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128 | .rm #[ #] #H #V #F C | |
129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLFORM 1" | |
132 | .TH PERLFORM 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlform \- Perl formats | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | Perl has a mechanism to help you generate simple reports and charts. To | |
138 | facilitate this, Perl helps you code up your output page close to how it | |
139 | will look when it's printed. It can keep track of things like how many | |
140 | lines are on a page, what page you're on, when to print page headers, | |
141 | etc. Keywords are borrowed from \s-1FORTRAN:\s0 \fIformat()\fR to declare and \fIwrite()\fR | |
142 | to execute; see their entries in perlfunc. Fortunately, the layout is | |
143 | much more legible, more like \s-1BASIC\s0's \s-1PRINT\s0 \s-1USING\s0 statement. Think of it | |
144 | as a poor man's \fInroff\fR\|(1). | |
145 | .PP | |
146 | Formats, like packages and subroutines, are declared rather than | |
147 | executed, so they may occur at any point in your program. (Usually it's | |
148 | best to keep them all together though.) They have their own namespace | |
149 | apart from all the other \*(L"types\*(R" in Perl. This means that if you have a | |
150 | function named \*(L"Foo\*(R", it is not the same thing as having a format named | |
151 | \&\*(L"Foo\*(R". However, the default name for the format associated with a given | |
152 | filehandle is the same as the name of the filehandle. Thus, the default | |
153 | format for \s-1STDOUT\s0 is named \*(L"\s-1STDOUT\s0\*(R", and the default format for filehandle | |
154 | \&\s-1TEMP\s0 is named \*(L"\s-1TEMP\s0\*(R". They just look the same. They aren't. | |
155 | .PP | |
156 | Output record formats are declared as follows: | |
157 | .PP | |
158 | .Vb 3 | |
159 | \& format NAME = | |
160 | \& FORMLIST | |
161 | \& . | |
162 | .Ve | |
163 | .PP | |
164 | If name is omitted, format \*(L"\s-1STDOUT\s0\*(R" is defined. \s-1FORMLIST\s0 consists of | |
165 | a sequence of lines, each of which may be one of three types: | |
166 | .IP "1." 4 | |
167 | A comment, indicated by putting a '#' in the first column. | |
168 | .IP "2." 4 | |
169 | A \*(L"picture\*(R" line giving the format for one output line. | |
170 | .IP "3." 4 | |
171 | An argument line supplying values to plug into the previous picture line. | |
172 | .PP | |
173 | Picture lines are printed exactly as they look, except for certain fields | |
174 | that substitute values into the line. Each field in a picture line starts | |
175 | with either \*(L"@\*(R" (at) or \*(L"^\*(R" (caret). These lines do not undergo any kind | |
176 | of variable interpolation. The at field (not to be confused with the array | |
177 | marker @) is the normal kind of field; the other kind, caret fields, are used | |
178 | to do rudimentary multi-line text block filling. The length of the field | |
179 | is supplied by padding out the field with multiple "<\*(L", \*(R">\*(L", or \*(R"|" | |
180 | characters to specify, respectively, left justification, right | |
181 | justification, or centering. If the variable would exceed the width | |
182 | specified, it is truncated. | |
183 | .PP | |
184 | As an alternate form of right justification, you may also use \*(L"#\*(R" | |
185 | characters (with an optional \*(L".\*(R") to specify a numeric field. This way | |
186 | you can line up the decimal points. With a \*(L"0\*(R" (zero) instead of the | |
187 | first \*(L"#\*(R", the formatted number will be padded with leading zeroes if | |
188 | necessary. If any value supplied for these fields contains a newline, | |
189 | only the text up to the newline is printed. Finally, the special field | |
190 | \&\*(L"@*\*(R" can be used for printing multi\-line, nontruncated values; it | |
191 | should appear by itself on a line. | |
192 | .PP | |
193 | The values are specified on the following line in the same order as | |
194 | the picture fields. The expressions providing the values should be | |
195 | separated by commas. The expressions are all evaluated in a list context | |
196 | before the line is processed, so a single list expression could produce | |
197 | multiple list elements. The expressions may be spread out to more than | |
198 | one line if enclosed in braces. If so, the opening brace must be the first | |
199 | token on the first line. If an expression evaluates to a number with a | |
200 | decimal part, and if the corresponding picture specifies that the decimal | |
201 | part should appear in the output (that is, any picture except multiple \*(L"#\*(R" | |
202 | characters \fBwithout\fR an embedded \*(L".\*(R"), the character used for the decimal | |
203 | point is \fBalways\fR determined by the current \s-1LC_NUMERIC\s0 locale. This | |
204 | means that, if, for example, the run-time environment happens to specify a | |
205 | German locale, \*(L",\*(R" will be used instead of the default \*(L".\*(R". See | |
206 | perllocale and \*(L"\s-1WARNINGS\s0\*(R" for more information. | |
207 | .PP | |
208 | Picture fields that begin with ^ rather than @ are treated specially. | |
209 | With a # field, the field is blanked out if the value is undefined. For | |
210 | other field types, the caret enables a kind of fill mode. Instead of an | |
211 | arbitrary expression, the value supplied must be a scalar variable name | |
212 | that contains a text string. Perl puts as much text as it can into the | |
213 | field, and then chops off the front of the string so that the next time | |
214 | the variable is referenced, more of the text can be printed. (Yes, this | |
215 | means that the variable itself is altered during execution of the \fIwrite()\fR | |
216 | call, and is not returned.) Normally you would use a sequence of fields | |
217 | in a vertical stack to print out a block of text. You might wish to end | |
218 | the final field with the text \*(L"...\*(R", which will appear in the output if | |
219 | the text was too long to appear in its entirety. You can change which | |
220 | characters are legal to break on by changing the variable \f(CW$:\fR (that's | |
221 | \&\f(CW$FORMAT_LINE_BREAK_CHARACTERS\fR if you're using the English module) to a | |
222 | list of the desired characters. | |
223 | .PP | |
224 | Using caret fields can produce variable length records. If the text | |
225 | to be formatted is short, you can suppress blank lines by putting a | |
226 | \&\*(L"~\*(R" (tilde) character anywhere in the line. The tilde will be translated | |
227 | to a space upon output. If you put a second tilde contiguous to the | |
228 | first, the line will be repeated until all the fields on the line are | |
229 | exhausted. (If you use a field of the at variety, the expression you | |
230 | supply had better not give the same value every time forever!) | |
231 | .PP | |
232 | Top-of-form processing is by default handled by a format with the | |
233 | same name as the current filehandle with \*(L"_TOP\*(R" concatenated to it. | |
234 | It's triggered at the top of each page. See \*(L"write\*(R" in perlfunc. | |
235 | .PP | |
236 | Examples: | |
237 | .PP | |
238 | .Vb 10 | |
239 | \& # a report on the /etc/passwd file | |
240 | \& format STDOUT_TOP = | |
241 | \& Passwd File | |
242 | \& Name Login Office Uid Gid Home | |
243 | \& ------------------------------------------------------------------ | |
244 | \& . | |
245 | \& format STDOUT = | |
246 | \& @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< | |
247 | \& $name, $login, $office,$uid,$gid, $home | |
248 | \& . | |
249 | .Ve | |
250 | .PP | |
251 | .Vb 29 | |
252 | \& # a report from a bug report form | |
253 | \& format STDOUT_TOP = | |
254 | \& Bug Reports | |
255 | \& @<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>> | |
256 | \& $system, $%, $date | |
257 | \& ------------------------------------------------------------------ | |
258 | \& . | |
259 | \& format STDOUT = | |
260 | \& Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
261 | \& $subject | |
262 | \& Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
263 | \& $index, $description | |
264 | \& Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
265 | \& $priority, $date, $description | |
266 | \& From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
267 | \& $from, $description | |
268 | \& Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
269 | \& $programmer, $description | |
270 | \& ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
271 | \& $description | |
272 | \& ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
273 | \& $description | |
274 | \& ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
275 | \& $description | |
276 | \& ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
277 | \& $description | |
278 | \& ~ ^<<<<<<<<<<<<<<<<<<<<<<<... | |
279 | \& $description | |
280 | \& . | |
281 | .Ve | |
282 | .PP | |
283 | It is possible to intermix \fIprint()\fRs with \fIwrite()\fRs on the same output | |
284 | channel, but you'll have to handle \f(CW\*(C`$\-\*(C'\fR (\f(CW$FORMAT_LINES_LEFT\fR) | |
285 | yourself. | |
286 | .Sh "Format Variables" | |
287 | .IX Subsection "Format Variables" | |
288 | The current format name is stored in the variable \f(CW$~\fR (\f(CW$FORMAT_NAME\fR), | |
289 | and the current top of form format name is in \f(CW$^\fR (\f(CW$FORMAT_TOP_NAME\fR). | |
290 | The current output page number is stored in \f(CW$%\fR (\f(CW$FORMAT_PAGE_NUMBER\fR), | |
291 | and the number of lines on the page is in \f(CW$=\fR (\f(CW$FORMAT_LINES_PER_PAGE\fR). | |
292 | Whether to autoflush output on this handle is stored in \f(CW$|\fR | |
293 | (\f(CW$OUTPUT_AUTOFLUSH\fR). The string output before each top of page (except | |
294 | the first) is stored in \f(CW$^L\fR (\f(CW$FORMAT_FORMFEED\fR). These variables are | |
295 | set on a per-filehandle basis, so you'll need to \fIselect()\fR into a different | |
296 | one to affect them: | |
297 | .PP | |
298 | .Vb 4 | |
299 | \& select((select(OUTF), | |
300 | \& $~ = "My_Other_Format", | |
301 | \& $^ = "My_Top_Format" | |
302 | \& )[0]); | |
303 | .Ve | |
304 | .PP | |
305 | Pretty ugly, eh? It's a common idiom though, so don't be too surprised | |
306 | when you see it. You can at least use a temporary variable to hold | |
307 | the previous filehandle: (this is a much better approach in general, | |
308 | because not only does legibility improve, you now have intermediary | |
309 | stage in the expression to single-step the debugger through): | |
310 | .PP | |
311 | .Vb 4 | |
312 | \& $ofh = select(OUTF); | |
313 | \& $~ = "My_Other_Format"; | |
314 | \& $^ = "My_Top_Format"; | |
315 | \& select($ofh); | |
316 | .Ve | |
317 | .PP | |
318 | If you use the English module, you can even read the variable names: | |
319 | .PP | |
320 | .Vb 5 | |
321 | \& use English '-no_match_vars'; | |
322 | \& $ofh = select(OUTF); | |
323 | \& $FORMAT_NAME = "My_Other_Format"; | |
324 | \& $FORMAT_TOP_NAME = "My_Top_Format"; | |
325 | \& select($ofh); | |
326 | .Ve | |
327 | .PP | |
328 | But you still have those funny \fIselect()\fRs. So just use the FileHandle | |
329 | module. Now, you can access these special variables using lowercase | |
330 | method names instead: | |
331 | .PP | |
332 | .Vb 3 | |
333 | \& use FileHandle; | |
334 | \& format_name OUTF "My_Other_Format"; | |
335 | \& format_top_name OUTF "My_Top_Format"; | |
336 | .Ve | |
337 | .PP | |
338 | Much better! | |
339 | .SH "NOTES" | |
340 | .IX Header "NOTES" | |
341 | Because the values line may contain arbitrary expressions (for at fields, | |
342 | not caret fields), you can farm out more sophisticated processing | |
343 | to other functions, like \fIsprintf()\fR or one of your own. For example: | |
344 | .PP | |
345 | .Vb 4 | |
346 | \& format Ident = | |
347 | \& @<<<<<<<<<<<<<<< | |
348 | \& &commify($n) | |
349 | \& . | |
350 | .Ve | |
351 | .PP | |
352 | To get a real at or caret into the field, do this: | |
353 | .PP | |
354 | .Vb 4 | |
355 | \& format Ident = | |
356 | \& I have an @ here. | |
357 | \& "@" | |
358 | \& . | |
359 | .Ve | |
360 | .PP | |
361 | To center a whole line of text, do something like this: | |
362 | .PP | |
363 | .Vb 4 | |
364 | \& format Ident = | |
365 | \& @||||||||||||||||||||||||||||||||||||||||||||||| | |
366 | \& "Some text line" | |
367 | \& . | |
368 | .Ve | |
369 | .PP | |
370 | There is no builtin way to say \*(L"float this to the right hand side | |
371 | of the page, however wide it is.\*(R" You have to specify where it goes. | |
372 | The truly desperate can generate their own format on the fly, based | |
373 | on the current number of columns, and then \fIeval()\fR it: | |
374 | .PP | |
375 | .Vb 9 | |
376 | \& $format = "format STDOUT = \en" | |
377 | \& . '^' . '<' x $cols . "\en" | |
378 | \& . '$entry' . "\en" | |
379 | \& . "\et^" . "<" x ($cols-8) . "~~\en" | |
380 | \& . '$entry' . "\en" | |
381 | \& . ".\en"; | |
382 | \& print $format if $Debugging; | |
383 | \& eval $format; | |
384 | \& die $@ if $@; | |
385 | .Ve | |
386 | .PP | |
387 | Which would generate a format looking something like this: | |
388 | .PP | |
389 | .Vb 6 | |
390 | \& format STDOUT = | |
391 | \& ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | |
392 | \& $entry | |
393 | \& ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~ | |
394 | \& $entry | |
395 | \& . | |
396 | .Ve | |
397 | .PP | |
398 | Here's a little program that's somewhat like \fIfmt\fR\|(1): | |
399 | .PP | |
400 | .Vb 3 | |
401 | \& format = | |
402 | \& ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ | |
403 | \& $_ | |
404 | .Ve | |
405 | .PP | |
406 | .Vb 1 | |
407 | \& . | |
408 | .Ve | |
409 | .PP | |
410 | .Vb 5 | |
411 | \& $/ = ''; | |
412 | \& while (<>) { | |
413 | \& s/\es*\en\es*/ /g; | |
414 | \& write; | |
415 | \& } | |
416 | .Ve | |
417 | .Sh "Footers" | |
418 | .IX Subsection "Footers" | |
419 | While \f(CW$FORMAT_TOP_NAME\fR contains the name of the current header format, | |
420 | there is no corresponding mechanism to automatically do the same thing | |
421 | for a footer. Not knowing how big a format is going to be until you | |
422 | evaluate it is one of the major problems. It's on the \s-1TODO\s0 list. | |
423 | .PP | |
424 | Here's one strategy: If you have a fixed-size footer, you can get footers | |
425 | by checking \f(CW$FORMAT_LINES_LEFT\fR before each \fIwrite()\fR and print the footer | |
426 | yourself if necessary. | |
427 | .PP | |
428 | Here's another strategy: Open a pipe to yourself, using \f(CW\*(C`open(MYSELF, "|\-")\*(C'\fR | |
429 | (see \*(L"\fIopen()\fR\*(R" in perlfunc) and always \fIwrite()\fR to \s-1MYSELF\s0 instead of \s-1STDOUT\s0. | |
430 | Have your child process massage its \s-1STDIN\s0 to rearrange headers and footers | |
431 | however you like. Not very convenient, but doable. | |
432 | .Sh "Accessing Formatting Internals" | |
433 | .IX Subsection "Accessing Formatting Internals" | |
434 | For low-level access to the formatting mechanism. you may use \fIformline()\fR | |
435 | and access \f(CW$^A\fR (the \f(CW$ACCUMULATOR\fR variable) directly. | |
436 | .PP | |
437 | For example: | |
438 | .PP | |
439 | .Vb 3 | |
440 | \& $str = formline <<'END', 1,2,3; | |
441 | \& @<<< @||| @>>> | |
442 | \& END | |
443 | .Ve | |
444 | .PP | |
445 | .Vb 1 | |
446 | \& print "Wow, I just stored `$^A' in the accumulator!\en"; | |
447 | .Ve | |
448 | .PP | |
449 | Or to make an \fIswrite()\fR subroutine, which is to \fIwrite()\fR what \fIsprintf()\fR | |
450 | is to \fIprintf()\fR, do this: | |
451 | .PP | |
452 | .Vb 8 | |
453 | \& use Carp; | |
454 | \& sub swrite { | |
455 | \& croak "usage: swrite PICTURE ARGS" unless @_; | |
456 | \& my $format = shift; | |
457 | \& $^A = ""; | |
458 | \& formline($format,@_); | |
459 | \& return $^A; | |
460 | \& } | |
461 | .Ve | |
462 | .PP | |
463 | .Vb 5 | |
464 | \& $string = swrite(<<'END', 1, 2, 3); | |
465 | \& Check me out | |
466 | \& @<<< @||| @>>> | |
467 | \& END | |
468 | \& print $string; | |
469 | .Ve | |
470 | .SH "WARNINGS" | |
471 | .IX Header "WARNINGS" | |
472 | The lone dot that ends a format can also prematurely end a mail | |
473 | message passing through a misconfigured Internet mailer (and based on | |
474 | experience, such misconfiguration is the rule, not the exception). So | |
475 | when sending format code through mail, you should indent it so that | |
476 | the format-ending dot is not on the left margin; this will prevent | |
477 | \&\s-1SMTP\s0 cutoff. | |
478 | .PP | |
479 | Lexical variables (declared with \*(L"my\*(R") are not visible within a | |
480 | format unless the format is declared within the scope of the lexical | |
481 | variable. (They weren't visible at all before version 5.001.) | |
482 | .PP | |
483 | Formats are the only part of Perl that unconditionally use information | |
484 | from a program's locale; if a program's environment specifies an | |
485 | \&\s-1LC_NUMERIC\s0 locale, it is always used to specify the decimal point | |
486 | character in formatted output. Perl ignores all other aspects of locale | |
487 | handling unless the \f(CW\*(C`use locale\*(C'\fR pragma is in effect. Formatted output | |
488 | cannot be controlled by \f(CW\*(C`use locale\*(C'\fR because the pragma is tied to the | |
489 | block structure of the program, and, for historical reasons, formats | |
490 | exist outside that block structure. See perllocale for further | |
491 | discussion of locale handling. | |
492 | .PP | |
493 | Inside of an expression, the whitespace characters \en, \et and \ef are | |
494 | considered to be equivalent to a single space. Thus, you could think | |
495 | of this filter being applied to each value in the format: | |
496 | .PP | |
497 | .Vb 1 | |
498 | \& $value =~ tr/\en\et\ef/ /; | |
499 | .Ve | |
500 | .PP | |
501 | The remaining whitespace character, \er, forces the printing of a new | |
502 | line if allowed by the picture line. |