=head1 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 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
-L<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
+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 L<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 name "STDOUT", and the default format for filehandle
-TEMP is name "TEMP". They just look the same. They aren't.
+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:
FORMLIST
.
-If name is omitted, format "STDOUT" is defined. FORMLIST consists of
-a sequence of lines, each of which may be of one of three types:
+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:
=over 4
=back
-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 "E<lt>", "E<gt>", 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,
-non-truncated 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
+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 adjustification
+ | pad character for centering
+ > pad character for right adjustificat
+ # 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.
+
+
+=head2 Text Fields
+
+The length of the field is supplied by padding out the field with multiple
+"E<lt>", "E<gt>", 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 L</Using Fill Mode> for details.
+
+ Example:
+ format STDOUT =
+ @<<<<<< @|||||| @>>>>>>
+ "left", "middle", "right"
+ .
+ Output:
+ left middle right
+
+
+=head2 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 ####
+
+
+=head2 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.
+
+
+=head2 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 I<not> be restored.
+
+ Example:
+ $text = "line 1\nline 2\nline 3";
+ format STDOUT =
+ Text: ^*
+ $text
+ ~~ ^*
+ $text
+ .
+ Output:
+ Text: line 1
+ line 2
+ line 3
+
+
+=head2 Specifying Values
+
+The values are specified on the following format line in the same order as
+the picture fields. The expressions providing the values must be
+separated by commas. They are all evaluated in a list context
before the line is processed, so a single list expression could produce
multiple list elements. The expressions may be spread out to more than
one line if enclosed in braces. If so, the opening brace must be the first
German locale, "," will be used instead of the default ".". See
L<perllocale> and L<"WARNINGS"> for more information.
-Picture fields that begin with ^ rather than @ are treated specially.
-With a # field, the field is blanked out if the value is undefined. For
-other field types, the caret enables a kind of fill mode. Instead of an
-arbitrary expression, the value supplied must be a scalar variable name
-that contains a text string. Perl puts as much text as it can into the
-field, and then chops off the front of the string so that the next time
+
+=head2 Using Fill Mode
+
+On text fields the caret enables a kind of fill mode. Instead of an
+arbitrary expression, the value supplied must be a scalar variable
+that contains a text string. Perl puts the next portion of the text into
+the field, and then chops off the front of the string so that the next time
the variable is referenced, more of the text can be printed. (Yes, this
means that the variable itself is altered during execution of the write()
-call, and is not returned.) Normally you would use a sequence of fields
-in a vertical stack to print out a block of text. You might wish to end
-the final field with the text "...", which will appear in the output if
-the text was too long to appear in its entirety. You can change which
-characters are legal to break on by changing the variable C<$:> (that's
-$FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a
+call, and is not restored.) The next portion of text is determined by
+a crude line breaking algorithm. You may use the carriage return character
+(C<\r>) to force a line break. You can change which characters are legal
+to break on by changing the variable C<$:> (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!)
+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.
+
+
+=head2 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.
+
+
+=head2 Repeating Format Lines
-Top-of-form processing is by default handled by a format with the
+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! (C<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.
+
+
+=head2 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 L<perlfunc/write>.
set on a per-filehandle basis, so you'll need to select() into a different
one to affect them:
- select((select(OUTF),
+ select((select(OUTF),
$~ = "My_Other_Format",
$^ = "My_Top_Format"
)[0]);
If you use the English module, you can even read the variable names:
- use English;
+ use English '-no_match_vars';
$ofh = select(OUTF);
$FORMAT_NAME = "My_Other_Format";
$FORMAT_TOP_NAME = "My_Top_Format";
=head1 NOTES
-Because the values line may contain arbitrary expressions (for at fields,
+Because the values line may contain arbitrary expressions (for at fields,
not caret fields), you can farm out more sophisticated processing
to other functions, like sprintf() or one of your own. For example:
- format Ident =
+ format Ident =
@<<<<<<<<<<<<<<<
&commify($n)
.
To get a real at or caret into the field, do this:
- format Ident =
+ format Ident =
I have an @ here.
"@"
.
To center a whole line of text, do something like this:
- format Ident =
+ format Ident =
@|||||||||||||||||||||||||||||||||||||||||||||||
"Some text line"
.
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";
+ $format = "format STDOUT = \n"
+ . '^' . '<' x $cols . "\n"
+ . '$entry' . "\n"
+ . "\t^" . "<" x ($cols-8) . "~~\n"
+ . '$entry' . "\n"
. ".\n";
print $format if $Debugging;
- eval $format;
+ eval $format;
die $@ if $@;
Which would generate a format looking something like this:
- format STDOUT =
+ format STDOUT =
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$entry
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
Here's a little program that's somewhat like fmt(1):
- format =
+ format =
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
$_
while (<>) {
s/\s*\n\s*/ /g;
write;
- }
+ }
=head2 Footers
by checking $FORMAT_LINES_LEFT before each write() and print the footer
yourself if necessary.
-Here's another strategy; open a pipe to yourself, using C<open(MESELF, "|-")>
-(see L<perlfunc/open()>) and always write() to MESELF instead of
-STDOUT. Have your child process massage its STDIN to rearrange
-headers and footers however you like. Not very convenient, but doable.
+Here's another strategy: Open a pipe to yourself, using C<open(MYSELF, "|-")>
+(see L<perlfunc/open()>) and always write() to MYSELF instead of STDOUT.
+Have your child process massage its STDIN to rearrange headers and footers
+however you like. Not very convenient, but doable.
=head2 Accessing Formatting Internals
print "Wow, I just stored `$^A' in the accumulator!\n";
-Or to make an swrite() subroutine which is to write() what sprintf()
+Or to make an swrite() subroutine, which is to write() what sprintf()
is to printf(), do this:
use Carp;
$^A = "";
formline($format,@_);
return $^A;
- }
+ }
$string = swrite(<<'END', 1, 2, 3);
Check me out
=head1 WARNINGS
-The lone dot that ends a format can also prematurely end an email
+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 email, you should indent it so that
+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
-email cutoff.
+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 which unconditionally use information
+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
block structure of the program, and, for historical reasons, formats
exist outside that block structure. See L<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 C<\r> when using fill mode.) This is done to avoid
+misalignment when control characters "disappear" on some output media.
+
https://perl5.git.perl.org / perl5.git / blobdiff