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PERLFORM(1)			 Perl Programmers Reference Guide		      PERLFORM(1)

       perlform - Perl formats

       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: format() to declare and
       write() to execute; see their entries in perlfunc.  Fortunately, the layout is much more
       legible, more like BASIC's PRINT USING statement.  Think of it as a poor man's nroff(1).

       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 "STD-
       OUT", 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 =

       If name is omitted, format "STDOUT" is defined.	FORMLIST consists of a sequence of lines,
       each of which may be one of three types:

       1.  A comment, indicated by putting a '#' in the first column.

       2.  A "picture" line giving the format for one output line.

       3.  An argument line supplying values to plug into the previous picture line.

       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.
       With a "0" (zero) instead of the first "#", the formatted number will be padded with lead-
       ing zeroes if necessary. 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 correspond-
       ing picture specifies that the decimal part should appear in the output (that is, any pic-
       ture 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 perllocale 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 write() call, and is not returned.)  Normally you would
       use a sequence of fields in a vertical stack to print out a block of text.  You might wish
       to end the final field with the text "...", which will appear in the output if the text
       was too long to appear in its entirety.	You can change which characters are legal to
       break on by changing the variable $: (that's $FORMAT_LINE_BREAK_CHARACTERS if you're using
       the English module) to a list of the desired characters.

       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" in perlfunc.


	# 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: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	       $index,			     $description
	Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
		  $priority,	    $date,   $description
	From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	      $from,			     $description
	Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
		     $programmer,	     $description
	~				     ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	~				     ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	~				     ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	~				     ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	~				     ^<<<<<<<<<<<<<<<<<<<<<<<...

       It is possible to intermix print()s with write()s on the same output channel, but you'll
       have to handle "$-" ($FORMAT_LINES_LEFT) yourself.

       Format Variables

       The current format name is stored in the variable $~ ($FORMAT_NAME), and the current top
       of form format name is in $^ ($FORMAT_TOP_NAME).  The current output page number is stored
       in $% ($FORMAT_PAGE_NUMBER), and the number of lines on the page is in $= ($FOR-
       MAT_LINES_PER_PAGE).  Whether to autoflush output on this handle is stored in $| ($OUT-
       PUT_AUTOFLUSH).	The string output before each top of page (except the first) is stored in
       $^L ($FORMAT_FORMFEED).	These variables are set on a per-filehandle basis, so you'll need
       to select() into a different one to affect them:

		   $~ = "My_Other_Format",
		   $^ = "My_Top_Format"

       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 inter-
       mediary stage in the expression to single-step the debugger through):

	   $ofh = select(OUTF);
	   $~ = "My_Other_Format";
	   $^ = "My_Top_Format";

       If you use the English module, you can even read the variable names:

	   use English '-no_match_vars';
	   $ofh = select(OUTF);
	   $FORMAT_NAME     = "My_Other_Format";
	   $FORMAT_TOP_NAME = "My_Top_Format";

       But you still have those funny select()s.  So just use the FileHandle module.  Now, you
       can access these special variables using lowercase method names instead:

	   use FileHandle;
	   format_name	   OUTF "My_Other_Format";
	   format_top_name OUTF "My_Top_Format";

       Much better!

       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 sprintf()
       or one of your own.  For example:

	   format Ident =

       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 eval() it:

	   $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 =

       Here's a little program that's somewhat like fmt(1):

	format =
	^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~


	$/ = '';
	while (<>) {
	    s/\s*\n\s*/ /g;


       While $FORMAT_TOP_NAME contains the name of the current header format, there is no corre-
       sponding mechanism to automatically do the same thing for a footer.  Not knowing how big a
       format is going to be until you evaluate it is one of the major problems.  It's on the
       TODO list.

       Here's one strategy:  If you have a fixed-size footer, you can get footers by checking
       $FORMAT_LINES_LEFT before each write() and print the footer yourself if necessary.

       Here's another strategy: Open a pipe to yourself, using "open(MYSELF, "|-")" (see "open()"
       in perlfunc) and always write() to MYSELF instead of STDOUT.  Have your child process mas-
       sage its STDIN to rearrange headers and footers however you like.  Not very convenient,
       but doable.

       Accessing Formatting Internals

       For low-level access to the formatting mechanism.  you may use formline() and access $^A
       (the $ACCUMULATOR variable) directly.

       For example:

	   $str = formline <<'END', 1,2,3;
	   @<<<  @|||  @>>>

	   print "Wow, I just stored `$^A' in the accumulator!\n";

       Or to make an swrite() subroutine, which is to write() what sprintf() is to printf(), do

	   use Carp;
	   sub swrite {
	       croak "usage: swrite PICTURE ARGS" unless @_;
	       my $format = shift;
	       $^A = "";
	       return $^A;

	   $string = swrite(<<'END', 1, 2, 3);
	Check me out
	@<<<  @|||  @>>>
	   print $string;

       The 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 "use locale" pragma is in effect.	Formatted output cannot
       be controlled by "use locale" because the pragma is tied to the block structure of the
       program, and, for historical reasons, formats exist outside that block structure.  See
       perllocale for further discussion of locale handling.

       Inside of an expression, the whitespace characters \n, \t and \f are considered to be
       equivalent to a single space.  Thus, you could think of this filter being applied to each
       value in the format:

	$value =~ tr/\n\t\f/ /;

       The remaining whitespace character, \r, forces the printing of a new line if allowed by
       the picture line.

perl v5.8.0				    2003-02-18				      PERLFORM(1)
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