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:
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:
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 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_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:
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):
If you use the English module, you can even read the variable names:
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:
Much better!
format Ident =
@<<<<<<<<<<<<<<<
&commify($n)
.
To get a real at or caret into the field, do this:
To center a whole line of text, do something like this:
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:
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;
}
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 massage its STDIN to rearrange
headers and footers however you like. Not very convenient, but doable.
$^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:
$string = swrite(<<'END', 1, 2, 3);
Check me out
@<<< @||| @>>>
END
print $string;