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1 | =head1 NAME |
2 | |
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3 | perlfaq4 - Data Manipulation ($Revision: 1.24 $, $Date: 1998/07/05 15:07:20 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | The section of the FAQ answers question related to the manipulation |
8 | of data as numbers, dates, strings, arrays, hashes, and miscellaneous |
9 | data issues. |
10 | |
11 | =head1 Data: Numbers |
12 | |
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13 | =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? |
14 | |
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15 | The infinite set that a mathematician thinks of as the real numbers can |
16 | only be approximate on a computer, since the computer only has a finite |
17 | number of bits to store an infinite number of, um, numbers. |
18 | |
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19 | Internally, your computer represents floating-point numbers in binary. |
20 | Floating-point numbers read in from a file, or appearing as literals |
21 | in your program, are converted from their decimal floating-point |
22 | representation (eg, 19.95) to the internal binary representation. |
23 | |
24 | However, 19.95 can't be precisely represented as a binary |
25 | floating-point number, just like 1/3 can't be exactly represented as a |
26 | decimal floating-point number. The computer's binary representation |
27 | of 19.95, therefore, isn't exactly 19.95. |
28 | |
29 | When a floating-point number gets printed, the binary floating-point |
30 | representation is converted back to decimal. These decimal numbers |
31 | are displayed in either the format you specify with printf(), or the |
32 | current output format for numbers (see L<perlvar/"$#"> 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 certain |
65 | number of digits, sprintf() or printf() is usually the easiest route. |
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66 | |
67 | The POSIX module (part of the standard perl distribution) implements |
68 | ceil(), floor(), and a number of other mathematical and trigonometric |
69 | functions. |
70 | |
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71 | In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex |
72 | module. With 5.004, the Math::Trig module (part of the standard perl |
73 | distribution) implements the trigonometric functions. Internally it |
74 | uses the Math::Complex module and some functions can break out from |
75 | the real axis into the complex plane, for example the inverse sine of |
76 | 2. |
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77 | |
78 | Rounding in financial applications can have serious implications, and |
79 | the rounding method used should be specified precisely. In these |
80 | cases, it probably pays not to trust whichever system rounding is |
81 | being used by Perl, but to instead implement the rounding function you |
82 | need yourself. |
83 | |
84 | =head2 How do I convert bits into ints? |
85 | |
86 | To turn a string of 1s and 0s like '10110110' into a scalar containing |
87 | its binary value, use the pack() function (documented in |
88 | L<perlfunc/"pack">): |
89 | |
90 | $decimal = pack('B8', '10110110'); |
91 | |
92 | Here's an example of going the other way: |
93 | |
94 | $binary_string = join('', unpack('B*', "\x29")); |
95 | |
96 | =head2 How do I multiply matrices? |
97 | |
98 | Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) |
99 | or the PDL extension (also available from CPAN). |
100 | |
101 | =head2 How do I perform an operation on a series of integers? |
102 | |
103 | To call a function on each element in an array, and collect the |
104 | results, use: |
105 | |
106 | @results = map { my_func($_) } @array; |
107 | |
108 | For example: |
109 | |
110 | @triple = map { 3 * $_ } @single; |
111 | |
112 | To call a function on each element of an array, but ignore the |
113 | results: |
114 | |
115 | foreach $iterator (@array) { |
116 | &my_func($iterator); |
117 | } |
118 | |
119 | To call a function on each integer in a (small) range, you B<can> use: |
120 | |
121 | @results = map { &my_func($_) } (5 .. 25); |
122 | |
123 | but you should be aware that the C<..> operator creates an array of |
124 | all integers in the range. This can take a lot of memory for large |
125 | ranges. Instead use: |
126 | |
127 | @results = (); |
128 | for ($i=5; $i < 500_005; $i++) { |
129 | push(@results, &my_func($i)); |
130 | } |
131 | |
132 | =head2 How can I output Roman numerals? |
133 | |
134 | Get the http://www.perl.com/CPAN/modules/by-module/Roman module. |
135 | |
136 | =head2 Why aren't my random numbers random? |
137 | |
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138 | John von Neumann said, ``Anyone who attempts to generate random numbers by |
139 | deterministic means is, of course, living in a state of sin.'' |
140 | |
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141 | The short explanation is that you're getting pseudorandom numbers, not |
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142 | random ones, because that's how these things work. A longer explanation |
143 | is available on http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy |
144 | of Tom Phoenix. |
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145 | |
146 | You should also check out the Math::TrulyRandom module from CPAN. |
147 | |
148 | =head1 Data: Dates |
149 | |
150 | =head2 How do I find the week-of-the-year/day-of-the-year? |
151 | |
152 | The day of the year is in the array returned by localtime() (see |
153 | L<perlfunc/"localtime">): |
154 | |
155 | $day_of_year = (localtime(time()))[7]; |
156 | |
157 | or more legibly (in 5.004 or higher): |
158 | |
159 | use Time::localtime; |
160 | $day_of_year = localtime(time())->yday; |
161 | |
162 | You can find the week of the year by dividing this by 7: |
163 | |
164 | $week_of_year = int($day_of_year / 7); |
165 | |
166 | Of course, this believes that weeks start at zero. |
167 | |
168 | =head2 How can I compare two date strings? |
169 | |
170 | Use the Date::Manip or Date::DateCalc modules from CPAN. |
171 | |
172 | =head2 How can I take a string and turn it into epoch seconds? |
173 | |
174 | If it's a regular enough string that it always has the same format, |
175 | you can split it up and pass the parts to timelocal in the standard |
176 | Time::Local module. Otherwise, you should look into one of the |
177 | Date modules from CPAN. |
178 | |
179 | =head2 How can I find the Julian Day? |
180 | |
181 | Neither Date::Manip nor Date::DateCalc deal with Julian days. |
182 | Instead, there is an example of Julian date calculation in |
183 | http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz, |
184 | which should help. |
185 | |
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186 | =head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant? |
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187 | |
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188 | Perl is just as Y2K compliant as your pencil--no more, and no less. |
189 | The date and time functions supplied with perl (gmtime and localtime) |
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190 | supply adequate information to determine the year well beyond 2000 |
191 | (2038 is when trouble strikes for 32-bit machines). The year returned |
192 | by these functions when used in an array context is the year minus 1900. |
193 | For years between 1910 and 1999 this I<happens> to be a 2-digit decimal |
194 | number. To avoid the year 2000 problem simply do not treat the year as |
195 | a 2-digit number. It isn't. |
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196 | |
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197 | When gmtime() and localtime() are used in scalar context they return |
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198 | a timestamp string that contains a fully-expanded year. For example, |
199 | C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 |
200 | 2001". There's no year 2000 problem here. |
201 | |
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202 | That doesn't mean that Perl can't be used to create non-Y2K compliant |
203 | programs. It can. But so can your pencil. It's the fault of the user, |
204 | not the language. At the risk of inflaming the NRA: ``Perl doesn't |
205 | break Y2K, people do.'' See http://language.perl.com/news/y2k.html for |
206 | a longer exposition. |
207 | |
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208 | =head1 Data: Strings |
209 | |
210 | =head2 How do I validate input? |
211 | |
212 | The answer to this question is usually a regular expression, perhaps |
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213 | with auxiliary logic. See the more specific questions (numbers, mail |
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214 | addresses, etc.) for details. |
215 | |
216 | =head2 How do I unescape a string? |
217 | |
218 | It depends just what you mean by "escape". URL escapes are dealt with |
219 | in L<perlfaq9>. Shell escapes with the backslash (\) |
220 | character are removed with: |
221 | |
222 | s/\\(.)/$1/g; |
223 | |
224 | Note that this won't expand \n or \t or any other special escapes. |
225 | |
226 | =head2 How do I remove consecutive pairs of characters? |
227 | |
228 | To turn "abbcccd" into "abccd": |
229 | |
230 | s/(.)\1/$1/g; |
231 | |
232 | =head2 How do I expand function calls in a string? |
233 | |
234 | This is documented in L<perlref>. In general, this is fraught with |
235 | quoting and readability problems, but it is possible. To interpolate |
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236 | a subroutine call (in list context) into a string: |
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237 | |
238 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
239 | |
240 | If you prefer scalar context, similar chicanery is also useful for |
241 | arbitrary expressions: |
242 | |
243 | print "That yields ${\($n + 5)} widgets\n"; |
244 | |
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245 | See also "How can I expand variables in text strings?" in this section |
246 | of the FAQ. |
247 | |
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248 | =head2 How do I find matching/nesting anything? |
249 | |
250 | This isn't something that can be tackled in one regular expression, no |
251 | matter how complicated. To find something between two single characters, |
252 | a pattern like C</x([^x]*)x/> will get the intervening bits in $1. For |
253 | multiple ones, then something more like C</alpha(.*?)omega/> would |
254 | be needed. But none of these deals with nested patterns, nor can they. |
255 | For that you'll have to write a parser. |
256 | |
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257 | One destructive, inside-out approach that you might try is to pull |
258 | out the smallest nesting parts one at a time: |
259 | |
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260 | while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) { |
5a964f20 |
261 | # do something with $1 |
262 | } |
263 | |
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264 | =head2 How do I reverse a string? |
265 | |
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266 | Use reverse() in scalar context, as documented in |
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267 | L<perlfunc/reverse>. |
268 | |
269 | $reversed = reverse $string; |
270 | |
271 | =head2 How do I expand tabs in a string? |
272 | |
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273 | You can do it yourself: |
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274 | |
275 | 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; |
276 | |
277 | Or you can just use the Text::Tabs module (part of the standard perl |
278 | distribution). |
279 | |
280 | use Text::Tabs; |
281 | @expanded_lines = expand(@lines_with_tabs); |
282 | |
283 | =head2 How do I reformat a paragraph? |
284 | |
285 | Use Text::Wrap (part of the standard perl distribution): |
286 | |
287 | use Text::Wrap; |
288 | print wrap("\t", ' ', @paragraphs); |
289 | |
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290 | The paragraphs you give to Text::Wrap may not contain embedded |
291 | newlines. Text::Wrap doesn't justify the lines (flush-right). |
292 | |
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293 | =head2 How can I access/change the first N letters of a string? |
294 | |
295 | There are many ways. If you just want to grab a copy, use |
296 | substr: |
297 | |
298 | $first_byte = substr($a, 0, 1); |
299 | |
300 | If you want to modify part of a string, the simplest way is often to |
301 | use substr() as an lvalue: |
302 | |
303 | substr($a, 0, 3) = "Tom"; |
304 | |
305 | Although those with a regexp kind of thought process will likely prefer |
306 | |
307 | $a =~ s/^.../Tom/; |
308 | |
309 | =head2 How do I change the Nth occurrence of something? |
310 | |
311 | You have to keep track. For example, let's say you want |
312 | to change the fifth occurrence of "whoever" or "whomever" |
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313 | into "whosoever" or "whomsoever", case insensitively. |
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314 | |
315 | $count = 0; |
316 | s{((whom?)ever)}{ |
317 | ++$count == 5 # is it the 5th? |
318 | ? "${2}soever" # yes, swap |
319 | : $1 # renege and leave it there |
320 | }igex; |
321 | |
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322 | In the more general case, you can use the C</g> modifier in a C<while> |
323 | loop, keeping count of matches. |
324 | |
325 | $WANT = 3; |
326 | $count = 0; |
327 | while (/(\w+)\s+fish\b/gi) { |
328 | if (++$count == $WANT) { |
329 | print "The third fish is a $1 one.\n"; |
330 | # Warning: don't `last' out of this loop |
331 | } |
332 | } |
333 | |
334 | That prints out: "The third fish is a red one." You can also use a |
335 | repetition count and repeated pattern like this: |
336 | |
337 | /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; |
338 | |
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339 | =head2 How can I count the number of occurrences of a substring within a string? |
340 | |
341 | There are a number of ways, with varying efficiency: If you want a |
342 | count of a certain single character (X) within a string, you can use the |
343 | C<tr///> function like so: |
344 | |
345 | $string = "ThisXlineXhasXsomeXx'sXinXit": |
346 | $count = ($string =~ tr/X//); |
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347 | print "There are $count X charcters in the string"; |
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348 | |
349 | This is fine if you are just looking for a single character. However, |
350 | if you are trying to count multiple character substrings within a |
351 | larger string, C<tr///> won't work. What you can do is wrap a while() |
352 | loop around a global pattern match. For example, let's count negative |
353 | integers: |
354 | |
355 | $string = "-9 55 48 -2 23 -76 4 14 -44"; |
356 | while ($string =~ /-\d+/g) { $count++ } |
357 | print "There are $count negative numbers in the string"; |
358 | |
359 | =head2 How do I capitalize all the words on one line? |
360 | |
361 | To make the first letter of each word upper case: |
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362 | |
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363 | $line =~ s/\b(\w)/\U$1/g; |
364 | |
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365 | This has the strange effect of turning "C<don't do it>" into "C<Don'T |
366 | Do It>". Sometimes you might want this, instead (Suggested by Brian |
367 | Foy E<lt>comdog@computerdog.comE<gt>): |
368 | |
369 | $string =~ s/ ( |
370 | (^\w) #at the beginning of the line |
371 | | # or |
372 | (\s\w) #preceded by whitespace |
373 | ) |
374 | /\U$1/xg; |
375 | $string =~ /([\w']+)/\u\L$1/g; |
376 | |
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377 | To make the whole line upper case: |
3fe9a6f1 |
378 | |
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379 | $line = uc($line); |
380 | |
381 | To force each word to be lower case, with the first letter upper case: |
3fe9a6f1 |
382 | |
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383 | $line =~ s/(\w+)/\u\L$1/g; |
384 | |
5a964f20 |
385 | You can (and probably should) enable locale awareness of those |
386 | characters by placing a C<use locale> pragma in your program. |
387 | See L<perllocale> for endless details. |
388 | |
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389 | =head2 How can I split a [character] delimited string except when inside |
390 | [character]? (Comma-separated files) |
391 | |
392 | Take the example case of trying to split a string that is comma-separated |
393 | into its different fields. (We'll pretend you said comma-separated, not |
394 | comma-delimited, which is different and almost never what you mean.) You |
395 | can't use C<split(/,/)> because you shouldn't split if the comma is inside |
396 | quotes. For example, take a data line like this: |
397 | |
398 | SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" |
399 | |
400 | Due to the restriction of the quotes, this is a fairly complex |
401 | problem. Thankfully, we have Jeffrey Friedl, author of a highly |
402 | recommended book on regular expressions, to handle these for us. He |
403 | suggests (assuming your string is contained in $text): |
404 | |
405 | @new = (); |
406 | push(@new, $+) while $text =~ m{ |
407 | "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes |
408 | | ([^,]+),? |
409 | | , |
410 | }gx; |
411 | push(@new, undef) if substr($text,-1,1) eq ','; |
412 | |
46fc3d4c |
413 | If you want to represent quotation marks inside a |
414 | quotation-mark-delimited field, escape them with backslashes (eg, |
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415 | C<"like \"this\"">. Unescaping them is a task addressed earlier in |
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416 | this section. |
417 | |
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418 | Alternatively, the Text::ParseWords module (part of the standard perl |
419 | distribution) lets you say: |
420 | |
421 | use Text::ParseWords; |
422 | @new = quotewords(",", 0, $text); |
423 | |
424 | =head2 How do I strip blank space from the beginning/end of a string? |
425 | |
5a964f20 |
426 | Although the simplest approach would seem to be: |
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427 | |
428 | $string =~ s/^\s*(.*?)\s*$/$1/; |
429 | |
5a964f20 |
430 | This is unneccesarily slow, destructive, and fails with embedded newlines. |
431 | It is much better faster to do this in two steps: |
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432 | |
433 | $string =~ s/^\s+//; |
434 | $string =~ s/\s+$//; |
435 | |
436 | Or more nicely written as: |
437 | |
438 | for ($string) { |
439 | s/^\s+//; |
440 | s/\s+$//; |
441 | } |
442 | |
5a964f20 |
443 | This idiom takes advantage of the for(each) loop's aliasing |
444 | behavior to factor out common code. You can do this |
445 | on several strings at once, or arrays, or even the |
446 | values of a hash if you use a slide: |
447 | |
448 | # trim whitespace in the scalar, the array, |
449 | # and all the values in the hash |
450 | foreach ($scalar, @array, @hash{keys %hash}) { |
451 | s/^\s+//; |
452 | s/\s+$//; |
453 | } |
454 | |
68dc0745 |
455 | =head2 How do I extract selected columns from a string? |
456 | |
457 | Use substr() or unpack(), both documented in L<perlfunc>. |
5a964f20 |
458 | If you prefer thinking in terms of columns instead of widths, |
459 | you can use this kind of thing: |
460 | |
461 | # determine the unpack format needed to split Linux ps output |
462 | # arguments are cut columns |
463 | my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); |
464 | |
465 | sub cut2fmt { |
466 | my(@positions) = @_; |
467 | my $template = ''; |
468 | my $lastpos = 1; |
469 | for my $place (@positions) { |
470 | $template .= "A" . ($place - $lastpos) . " "; |
471 | $lastpos = $place; |
472 | } |
473 | $template .= "A*"; |
474 | return $template; |
475 | } |
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476 | |
477 | =head2 How do I find the soundex value of a string? |
478 | |
479 | Use the standard Text::Soundex module distributed with perl. |
480 | |
481 | =head2 How can I expand variables in text strings? |
482 | |
483 | Let's assume that you have a string like: |
484 | |
485 | $text = 'this has a $foo in it and a $bar'; |
5a964f20 |
486 | |
487 | If those were both global variables, then this would |
488 | suffice: |
489 | |
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490 | $text =~ s/\$(\w+)/${$1}/g; |
491 | |
5a964f20 |
492 | But since they are probably lexicals, or at least, they could |
493 | be, you'd have to do this: |
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494 | |
495 | $text =~ s/(\$\w+)/$1/eeg; |
5a964f20 |
496 | die if $@; # needed on /ee, not /e |
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497 | |
5a964f20 |
498 | It's probably better in the general case to treat those |
499 | variables as entries in some special hash. For example: |
500 | |
501 | %user_defs = ( |
502 | foo => 23, |
503 | bar => 19, |
504 | ); |
505 | $text =~ s/\$(\w+)/$user_defs{$1}/g; |
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506 | |
46fc3d4c |
507 | See also "How do I expand function calls in a string?" in this section |
508 | of the FAQ. |
509 | |
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510 | =head2 What's wrong with always quoting "$vars"? |
511 | |
512 | The problem is that those double-quotes force stringification, |
513 | coercing numbers and references into strings, even when you |
514 | don't want them to be. |
515 | |
516 | If you get used to writing odd things like these: |
517 | |
518 | print "$var"; # BAD |
519 | $new = "$old"; # BAD |
520 | somefunc("$var"); # BAD |
521 | |
522 | You'll be in trouble. Those should (in 99.8% of the cases) be |
523 | the simpler and more direct: |
524 | |
525 | print $var; |
526 | $new = $old; |
527 | somefunc($var); |
528 | |
529 | Otherwise, besides slowing you down, you're going to break code when |
530 | the thing in the scalar is actually neither a string nor a number, but |
531 | a reference: |
532 | |
533 | func(\@array); |
534 | sub func { |
535 | my $aref = shift; |
536 | my $oref = "$aref"; # WRONG |
537 | } |
538 | |
539 | You can also get into subtle problems on those few operations in Perl |
540 | that actually do care about the difference between a string and a |
541 | number, such as the magical C<++> autoincrement operator or the |
542 | syscall() function. |
543 | |
5a964f20 |
544 | Stringification also destroys arrays. |
545 | |
546 | @lines = `command`; |
547 | print "@lines"; # WRONG - extra blanks |
548 | print @lines; # right |
549 | |
68dc0745 |
550 | =head2 Why don't my <<HERE documents work? |
551 | |
552 | Check for these three things: |
553 | |
554 | =over 4 |
555 | |
556 | =item 1. There must be no space after the << part. |
557 | |
558 | =item 2. There (probably) should be a semicolon at the end. |
559 | |
560 | =item 3. You can't (easily) have any space in front of the tag. |
561 | |
562 | =back |
563 | |
5a964f20 |
564 | If you want to indent the text in the here document, you |
565 | can do this: |
566 | |
567 | # all in one |
568 | ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; |
569 | your text |
570 | goes here |
571 | HERE_TARGET |
572 | |
573 | But the HERE_TARGET must still be flush against the margin. |
574 | If you want that indented also, you'll have to quote |
575 | in the indentation. |
576 | |
577 | ($quote = <<' FINIS') =~ s/^\s+//gm; |
578 | ...we will have peace, when you and all your works have |
579 | perished--and the works of your dark master to whom you |
580 | would deliver us. You are a liar, Saruman, and a corrupter |
581 | of men's hearts. --Theoden in /usr/src/perl/taint.c |
582 | FINIS |
583 | $quote =~ s/\s*--/\n--/; |
584 | |
585 | A nice general-purpose fixer-upper function for indented here documents |
586 | follows. It expects to be called with a here document as its argument. |
587 | It looks to see whether each line begins with a common substring, and |
588 | if so, strips that off. Otherwise, it takes the amount of leading |
589 | white space found on the first line and removes that much off each |
590 | subsequent line. |
591 | |
592 | sub fix { |
593 | local $_ = shift; |
594 | my ($white, $leader); # common white space and common leading string |
595 | if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { |
596 | ($white, $leader) = ($2, quotemeta($1)); |
597 | } else { |
598 | ($white, $leader) = (/^(\s+)/, ''); |
599 | } |
600 | s/^\s*?$leader(?:$white)?//gm; |
601 | return $_; |
602 | } |
603 | |
c8db1d39 |
604 | This works with leading special strings, dynamically determined: |
5a964f20 |
605 | |
606 | $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; |
607 | @@@ int |
608 | @@@ runops() { |
609 | @@@ SAVEI32(runlevel); |
610 | @@@ runlevel++; |
611 | @@@ while ( op = (*op->op_ppaddr)() ) ; |
612 | @@@ TAINT_NOT; |
613 | @@@ return 0; |
614 | @@@ } |
615 | MAIN_INTERPRETER_LOOP |
616 | |
617 | Or with a fixed amount of leading white space, with remaining |
618 | indentation correctly preserved: |
619 | |
620 | $poem = fix<<EVER_ON_AND_ON; |
621 | Now far ahead the Road has gone, |
622 | And I must follow, if I can, |
623 | Pursuing it with eager feet, |
624 | Until it joins some larger way |
625 | Where many paths and errands meet. |
626 | And whither then? I cannot say. |
627 | --Bilbo in /usr/src/perl/pp_ctl.c |
628 | EVER_ON_AND_ON |
629 | |
68dc0745 |
630 | =head1 Data: Arrays |
631 | |
632 | =head2 What is the difference between $array[1] and @array[1]? |
633 | |
634 | The former is a scalar value, the latter an array slice, which makes |
635 | it a list with one (scalar) value. You should use $ when you want a |
636 | scalar value (most of the time) and @ when you want a list with one |
637 | scalar value in it (very, very rarely; nearly never, in fact). |
638 | |
639 | Sometimes it doesn't make a difference, but sometimes it does. |
640 | For example, compare: |
641 | |
642 | $good[0] = `some program that outputs several lines`; |
643 | |
644 | with |
645 | |
646 | @bad[0] = `same program that outputs several lines`; |
647 | |
648 | The B<-w> flag will warn you about these matters. |
649 | |
650 | =head2 How can I extract just the unique elements of an array? |
651 | |
652 | There are several possible ways, depending on whether the array is |
653 | ordered and whether you wish to preserve the ordering. |
654 | |
655 | =over 4 |
656 | |
657 | =item a) If @in is sorted, and you want @out to be sorted: |
5a964f20 |
658 | (this assumes all true values in the array) |
68dc0745 |
659 | |
660 | $prev = 'nonesuch'; |
661 | @out = grep($_ ne $prev && ($prev = $_), @in); |
662 | |
c8db1d39 |
663 | This is nice in that it doesn't use much extra memory, simulating |
664 | uniq(1)'s behavior of removing only adjacent duplicates. It's less |
665 | nice in that it won't work with false values like undef, 0, or ""; |
666 | "0 but true" is ok, though. |
68dc0745 |
667 | |
668 | =item b) If you don't know whether @in is sorted: |
669 | |
670 | undef %saw; |
671 | @out = grep(!$saw{$_}++, @in); |
672 | |
673 | =item c) Like (b), but @in contains only small integers: |
674 | |
675 | @out = grep(!$saw[$_]++, @in); |
676 | |
677 | =item d) A way to do (b) without any loops or greps: |
678 | |
679 | undef %saw; |
680 | @saw{@in} = (); |
681 | @out = sort keys %saw; # remove sort if undesired |
682 | |
683 | =item e) Like (d), but @in contains only small positive integers: |
684 | |
685 | undef @ary; |
686 | @ary[@in] = @in; |
687 | @out = @ary; |
688 | |
689 | =back |
690 | |
5a964f20 |
691 | =head2 How can I tell whether a list or array contains a certain element? |
692 | |
693 | Hearing the word "in" is an I<in>dication that you probably should have |
694 | used a hash, not a list or array, to store your data. Hashes are |
695 | designed to answer this question quickly and efficiently. Arrays aren't. |
68dc0745 |
696 | |
5a964f20 |
697 | That being said, there are several ways to approach this. If you |
698 | are going to make this query many times over arbitrary string values, |
699 | the fastest way is probably to invert the original array and keep an |
68dc0745 |
700 | associative array lying about whose keys are the first array's values. |
701 | |
702 | @blues = qw/azure cerulean teal turquoise lapis-lazuli/; |
703 | undef %is_blue; |
704 | for (@blues) { $is_blue{$_} = 1 } |
705 | |
706 | Now you can check whether $is_blue{$some_color}. It might have been a |
707 | good idea to keep the blues all in a hash in the first place. |
708 | |
709 | If the values are all small integers, you could use a simple indexed |
710 | array. This kind of an array will take up less space: |
711 | |
712 | @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); |
713 | undef @is_tiny_prime; |
714 | for (@primes) { $is_tiny_prime[$_] = 1; } |
715 | |
716 | Now you check whether $is_tiny_prime[$some_number]. |
717 | |
718 | If the values in question are integers instead of strings, you can save |
719 | quite a lot of space by using bit strings instead: |
720 | |
721 | @articles = ( 1..10, 150..2000, 2017 ); |
722 | undef $read; |
7b8d334a |
723 | for (@articles) { vec($read,$_,1) = 1 } |
68dc0745 |
724 | |
725 | Now check whether C<vec($read,$n,1)> is true for some C<$n>. |
726 | |
727 | Please do not use |
728 | |
729 | $is_there = grep $_ eq $whatever, @array; |
730 | |
731 | or worse yet |
732 | |
733 | $is_there = grep /$whatever/, @array; |
734 | |
735 | These are slow (checks every element even if the first matches), |
736 | inefficient (same reason), and potentially buggy (what if there are |
737 | regexp characters in $whatever?). |
738 | |
739 | =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? |
740 | |
741 | Use a hash. Here's code to do both and more. It assumes that |
742 | each element is unique in a given array: |
743 | |
744 | @union = @intersection = @difference = (); |
745 | %count = (); |
746 | foreach $element (@array1, @array2) { $count{$element}++ } |
747 | foreach $element (keys %count) { |
748 | push @union, $element; |
749 | push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; |
750 | } |
751 | |
752 | =head2 How do I find the first array element for which a condition is true? |
753 | |
754 | You can use this if you care about the index: |
755 | |
756 | for ($i=0; $i < @array; $i++) { |
757 | if ($array[$i] eq "Waldo") { |
758 | $found_index = $i; |
759 | last; |
760 | } |
761 | } |
762 | |
763 | Now C<$found_index> has what you want. |
764 | |
765 | =head2 How do I handle linked lists? |
766 | |
767 | In general, you usually don't need a linked list in Perl, since with |
768 | regular arrays, you can push and pop or shift and unshift at either end, |
5a964f20 |
769 | or you can use splice to add and/or remove arbitrary number of elements at |
770 | arbitrary points. Both pop and shift are both O(1) operations on perl's |
771 | dynamic arrays. In the absence of shifts and pops, push in general |
772 | needs to reallocate on the order every log(N) times, and unshift will |
773 | need to copy pointers each time. |
68dc0745 |
774 | |
775 | If you really, really wanted, you could use structures as described in |
776 | L<perldsc> or L<perltoot> and do just what the algorithm book tells you |
777 | to do. |
778 | |
779 | =head2 How do I handle circular lists? |
780 | |
781 | Circular lists could be handled in the traditional fashion with linked |
782 | lists, or you could just do something like this with an array: |
783 | |
784 | unshift(@array, pop(@array)); # the last shall be first |
785 | push(@array, shift(@array)); # and vice versa |
786 | |
787 | =head2 How do I shuffle an array randomly? |
788 | |
5a964f20 |
789 | Use this: |
790 | |
791 | # fisher_yates_shuffle( \@array ) : |
792 | # generate a random permutation of @array in place |
793 | sub fisher_yates_shuffle { |
794 | my $array = shift; |
795 | my $i; |
796 | for ($i = @$array; --$i; ) { |
797 | my $j = int rand ($i+1); |
798 | next if $i == $j; |
799 | @$array[$i,$j] = @$array[$j,$i]; |
800 | } |
801 | } |
802 | |
803 | fisher_yates_shuffle( \@array ); # permutes @array in place |
804 | |
805 | You've probably seen shuffling algorithms that works using splice, |
68dc0745 |
806 | randomly picking another element to swap the current element with: |
807 | |
808 | srand; |
809 | @new = (); |
810 | @old = 1 .. 10; # just a demo |
811 | while (@old) { |
812 | push(@new, splice(@old, rand @old, 1)); |
813 | } |
814 | |
5a964f20 |
815 | This is bad because splice is already O(N), and since you do it N times, |
816 | you just invented a quadratic algorithm; that is, O(N**2). This does |
817 | not scale, although Perl is so efficient that you probably won't notice |
818 | this until you have rather largish arrays. |
68dc0745 |
819 | |
820 | =head2 How do I process/modify each element of an array? |
821 | |
822 | Use C<for>/C<foreach>: |
823 | |
824 | for (@lines) { |
5a964f20 |
825 | s/foo/bar/; # change that word |
826 | y/XZ/ZX/; # swap those letters |
68dc0745 |
827 | } |
828 | |
829 | Here's another; let's compute spherical volumes: |
830 | |
5a964f20 |
831 | for (@volumes = @radii) { # @volumes has changed parts |
68dc0745 |
832 | $_ **= 3; |
833 | $_ *= (4/3) * 3.14159; # this will be constant folded |
834 | } |
835 | |
5a964f20 |
836 | If you want to do the same thing to modify the values of the hash, |
837 | you may not use the C<values> function, oddly enough. You need a slice: |
838 | |
839 | for $orbit ( @orbits{keys %orbits} ) { |
840 | ($orbit **= 3) *= (4/3) * 3.14159; |
841 | } |
842 | |
68dc0745 |
843 | =head2 How do I select a random element from an array? |
844 | |
845 | Use the rand() function (see L<perlfunc/rand>): |
846 | |
5a964f20 |
847 | # at the top of the program: |
68dc0745 |
848 | srand; # not needed for 5.004 and later |
5a964f20 |
849 | |
850 | # then later on |
68dc0745 |
851 | $index = rand @array; |
852 | $element = $array[$index]; |
853 | |
5a964f20 |
854 | Make sure you I<only call srand once per program, if then>. |
855 | If you are calling it more than once (such as before each |
856 | call to rand), you're almost certainly doing something wrong. |
857 | |
68dc0745 |
858 | =head2 How do I permute N elements of a list? |
859 | |
860 | Here's a little program that generates all permutations |
861 | of all the words on each line of input. The algorithm embodied |
5a964f20 |
862 | in the permute() function should work on any list: |
68dc0745 |
863 | |
864 | #!/usr/bin/perl -n |
5a964f20 |
865 | # tsc-permute: permute each word of input |
866 | permute([split], []); |
867 | sub permute { |
868 | my @items = @{ $_[0] }; |
869 | my @perms = @{ $_[1] }; |
870 | unless (@items) { |
871 | print "@perms\n"; |
68dc0745 |
872 | } else { |
5a964f20 |
873 | my(@newitems,@newperms,$i); |
874 | foreach $i (0 .. $#items) { |
875 | @newitems = @items; |
876 | @newperms = @perms; |
877 | unshift(@newperms, splice(@newitems, $i, 1)); |
878 | permute([@newitems], [@newperms]); |
68dc0745 |
879 | } |
880 | } |
881 | } |
882 | |
883 | =head2 How do I sort an array by (anything)? |
884 | |
885 | Supply a comparison function to sort() (described in L<perlfunc/sort>): |
886 | |
887 | @list = sort { $a <=> $b } @list; |
888 | |
889 | The default sort function is cmp, string comparison, which would |
890 | sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is |
891 | the numerical comparison operator. |
892 | |
893 | If you have a complicated function needed to pull out the part you |
894 | want to sort on, then don't do it inside the sort function. Pull it |
895 | out first, because the sort BLOCK can be called many times for the |
896 | same element. Here's an example of how to pull out the first word |
897 | after the first number on each item, and then sort those words |
898 | case-insensitively. |
899 | |
900 | @idx = (); |
901 | for (@data) { |
902 | ($item) = /\d+\s*(\S+)/; |
903 | push @idx, uc($item); |
904 | } |
905 | @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; |
906 | |
907 | Which could also be written this way, using a trick |
908 | that's come to be known as the Schwartzian Transform: |
909 | |
910 | @sorted = map { $_->[0] } |
911 | sort { $a->[1] cmp $b->[1] } |
46fc3d4c |
912 | map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data; |
68dc0745 |
913 | |
914 | If you need to sort on several fields, the following paradigm is useful. |
915 | |
916 | @sorted = sort { field1($a) <=> field1($b) || |
917 | field2($a) cmp field2($b) || |
918 | field3($a) cmp field3($b) |
919 | } @data; |
920 | |
921 | This can be conveniently combined with precalculation of keys as given |
922 | above. |
923 | |
924 | See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about |
925 | this approach. |
926 | |
927 | See also the question below on sorting hashes. |
928 | |
929 | =head2 How do I manipulate arrays of bits? |
930 | |
931 | Use pack() and unpack(), or else vec() and the bitwise operations. |
932 | |
933 | For example, this sets $vec to have bit N set if $ints[N] was set: |
934 | |
935 | $vec = ''; |
936 | foreach(@ints) { vec($vec,$_,1) = 1 } |
937 | |
938 | And here's how, given a vector in $vec, you can |
939 | get those bits into your @ints array: |
940 | |
941 | sub bitvec_to_list { |
942 | my $vec = shift; |
943 | my @ints; |
944 | # Find null-byte density then select best algorithm |
945 | if ($vec =~ tr/\0// / length $vec > 0.95) { |
946 | use integer; |
947 | my $i; |
948 | # This method is faster with mostly null-bytes |
949 | while($vec =~ /[^\0]/g ) { |
950 | $i = -9 + 8 * pos $vec; |
951 | push @ints, $i if vec($vec, ++$i, 1); |
952 | push @ints, $i if vec($vec, ++$i, 1); |
953 | push @ints, $i if vec($vec, ++$i, 1); |
954 | push @ints, $i if vec($vec, ++$i, 1); |
955 | push @ints, $i if vec($vec, ++$i, 1); |
956 | push @ints, $i if vec($vec, ++$i, 1); |
957 | push @ints, $i if vec($vec, ++$i, 1); |
958 | push @ints, $i if vec($vec, ++$i, 1); |
959 | } |
960 | } else { |
961 | # This method is a fast general algorithm |
962 | use integer; |
963 | my $bits = unpack "b*", $vec; |
964 | push @ints, 0 if $bits =~ s/^(\d)// && $1; |
965 | push @ints, pos $bits while($bits =~ /1/g); |
966 | } |
967 | return \@ints; |
968 | } |
969 | |
970 | This method gets faster the more sparse the bit vector is. |
971 | (Courtesy of Tim Bunce and Winfried Koenig.) |
972 | |
973 | =head2 Why does defined() return true on empty arrays and hashes? |
974 | |
975 | See L<perlfunc/defined> in the 5.004 release or later of Perl. |
976 | |
977 | =head1 Data: Hashes (Associative Arrays) |
978 | |
979 | =head2 How do I process an entire hash? |
980 | |
981 | Use the each() function (see L<perlfunc/each>) if you don't care |
982 | whether it's sorted: |
983 | |
5a964f20 |
984 | while ( ($key, $value) = each %hash) { |
68dc0745 |
985 | print "$key = $value\n"; |
986 | } |
987 | |
988 | If you want it sorted, you'll have to use foreach() on the result of |
989 | sorting the keys as shown in an earlier question. |
990 | |
991 | =head2 What happens if I add or remove keys from a hash while iterating over it? |
992 | |
993 | Don't do that. |
994 | |
995 | =head2 How do I look up a hash element by value? |
996 | |
997 | Create a reverse hash: |
998 | |
999 | %by_value = reverse %by_key; |
1000 | $key = $by_value{$value}; |
1001 | |
1002 | That's not particularly efficient. It would be more space-efficient |
1003 | to use: |
1004 | |
1005 | while (($key, $value) = each %by_key) { |
1006 | $by_value{$value} = $key; |
1007 | } |
1008 | |
1009 | If your hash could have repeated values, the methods above will only |
1010 | find one of the associated keys. This may or may not worry you. |
1011 | |
1012 | =head2 How can I know how many entries are in a hash? |
1013 | |
1014 | If you mean how many keys, then all you have to do is |
1015 | take the scalar sense of the keys() function: |
1016 | |
3fe9a6f1 |
1017 | $num_keys = scalar keys %hash; |
68dc0745 |
1018 | |
1019 | In void context it just resets the iterator, which is faster |
1020 | for tied hashes. |
1021 | |
1022 | =head2 How do I sort a hash (optionally by value instead of key)? |
1023 | |
1024 | Internally, hashes are stored in a way that prevents you from imposing |
1025 | an order on key-value pairs. Instead, you have to sort a list of the |
1026 | keys or values: |
1027 | |
1028 | @keys = sort keys %hash; # sorted by key |
1029 | @keys = sort { |
1030 | $hash{$a} cmp $hash{$b} |
1031 | } keys %hash; # and by value |
1032 | |
1033 | Here we'll do a reverse numeric sort by value, and if two keys are |
1034 | identical, sort by length of key, and if that fails, by straight ASCII |
1035 | comparison of the keys (well, possibly modified by your locale -- see |
1036 | L<perllocale>). |
1037 | |
1038 | @keys = sort { |
1039 | $hash{$b} <=> $hash{$a} |
1040 | || |
1041 | length($b) <=> length($a) |
1042 | || |
1043 | $a cmp $b |
1044 | } keys %hash; |
1045 | |
1046 | =head2 How can I always keep my hash sorted? |
1047 | |
1048 | You can look into using the DB_File module and tie() using the |
1049 | $DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. |
5a964f20 |
1050 | The Tie::IxHash module from CPAN might also be instructive. |
68dc0745 |
1051 | |
1052 | =head2 What's the difference between "delete" and "undef" with hashes? |
1053 | |
1054 | Hashes are pairs of scalars: the first is the key, the second is the |
1055 | value. The key will be coerced to a string, although the value can be |
1056 | any kind of scalar: string, number, or reference. If a key C<$key> is |
1057 | present in the array, C<exists($key)> will return true. The value for |
1058 | a given key can be C<undef>, in which case C<$array{$key}> will be |
1059 | C<undef> while C<$exists{$key}> will return true. This corresponds to |
1060 | (C<$key>, C<undef>) being in the hash. |
1061 | |
1062 | Pictures help... here's the C<%ary> table: |
1063 | |
1064 | keys values |
1065 | +------+------+ |
1066 | | a | 3 | |
1067 | | x | 7 | |
1068 | | d | 0 | |
1069 | | e | 2 | |
1070 | +------+------+ |
1071 | |
1072 | And these conditions hold |
1073 | |
1074 | $ary{'a'} is true |
1075 | $ary{'d'} is false |
1076 | defined $ary{'d'} is true |
1077 | defined $ary{'a'} is true |
1078 | exists $ary{'a'} is true (perl5 only) |
1079 | grep ($_ eq 'a', keys %ary) is true |
1080 | |
1081 | If you now say |
1082 | |
1083 | undef $ary{'a'} |
1084 | |
1085 | your table now reads: |
1086 | |
1087 | |
1088 | keys values |
1089 | +------+------+ |
1090 | | a | undef| |
1091 | | x | 7 | |
1092 | | d | 0 | |
1093 | | e | 2 | |
1094 | +------+------+ |
1095 | |
1096 | and these conditions now hold; changes in caps: |
1097 | |
1098 | $ary{'a'} is FALSE |
1099 | $ary{'d'} is false |
1100 | defined $ary{'d'} is true |
1101 | defined $ary{'a'} is FALSE |
1102 | exists $ary{'a'} is true (perl5 only) |
1103 | grep ($_ eq 'a', keys %ary) is true |
1104 | |
1105 | Notice the last two: you have an undef value, but a defined key! |
1106 | |
1107 | Now, consider this: |
1108 | |
1109 | delete $ary{'a'} |
1110 | |
1111 | your table now reads: |
1112 | |
1113 | keys values |
1114 | +------+------+ |
1115 | | x | 7 | |
1116 | | d | 0 | |
1117 | | e | 2 | |
1118 | +------+------+ |
1119 | |
1120 | and these conditions now hold; changes in caps: |
1121 | |
1122 | $ary{'a'} is false |
1123 | $ary{'d'} is false |
1124 | defined $ary{'d'} is true |
1125 | defined $ary{'a'} is false |
1126 | exists $ary{'a'} is FALSE (perl5 only) |
1127 | grep ($_ eq 'a', keys %ary) is FALSE |
1128 | |
1129 | See, the whole entry is gone! |
1130 | |
1131 | =head2 Why don't my tied hashes make the defined/exists distinction? |
1132 | |
1133 | They may or may not implement the EXISTS() and DEFINED() methods |
1134 | differently. For example, there isn't the concept of undef with hashes |
1135 | that are tied to DBM* files. This means the true/false tables above |
1136 | will give different results when used on such a hash. It also means |
1137 | that exists and defined do the same thing with a DBM* file, and what |
1138 | they end up doing is not what they do with ordinary hashes. |
1139 | |
1140 | =head2 How do I reset an each() operation part-way through? |
1141 | |
5a964f20 |
1142 | Using C<keys %hash> in scalar context returns the number of keys in |
68dc0745 |
1143 | the hash I<and> resets the iterator associated with the hash. You may |
1144 | need to do this if you use C<last> to exit a loop early so that when you |
46fc3d4c |
1145 | re-enter it, the hash iterator has been reset. |
68dc0745 |
1146 | |
1147 | =head2 How can I get the unique keys from two hashes? |
1148 | |
1149 | First you extract the keys from the hashes into arrays, and then solve |
1150 | the uniquifying the array problem described above. For example: |
1151 | |
1152 | %seen = (); |
1153 | for $element (keys(%foo), keys(%bar)) { |
1154 | $seen{$element}++; |
1155 | } |
1156 | @uniq = keys %seen; |
1157 | |
1158 | Or more succinctly: |
1159 | |
1160 | @uniq = keys %{{%foo,%bar}}; |
1161 | |
1162 | Or if you really want to save space: |
1163 | |
1164 | %seen = (); |
1165 | while (defined ($key = each %foo)) { |
1166 | $seen{$key}++; |
1167 | } |
1168 | while (defined ($key = each %bar)) { |
1169 | $seen{$key}++; |
1170 | } |
1171 | @uniq = keys %seen; |
1172 | |
1173 | =head2 How can I store a multidimensional array in a DBM file? |
1174 | |
1175 | Either stringify the structure yourself (no fun), or else |
1176 | get the MLDBM (which uses Data::Dumper) module from CPAN and layer |
1177 | it on top of either DB_File or GDBM_File. |
1178 | |
1179 | =head2 How can I make my hash remember the order I put elements into it? |
1180 | |
1181 | Use the Tie::IxHash from CPAN. |
1182 | |
46fc3d4c |
1183 | use Tie::IxHash; |
1184 | tie(%myhash, Tie::IxHash); |
1185 | for ($i=0; $i<20; $i++) { |
1186 | $myhash{$i} = 2*$i; |
1187 | } |
1188 | @keys = keys %myhash; |
1189 | # @keys = (0,1,2,3,...) |
1190 | |
68dc0745 |
1191 | =head2 Why does passing a subroutine an undefined element in a hash create it? |
1192 | |
1193 | If you say something like: |
1194 | |
1195 | somefunc($hash{"nonesuch key here"}); |
1196 | |
1197 | Then that element "autovivifies"; that is, it springs into existence |
1198 | whether you store something there or not. That's because functions |
1199 | get scalars passed in by reference. If somefunc() modifies C<$_[0]>, |
1200 | it has to be ready to write it back into the caller's version. |
1201 | |
1202 | This has been fixed as of perl5.004. |
1203 | |
1204 | Normally, merely accessing a key's value for a nonexistent key does |
1205 | I<not> cause that key to be forever there. This is different than |
1206 | awk's behavior. |
1207 | |
fc36a67e |
1208 | =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? |
68dc0745 |
1209 | |
1210 | Use references (documented in L<perlref>). Examples of complex data |
1211 | structures are given in L<perldsc> and L<perllol>. Examples of |
1212 | structures and object-oriented classes are in L<perltoot>. |
1213 | |
1214 | =head2 How can I use a reference as a hash key? |
1215 | |
1216 | You can't do this directly, but you could use the standard Tie::Refhash |
1217 | module distributed with perl. |
1218 | |
1219 | =head1 Data: Misc |
1220 | |
1221 | =head2 How do I handle binary data correctly? |
1222 | |
1223 | Perl is binary clean, so this shouldn't be a problem. For example, |
1224 | this works fine (assuming the files are found): |
1225 | |
1226 | if (`cat /vmunix` =~ /gzip/) { |
1227 | print "Your kernel is GNU-zip enabled!\n"; |
1228 | } |
1229 | |
1230 | On some systems, however, you have to play tedious games with "text" |
1231 | versus "binary" files. See L<perlfunc/"binmode">. |
1232 | |
1233 | If you're concerned about 8-bit ASCII data, then see L<perllocale>. |
1234 | |
54310121 |
1235 | If you want to deal with multibyte characters, however, there are |
68dc0745 |
1236 | some gotchas. See the section on Regular Expressions. |
1237 | |
1238 | =head2 How do I determine whether a scalar is a number/whole/integer/float? |
1239 | |
1240 | Assuming that you don't care about IEEE notations like "NaN" or |
1241 | "Infinity", you probably just want to use a regular expression. |
1242 | |
1243 | warn "has nondigits" if /\D/; |
5a964f20 |
1244 | warn "not a natural number" unless /^\d+$/; # rejects -3 |
1245 | warn "not an integer" unless /^-?\d+$/; # rejects +3 |
54310121 |
1246 | warn "not an integer" unless /^[+-]?\d+$/; |
68dc0745 |
1247 | warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 |
1248 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; |
1249 | warn "not a C float" |
1250 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
1251 | |
5a964f20 |
1252 | If you're on a POSIX system, Perl's supports the C<POSIX::strtod> |
1253 | function. Its semantics are somewhat cumbersome, so here's a C<getnum> |
1254 | wrapper function for more convenient access. This function takes |
1255 | a string and returns the number it found, or C<undef> for input that |
1256 | isn't a C float. The C<is_numeric> function is a front end to C<getnum> |
1257 | if you just want to say, ``Is this a float?'' |
1258 | |
1259 | sub getnum { |
1260 | use POSIX qw(strtod); |
1261 | my $str = shift; |
1262 | $str =~ s/^\s+//; |
1263 | $str =~ s/\s+$//; |
1264 | $! = 0; |
1265 | my($num, $unparsed) = strtod($str); |
1266 | if (($str eq '') || ($unparsed != 0) || $!) { |
1267 | return undef; |
1268 | } else { |
1269 | return $num; |
1270 | } |
1271 | } |
1272 | |
1273 | sub is_numeric { defined &getnum } |
1274 | |
68dc0745 |
1275 | Or you could check out |
1276 | http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz |
1277 | instead. The POSIX module (part of the standard Perl distribution) |
1278 | provides the C<strtol> and C<strtod> for converting strings to double |
1279 | and longs, respectively. |
1280 | |
1281 | =head2 How do I keep persistent data across program calls? |
1282 | |
1283 | For some specific applications, you can use one of the DBM modules. |
1284 | See L<AnyDBM_File>. More generically, you should consult the |
1285 | FreezeThaw, Storable, or Class::Eroot modules from CPAN. |
1286 | |
1287 | =head2 How do I print out or copy a recursive data structure? |
1288 | |
1289 | The Data::Dumper module on CPAN is nice for printing out |
1290 | data structures, and FreezeThaw for copying them. For example: |
1291 | |
1292 | use FreezeThaw qw(freeze thaw); |
1293 | $new = thaw freeze $old; |
1294 | |
1295 | Where $old can be (a reference to) any kind of data structure you'd like. |
1296 | It will be deeply copied. |
1297 | |
1298 | =head2 How do I define methods for every class/object? |
1299 | |
1300 | Use the UNIVERSAL class (see L<UNIVERSAL>). |
1301 | |
1302 | =head2 How do I verify a credit card checksum? |
1303 | |
1304 | Get the Business::CreditCard module from CPAN. |
1305 | |
1306 | =head1 AUTHOR AND COPYRIGHT |
1307 | |
5a964f20 |
1308 | Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington. |
1309 | All rights reserved. |
1310 | |
1311 | When included as part of the Standard Version of Perl, or as part of |
1312 | its complete documentation whether printed or otherwise, this work |
1313 | may be distributed only under the terms of Perl's Artistic License. |
1314 | Any distribution of this file or derivatives thereof I<outside> |
1315 | of that package require that special arrangements be made with |
1316 | copyright holder. |
1317 | |
1318 | Irrespective of its distribution, all code examples in this file |
1319 | are hereby placed into the public domain. You are permitted and |
1320 | encouraged to use this code in your own programs for fun |
1321 | or for profit as you see fit. A simple comment in the code giving |
1322 | credit would be courteous but is not required. |