发信人: saka.bbs@bbs.neu.edu.cn (机器猫), 信区: cnlinux
标 题: perl(9)
发信站: 白山黑水BBS (Tue Apr 1 11:21:19 1997)
转信站: Lilac!ustcnews!ustcnews!sjtunews!neubbs
出 处: conger.neu.edu.cn
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NAME
perlform - Perl formats
---------------------------------------------------------------------------
DESCRIPTION
Perl has a mechanism to help you generate simple reports and charts. To
facilitate this, Perl helps you lay out your output page in your code in a
fashion that's 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, when to
print page headers, etc. Keywords are borrowed from FORTRAN: format() to
declare and write() to execute; see their entries in the perlfunc manpage .
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.
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 of 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, 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 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.
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 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 <perlfunc/ write() >.
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 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 lower-case method
names instead:
use FileHandle;
format_name OUTF "My_Other_Format";
format_top_name OUTF "My_Top_Format";
Much better!
---------------------------------------------------------------------------
NOTES
Since 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(MESELF,
``|-'') (see open ) and always write() to MESELF instead of STDOUT. Have
your child process postprocesses 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;
---------------------------------------------------------------------------
WARNING
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.) Furthermore, lexical
aliases will not be compiled correctly: see my for other issues.
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