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

NAME
       perlform	- 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: 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 "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 the name is omitted, format "STDOUT" is defined. A single "."	in
       column 1	is used	to terminate a format.	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 contain output field definitions, intermingled with
       literal text. These lines do not	undergo	any kind of variable
       interpolation.  Field definitions are made up from a set	of characters,
       for starting and	extending a field to its desired width.	This is	the
       complete	set of characters for field definitions:

	  @    start of	regular	field
	  ^    start of	special	field
	  <    pad character for left justification
	  |    pad character for centering
	  >    pad character for right justification
	  #    pad character for a right-justified numeric field
	  0    instead of first	#: pad number with leading zeroes
	  .    decimal point within a numeric field
	  ...  terminate a text	field, show "..." as truncation	evidence
	  @*   variable	width field for	a multi-line value
	  ^*   variable	width field for	next line of a multi-line value
	  ~    suppress	line with all fields empty
	  ~~   repeat line until all fields are	exhausted

       Each field in a picture line starts with	either "@" (at)	or "^"
       (caret),	indicating what	we'll call, respectively, a "regular" or
       "special" field.	 The choice of pad characters determines whether a
       field is	textual	or numeric. The	tilde operators	are not	part of	a
       field.  Let's look at the various possibilities in detail.

   Text	Fields
       The length of the field is supplied by padding out the field with
       multiple	"<", ">", or "|" characters to specify a non-numeric field
       with, respectively, left	justification, right justification, or
       centering.  For a regular field,	the value (up to the first newline) is
       taken and printed according to the selected justification, truncating
       excess characters.  If you terminate a text field with "...", three
       dots will be shown if the value is truncated. A special text field may
       be used to do rudimentary multi-line text block filling;	see "Using
       Fill Mode" for details.

	  Example:
	     format STDOUT =
	     @<<<<<<   @||||||	 @>>>>>>
	     "left",   "middle", "right"
	     .
	  Output:
	     left      middle	 right

   Numeric Fields
       Using "#" as a padding character	specifies a numeric field, with	right
       justification. An optional "." defines the position of the decimal
       point. With a "0" (zero)	instead	of the first "#", the formatted	number
       will be padded with leading zeroes if necessary.	 A special numeric
       field is	blanked	out if the value is undefined.	If the resulting value
       would exceed the	width specified	the field is filled with "#" as
       overflow	evidence.

	  Example:
	     format STDOUT =
	     @###   @.###   @##.###  @###   @###   ^####
	      42,   3.1415,  undef,    0, 10000,   undef
	     .
	  Output:
	       42   3.142     0.000	0   ####

   The Field @*	for Variable-Width Multi-Line Text
       The field "@*" can be used for printing multi-line, nontruncated
       values; it should (but need not)	appear by itself on a line. A final
       line feed is chomped off, but all other characters are emitted
       verbatim.

   The Field ^*	for Variable-Width One-line-at-a-time Text
       Like "@*", this is a variable-width field. The value supplied must be a
       scalar variable.	Perl puts the first line (up to	the first "\n")	of the
       text 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.	 The variable will not be restored.

	  Example:
	     $text = "line 1\nline 2\nline 3";
	     format STDOUT =
	     Text: ^*
		   $text
	     ~~	   ^*
		   $text
	     .
	  Output:
	     Text: line	1
		   line	2
		   line	3

   Specifying Values
       The values are specified	on the following format	line in	the same order
       as the picture fields.  The expressions providing the values must be
       separated by commas.  They 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 determined	by the current LC_NUMERIC
       locale if "use locale" is in effect.  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.

   Using Fill Mode
       On text fields the caret	enables	a kind of fill mode.  Instead of an
       arbitrary expression, the value supplied	must be	a scalar variable that
       contains	a text string.	Perl puts the next portion of the text 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 restored.)  The next portion of text is
       determined by a crude line-breaking algorithm. You may use the carriage
       return character	("\r") to force	a line break. 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.

       Normally	you would use a	sequence of fields in a	vertical stack
       associated with the same	scalar variable	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.

   Suppressing Lines Where All Fields Are Void
       Using caret fields can produce lines where all fields are blank.	You
       can suppress such lines by putting a "~"	(tilde)	character anywhere in
       the line.  The tilde will be translated to a space upon output.

   Repeating Format Lines
       If you put two contiguous tilde characters "~~" anywhere	into a line,
       the line	will be	repeated until all the fields on the line are
       exhausted, i.e. undefined. For special (caret) text fields this will
       occur sooner or later, but if you use a text field of the at variety,
       the  expression you supply had better not give the same value every
       time forever! ("shift(@f)" is a simple example that would work.)	 Don't
       use a regular (at) numeric field	in such	lines, because it will never
       go blank.

   Top of Form Processing
       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.

       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 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 $= ($FORMAT_LINES_PER_PAGE).
       Whether to autoflush output on this handle is stored in $|
       ($OUTPUT_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:

	   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 an 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 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!

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

	   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 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 =
	^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	$entry
		^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
	$entry
	.

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

	format =
	^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<	~~
	$_

	.

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

   Footers
       While $FORMAT_TOP_NAME contains the name	of the current header format,
       there is	no corresponding 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	massage	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;
	   @<<<	 @|||  @>>>
	   END

	   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 this:

	   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;

WARNINGS
       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.

       If a program's environment specifies an LC_NUMERIC locale and "use
       locale" is in effect when the format is declared, the locale is used to
       specify the decimal point character in formatted	output.	 Formatted
       output cannot be	controlled by "use locale" at the time when write() is
       called. See perllocale for further discussion of	locale handling.

       Within strings that are to be displayed in a fixed-length text field,
       each control character is substituted by	a space. (But remember the
       special meaning of "\r" when using fill mode.) This is done to avoid
       misalignment when control characters "disappear"	on some	output media.

perl v5.28.3			  2020-05-14			   PERLFORM(1)

NAME | DESCRIPTION | NOTES | WARNINGS

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