Syntactic Confectionery Delight | |
PerlMonks |
perlman:perlformby gods (Initiate) |
on Aug 25, 1999 at 06:29 UTC ( [id://390]=perlman: print w/replies, xml ) | Need Help?? |
perlformCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
NAMEperlform - Perl formats
DESCRIPTION
Perl has a mechanism to help you generate simple reports and charts. To facilitate this, Perl helps you code up your output page close to how it will look when it's printed. It can keep track of things like how many lines are on a page, what page you're on, when to print page headers, etc. Keywords are borrowed from
FORTRAN:
Formats, like packages and subroutines, are declared rather than executed, so they may occur at any point in your program. (Usually it's best to keep them all together though.) They have their own namespace apart from all the other ``types'' in Perl. This means that if you have a function named ``Foo'', it is not the same thing as having a format named ``Foo''. However, the default name for the format associated with a given filehandle is the same as the name of the filehandle. Thus, the default format for STDOUT is named ``STDOUT'', and the default format for filehandle TEMP is named ``TEMP''. They just look the same. They aren't. Output record formats are declared as follows:
format NAME = FORMLIST . If name is omitted, format ``STDOUT'' is defined. FORMLIST consists of a sequence of lines, each of which may be one of three types:
Picture lines are printed exactly as they look, except for certain fields that substitute values into the line. Each field in a picture line starts with either ``@'' (at) or ``^'' (caret). These lines do not undergo any kind of variable interpolation. The at field (not to be confused with the array marker @) is the normal kind of field; the other kind, caret fields, are used to do rudimentary multi-line text block filling. The length of the field is supplied by padding out the field with multiple ``<'', ``>'', or ``|'' characters to specify, respectively, left justification, right justification, or centering. If the variable would exceed the width specified, it is truncated. As an alternate form of right justification, you may also use ``#'' characters (with an optional ``.'') to specify a numeric field. This way you can line up the decimal points. If any value supplied for these fields contains a newline, only the text up to the newline is printed. Finally, the special field ``@*'' can be used for printing multi-line, nontruncated values; it should appear by itself on a line. The values are specified on the following line in the same order as the picture fields. The expressions providing the values should be separated by commas. The expressions are all evaluated in a list context before the line is processed, so a single list expression could produce multiple list elements. The expressions may be spread out to more than one line if enclosed in braces. If so, the opening brace must be the first token on the first line. If an expression evaluates to a number with a decimal part, and if the corresponding picture specifies that the decimal part should appear in the output (that is, any picture except multiple ``#'' characters without an embedded ``.''), the character used for the decimal point is always determined by the current LC_NUMERIC locale. This means that, if, for example, the run-time environment happens to specify a German locale, ``,'' will be used instead of the default ``.''. See the perllocale manpage and WARNINGS for more information.
Picture fields that begin with ^ rather than @ are treated specially. With a # field, the field is blanked out if the value is undefined. For other field types, the caret enables a kind of fill mode. Instead of an arbitrary expression, the value supplied must be a scalar variable name that contains a text string. Perl puts as much text as it can into the field, and then chops off the front of the string so that the next time the variable is referenced, more of the text can be printed. (Yes, this means that the variable itself is altered during execution of the
Using caret fields can produce variable length records. If the text to be formatted is short, you can suppress blank lines by putting a ``~'' (tilde) character anywhere in the line. The tilde will be translated to a space upon output. If you put a second tilde contiguous to the first, the line will be repeated until all the fields on the line are exhausted. (If you use a field of the at variety, the expression you supply had better not give the same value every time forever!) Top-of-form processing is by default handled by a format with the same name as the current filehandle with ``_TOP'' concatenated to it. It's triggered at the top of each page. See write. Examples:
# a report on the /etc/passwd file format STDOUT_TOP = Passwd File Name Login Office Uid Gid Home ------------------------------------------------------------------ . format STDOUT = @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< $name, $login, $office,$uid,$gid, $home .
# a report from a bug report form format STDOUT_TOP = Bug Reports @<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>> $system, $%, $date ------------------------------------------------------------------ . format STDOUT = Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< $subject Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $index, $description Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $priority, $date, $description From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $from, $description Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $programmer, $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< $description ~ ^<<<<<<<<<<<<<<<<<<<<<<<... $description .
It is possible to intermix
Format Variables
The current format name is stored in the variable
select((select(OUTF), $~ = "My_Other_Format", $^ = "My_Top_Format" )[0]); Pretty ugly, eh? It's a common idiom though, so don't be too surprised when you see it. You can at least use a temporary variable to hold the previous filehandle: (this is a much better approach in general, because not only does legibility improve, you now have intermediary stage in the expression to single-step the debugger through):
$ofh = select(OUTF); $~ = "My_Other_Format"; $^ = "My_Top_Format"; select($ofh); If you use the English module, you can even read the variable names:
use English; $ofh = select(OUTF); $FORMAT_NAME = "My_Other_Format"; $FORMAT_TOP_NAME = "My_Top_Format"; select($ofh);
But you still have those funny
use FileHandle; format_name OUTF "My_Other_Format"; format_top_name OUTF "My_Top_Format"; Much better!
NOTES
Because the values line may contain arbitrary expressions (for at fields, not caret fields), you can farm out more sophisticated processing to other functions, like
format Ident = @<<<<<<<<<<<<<<< &commify($n) . To get a real at or caret into the field, do this:
format Ident = I have an @ here. "@" . To center a whole line of text, do something like this:
format Ident = @||||||||||||||||||||||||||||||||||||||||||||||| "Some text line" .
There is no builtin way to say ``float this to the right hand side of the page, however wide it is.'' You have to specify where it goes. The truly desperate can generate their own format on the fly, based on the current number of columns, and then
$format = "format STDOUT = \n" . '^' . '<' x $cols . "\n" . '$entry' . "\n" . "\t^" . "<" x ($cols-8) . "~~\n" . '$entry' . "\n" . ".\n"; print $format if $Debugging; eval $format; die $@ if $@; Which would generate a format looking something like this:
format STDOUT = ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< $entry ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~ $entry .
Here's a little program that's somewhat like
format = ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ $_
.
$/ = ''; while (<>) { s/\s*\n\s*/ /g; write; }
Footers
While
Here's one strategy: If you have a fixed-size footer, you can get footers by checking
Here's another strategy: Open a pipe to yourself, using open(MYSELF, "|-")
(see open()) and always
Accessing Formatting Internals
For low-level access to the formatting mechanism. you may use
For example:
$str = formline <<'END', 1,2,3; @<<< @||| @>>> END
print "Wow, I just stored `$^A' in the accumulator!\n";
Or to make an
use Carp; sub swrite { croak "usage: swrite PICTURE ARGS" unless @_; my $format = shift; $^A = ""; formline($format,@_); return $^A; }
$string = swrite(<<'END', 1, 2, 3); Check me out @<<< @||| @>>> END print $string;
WARNINGSThe lone dot that ends a format can also prematurely end a mail message passing through a misconfigured Internet mailer (and based on experience, such misconfiguration is the rule, not the exception). So when sending format code through mail, you should indent it so that the format-ending dot is not on the left margin; this will prevent SMTP cutoff. Lexical variables (declared with ``my'') are not visible within a format unless the format is declared within the scope of the lexical variable. (They weren't visible at all before version 5.001.)
Formats are the only part of Perl that unconditionally use information from a program's locale; if a program's environment specifies an
LC_NUMERIC locale, it is always used to specify the decimal point character in formatted output. Perl ignores all other aspects of locale handling unless the
Return to the Library |
|