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 state-
ment. Think of it as a poor man's nroff(1).
Formats, like packages and subroutines, are declared rather than exe-
cuted, 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 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, respec-
tively, 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 "#" char-
acters (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 leading zeroes if neces-
sary. 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 con-
text 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 spec-
ifies 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 cur-
rent 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 vari-
able 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.
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 $| ($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:
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 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 process-
ing to other functions, like sprintf() or one of your own. For exam-
ple:
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 form-
line() 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 experi-
ence, such misconfiguration is the rule, not the exception). So when
sending format code through mail, you should indent it so that the for-
mat-ending dot is not on the left margin; this will prevent SMTP cut-
off.
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 char-
acter 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 2002-06-08 PERLFORM(1)
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