hashes? |
1293 | |
65acb1b1 |
1294 | The short story is that you should probably only use defined on scalars or |
1295 | functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> |
1296 | in the 5.004 release or later of Perl for more detail. |
68dc0745 |
1297 | |
1298 | =head1 Data: Hashes (Associative Arrays) |
1299 | |
1300 | =head2 How do I process an entire hash? |
1301 | |
1302 | Use the each() function (see L<perlfunc/each>) if you don't care |
1303 | whether it's sorted: |
1304 | |
5a964f20 |
1305 | while ( ($key, $value) = each %hash) { |
68dc0745 |
1306 | print "$key = $value\n"; |
1307 | } |
1308 | |
1309 | If you want it sorted, you'll have to use foreach() on the result of |
1310 | sorting the keys as shown in an earlier question. |
1311 | |
1312 | =head2 What happens if I add or remove keys from a hash while iterating over it? |
1313 | |
1314 | Don't do that. |
1315 | |
1316 | =head2 How do I look up a hash element by value? |
1317 | |
1318 | Create a reverse hash: |
1319 | |
1320 | %by_value = reverse %by_key; |
1321 | $key = $by_value{$value}; |
1322 | |
1323 | That's not particularly efficient. It would be more space-efficient |
1324 | to use: |
1325 | |
1326 | while (($key, $value) = each %by_key) { |
1327 | $by_value{$value} = $key; |
1328 | } |
1329 | |
1330 | If your hash could have repeated values, the methods above will only |
1331 | find one of the associated keys. This may or may not worry you. |
1332 | |
1333 | =head2 How can I know how many entries are in a hash? |
1334 | |
1335 | If you mean how many keys, then all you have to do is |
1336 | take the scalar sense of the keys() function: |
1337 | |
3fe9a6f1 |
1338 | $num_keys = scalar keys %hash; |
68dc0745 |
1339 | |
1340 | In void context it just resets the iterator, which is faster |
1341 | for tied hashes. |
1342 | |
1343 | =head2 How do I sort a hash (optionally by value instead of key)? |
1344 | |
1345 | Internally, hashes are stored in a way that prevents you from imposing |
1346 | an order on key-value pairs. Instead, you have to sort a list of the |
1347 | keys or values: |
1348 | |
1349 | @keys = sort keys %hash; # sorted by key |
1350 | @keys = sort { |
1351 | $hash{$a} cmp $hash{$b} |
1352 | } keys %hash; # and by value |
1353 | |
1354 | Here we'll do a reverse numeric sort by value, and if two keys are |
1355 | identical, sort by length of key, and if that fails, by straight ASCII |
1356 | comparison of the keys (well, possibly modified by your locale -- see |
1357 | L<perllocale>). |
1358 | |
1359 | @keys = sort { |
1360 | $hash{$b} <=> $hash{$a} |
1361 | || |
1362 | length($b) <=> length($a) |
1363 | || |
1364 | $a cmp $b |
1365 | } keys %hash; |
1366 | |
1367 | =head2 How can I always keep my hash sorted? |
1368 | |
1369 | You can look into using the DB_File module and tie() using the |
1370 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. |
5a964f20 |
1371 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 |
1372 | |
1373 | =head2 What's the difference between "delete" and "undef" with hashes? |
1374 | |
1375 | Hashes are pairs of scalars: the first is the key, the second is the |
1376 | value. The key will be coerced to a string, although the value can be |
1377 | any kind of scalar: string, number, or reference. If a key C<$key> is |
1378 | present in the array, C<exists($key)> will return true. The value for |
1379 | a given key can be C<undef>, in which case C<$array{$key}> will be |
1380 | C<undef> while C<$exists{$key}> will return true. This corresponds to |
1381 | (C<$key>, C<undef>) being in the hash. |
1382 | |
1383 | Pictures help... here's the C<%ary> table: |
1384 | |
1385 | keys values |
1386 | +------+------+ |
1387 | | a | 3 | |
1388 | | x | 7 | |
1389 | | d | 0 | |
1390 | | e | 2 | |
1391 | +------+------+ |
1392 | |
1393 | And these conditions hold |
1394 | |
1395 | $ary{'a'} is true |
1396 | $ary{'d'} is false |
1397 | defined $ary{'d'} is true |
1398 | defined $ary{'a'} is true |
1399 | exists $ary{'a'} is true (perl5 only) |
1400 | grep ($_ eq 'a', keys %ary) is true |
1401 | |
1402 | If you now say |
1403 | |
1404 | undef $ary{'a'} |
1405 | |
1406 | your table now reads: |
1407 | |
1408 | |
1409 | keys values |
1410 | +------+------+ |
1411 | | a | undef| |
1412 | | x | 7 | |
1413 | | d | 0 | |
1414 | | e | 2 | |
1415 | +------+------+ |
1416 | |
1417 | and these conditions now hold; changes in caps: |
1418 | |
1419 | $ary{'a'} is FALSE |
1420 | $ary{'d'} is false |
1421 | defined $ary{'d'} is true |
1422 | defined $ary{'a'} is FALSE |
1423 | exists $ary{'a'} is true (perl5 only) |
1424 | grep ($_ eq 'a', keys %ary) is true |
1425 | |
1426 | Notice the last two: you have an undef value, but a defined key! |
1427 | |
1428 | Now, consider this: |
1429 | |
1430 | delete $ary{'a'} |
1431 | |
1432 | your table now reads: |
1433 | |
1434 | keys values |
1435 | +------+------+ |
1436 | | x | 7 | |
1437 | | d | 0 | |
1438 | | e | 2 | |
1439 | +------+------+ |
1440 | |
1441 | and these conditions now hold; changes in caps: |
1442 | |
1443 | $ary{'a'} is false |
1444 | $ary{'d'} is false |
1445 | defined $ary{'d'} is true |
1446 | defined $ary{'a'} is false |
1447 | exists $ary{'a'} is FALSE (perl5 only) |
1448 | grep ($_ eq 'a', keys %ary) is FALSE |
1449 | |
1450 | See, the whole entry is gone! |
1451 | |
1452 | =head2 Why don't my tied hashes make the defined/exists distinction? |
1453 | |
1454 | They may or may not implement the EXISTS() and DEFINED() methods |
1455 | differently. For example, there isn't the concept of undef with hashes |
1456 | that are tied to DBM* files. This means the true/false tables above |
1457 | will give different results when used on such a hash. It also means |
1458 | that exists and defined do the same thing with a DBM* file, and what |
1459 | they end up doing is not what they do with ordinary hashes. |
1460 | |
1461 | =head2 How do I reset an each() operation part-way through? |
1462 | |
5a964f20 |
1463 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 |
1464 | the hash I<and> resets the iterator associated with the hash. You may |
1465 | need to do this if you use C<last> to exit a loop early so that when you |
46fc3d4c |
1466 | re-enter it, the hash iterator has been reset. |
68dc0745 |
1467 | |
1468 | =head2 How can I get the unique keys from two hashes? |
1469 | |
1470 | First you extract the keys from the hashes into arrays, and then solve |
1471 | the uniquifying the array problem described above. For example: |
1472 | |
1473 | %seen = (); |
1474 | for $element (keys(%foo), keys(%bar)) { |
1475 | $seen{$element}++; |
1476 | } |
1477 | @uniq = keys %seen; |
1478 | |
1479 | Or more succinctly: |
1480 | |
1481 | @uniq = keys %{{%foo,%bar}}; |
1482 | |
1483 | Or if you really want to save space: |
1484 | |
1485 | %seen = (); |
1486 | while (defined ($key = each %foo)) { |
1487 | $seen{$key}++; |
1488 | } |
1489 | while (defined ($key = each %bar)) { |
1490 | $seen{$key}++; |
1491 | } |
1492 | @uniq = keys %seen; |
1493 | |
1494 | =head2 How can I store a multidimensional array in a DBM file? |
1495 | |
1496 | Either stringify the structure yourself (no fun), or else |
1497 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer |
1498 | it on top of either DB_File or GDBM_File. |
1499 | |
1500 | =head2 How can I make my hash remember the order I put elements into it? |
1501 | |
1502 | Use the Tie::IxHash from CPAN. |
1503 | |
46fc3d4c |
1504 | use Tie::IxHash; |
1505 | tie(%myhash, Tie::IxHash); |
1506 | for ($i=0; $i<20; $i++) { |
1507 | $myhash{$i} = 2*$i; |
1508 | } |
1509 | @keys = keys %myhash; |
1510 | # @keys = (0,1,2,3,...) |
1511 | |
68dc0745 |
1512 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1513 | |
1514 | If you say something like: |
1515 | |
1516 | somefunc($hash{"nonesuch key here"}); |
1517 | |
1518 | Then that element "autovivifies"; that is, it springs into existence |
1519 | whether you store something there or not. That's because functions |
1520 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, |
1521 | it has to be ready to write it back into the caller's version. |
1522 | |
1523 | This has been fixed as of perl5.004. |
1524 | |
1525 | Normally, merely accessing a key's value for a nonexistent key does |
1526 | I<not> cause that key to be forever there. This is different than |
1527 | awk's behavior. |
1528 | |
fc36a67e |
1529 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 |
1530 | |
65acb1b1 |
1531 | Usually a hash ref, perhaps like this: |
1532 | |
1533 | $record = { |
1534 | NAME => "Jason", |
1535 | EMPNO => 132, |
1536 | TITLE => "deputy peon", |
1537 | AGE => 23, |
1538 | SALARY => 37_000, |
1539 | PALS => [ "Norbert", "Rhys", "Phineas"], |
1540 | }; |
1541 | |
1542 | References are documented in L<perlref> and the upcoming L<perlreftut>. |
1543 | Examples of complex data structures are given in L<perldsc> and |
1544 | L<perllol>. Examples of structures and object-oriented classes are |
1545 | in L<perltoot>. |
68dc0745 |
1546 | |
1547 | =head2 How can I use a reference as a hash key? |
1548 | |
1549 | You can't do this directly, but you could use the standard Tie::Refhash |
1550 | module distributed with perl. |
1551 | |
1552 | =head1 Data: Misc |
1553 | |
1554 | =head2 How do I handle binary data correctly? |
1555 | |
1556 | Perl is binary clean, so this shouldn't be a problem. For example, |
1557 | this works fine (assuming the files are found): |
1558 | |
1559 | if (`cat /vmunix` =~ /gzip/) { |
1560 | print "Your kernel is GNU-zip enabled!\n"; |
1561 | } |
1562 | |
65acb1b1 |
1563 | On some legacy systems, however, you have to play tedious games with |
1564 | "text" versus "binary" files. See L<perlfunc/"binmode">, or the upcoming |
1565 | L<perlopentut> manpage. |
68dc0745 |
1566 | |
1567 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. |
1568 | |
54310121 |
1569 | If you want to deal with multibyte characters, however, there are |
68dc0745 |
1570 | some gotchas. See the section on Regular Expressions. |
1571 | |
1572 | =head2 How do I determine whether a scalar is a number/whole/integer/float? |
1573 | |
1574 | Assuming that you don't care about IEEE notations like "NaN" or |
1575 | "Infinity", you probably just want to use a regular expression. |
1576 | |
65acb1b1 |
1577 | if (/\D/) { print "has nondigits\n" } |
1578 | if (/^\d+$/) { print "is a whole number\n" } |
1579 | if (/^-?\d+$/) { print "is an integer\n" } |
1580 | if (/^[+-]?\d+$/) { print "is a +/- integer\n" } |
1581 | if (/^-?\d+\.?\d*$/) { print "is a real number\n" } |
1582 | if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" } |
1583 | if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) |
1584 | { print "a C float" } |
68dc0745 |
1585 | |
5a964f20 |
1586 | If you're on a POSIX system, Perl's supports the C<POSIX::strtod> |
1587 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> |
1588 | wrapper function for more convenient access. This function takes |
1589 | a string and returns the number it found, or C<undef> for input that |
1590 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> |
1591 | if you just want to say, ``Is this a float?'' |
1592 | |
1593 | sub getnum { |
1594 | use POSIX qw(strtod); |
1595 | my $str = shift; |
1596 | $str =~ s/^\s+//; |
1597 | $str =~ s/\s+$//; |
1598 | $! = 0; |
1599 | my($num, $unparsed) = strtod($str); |
1600 | if (($str eq '') || ($unparsed != 0) || $!) { |
1601 | return undef; |
1602 | } else { |
1603 | return $num; |
1604 | } |
1605 | } |
1606 | |
1607 | sub is_numeric { defined &getnum } |
1608 | |
be94a901 |
1609 | Or you could check out String::Scanf which can be found at |
1610 | http://www.perl.com/CPAN/modules/by-module/String/. |
1611 | The POSIX module (part of the standard Perl distribution) provides |
1612 | the C<strtol> and C<strtod> for converting strings to double |
68dc0745 |
1613 | and longs, respectively. |
1614 | |
1615 | =head2 How do I keep persistent data across program calls? |
1616 | |
1617 | For some specific applications, you can use one of the DBM modules. |
65acb1b1 |
1618 | See L<AnyDBM_File>. More generically, you should consult the FreezeThaw, |
1619 | Storable, or Class::Eroot modules from CPAN. Here's one example using |
1620 | Storable's C<store> and C<retrieve> functions: |
1621 | |
1622 | use Storable; |
1623 | store(\%hash, "filename"); |
1624 | |
1625 | # later on... |
1626 | $href = retrieve("filename"); # by ref |
1627 | %hash = %{ retrieve("filename") }; # direct to hash |
68dc0745 |
1628 | |
1629 | =head2 How do I print out or copy a recursive data structure? |
1630 | |
65acb1b1 |
1631 | The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great |
1632 | for printing out data structures. The Storable module, found on CPAN, |
1633 | provides a function called C<dclone> that recursively copies its argument. |
1634 | |
1635 | use Storable qw(dclone); |
1636 | $r2 = dclone($r1); |
68dc0745 |
1637 | |
65acb1b1 |
1638 | Where $r1 can be a reference to any kind of data structure you'd like. |
1639 | It will be deeply copied. Because C<dclone> takes and returns references, |
1640 | you'd have to add extra punctuation if you had a hash of arrays that |
1641 | you wanted to copy. |
68dc0745 |
1642 | |
65acb1b1 |
1643 | %newhash = %{ dclone(\%oldhash) }; |
68dc0745 |
1644 | |
1645 | =head2 How do I define methods for every class/object? |
1646 | |
1647 | Use the UNIVERSAL class (see L<UNIVERSAL>). |
1648 | |
1649 | =head2 How do I verify a credit card checksum? |
1650 | |
1651 | Get the Business::CreditCard module from CPAN. |
1652 | |
65acb1b1 |
1653 | =head2 How do I pack arrays of doubles or floats for XS code? |
1654 | |
1655 | The kgbpack.c code in the PGPLOT module on CPAN does just this. |
1656 | If you're doing a lot of float or double processing, consider using |
1657 | the PDL module from CPAN instead--it makes number-crunching easy. |
1658 | |
68dc0745 |
1659 | =head1 AUTHOR AND COPYRIGHT |
1660 | |
65acb1b1 |
1661 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
1662 | All rights reserved. |
1663 | |
1664 | When included as part of the Standard Version of Perl, or as part of |
1665 | its complete documentation whether printed or otherwise, this work |
c2611fb3 |
1666 | may be distributed only under the terms of Perl's Artistic Licence. |
5a964f20 |
1667 | Any distribution of this file or derivatives thereof I<outside> |
1668 | of that package require that special arrangements be made with |
1669 | copyright holder. |
1670 | |
1671 | Irrespective of its distribution, all code examples in this file |
1672 | are hereby placed into the public domain. You are permitted and |
1673 | encouraged to use this code in your own programs for fun |
1674 | or for profit as you see fit. A simple comment in the code giving |
1675 | credit would be courteous but is not required. |
65acb1b1 |
1676 | |