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
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,
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
$^L ($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 '-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 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($n)
.
To get a real at or caret into the field, do this:
format Ident =
I have an @ here.
"@"
.
To center a whole line of text, do something like this:
format Ident =
. '$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:
@<<< @||| @>>>
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.)
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 v5.16.3 2013-03-04 PERLFORM(1)
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