perlform - Perl formats
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: for-
mat() 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 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 various possibilities in detail.
Text Fields [Toc] [Back]
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 multiline
text block filling; see "Using Fill Mode" for
details.
Example:
format STDOUT =
@<<<<<< @|||||| @>>>>>>
"left", "middle", "right"
.
Output:
left middle right
Numeric Fields [Toc] [Back]
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 "0) 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 10ine 20ine 3";
format STDOUT =
Text: ^*
$text
~~ ^*
$text
.
Output:
Text: line 1
line 2
line 3
Specifying Values [Toc] [Back]
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 always
determined by the current 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 [Toc] [Back]
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 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
restored.) The next portion of text is determined by a
crude line breaking")ltorforce aolineybreak.hYouacanage
return character ("
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 '-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!
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 =
@|||||||||||||||||||||||||||||||||||||||||||||||
"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 = 0
. '^' . '<' x $cols . "0
. '$entry' . "0
. "^" . "<" x ($cols-8) . "~~0
. '$entry' . "0
. ".0;
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/0s*/ /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 [Toc] [Back]
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!0;
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;
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 environment
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 reasons, formats exist outside that block
structure. 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 substit"twheny a
space. (But remember the special meaning of "
using fill mode.) This is done to avoid misalignment when
control characters "disappear" on some output media.
perl v5.8.5 2002-11-06 9 [ Back ] |
