PERLFORM(1) Perl Programmers Reference Guide PERLFORM(1)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. Key-
words are borrowed from FORTRAN: format() to declare and
write() to execute; see their entries in perlfunc. For-
tunately, 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 pro-
gram. (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 asso-
ciated 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 pre-
vious 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:
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@ 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 vari-
ous 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 posi-
tion 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.
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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, nontrun-
cated values; it should (but need not) appear by itself on a
line. A final line feed is chomped off, but all other char-
acters are emitted verbatim.
The Field ^* for Variable Width One-line-at-a-time Text
Like "@*", this is a variable width field. The value sup-
plied 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 ele-
ments. 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
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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.
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 vari-
able is referenced, more of the text can be printed. (Yes,
this means that the variable itself is altered during execu-
tion of the write() call, and is not restored.) The next
portion of text is determined by a crude line breaking algo-
rithm. 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 for-
ever! ("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.
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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.
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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 tem-
porary variable to hold the previous filehandle: (this is a
much better approach in general, because not only does legi-
bility 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 vari-
able names:
use English '-no_match_vars';
$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 vari-
ables 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
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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 =
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
$_
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.
$/ = '';
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 $^A;
}
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$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. (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 environ-
ment 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 rea-
sons, formats exist outside that block structure. See perl-
locale 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.
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