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, the current page number, when to
print page headers, and so on. Keywords are borrowed from FORTRAN:
format
to declare and write
to
execute; see the relevant entries in Chapter 29. Fortunately, the layout is
much more legible, more like the PRINT USING
statement of BASIC. Think of it as a poor man's
nroff (1). (If you know
nroff, that may not sound like a
recommendation.)
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.) 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 a format named "Foo
". However, the default name
for the format associated with a given filehandle is the same as the
name of that 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:
formatNAME
=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:
A comment, indicated by putting a #
in the
first column.
A "picture" line giving the format for one output line.
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.[1] Each substitution field in a picture line starts with
either @
(at) or ^
(caret). These
lines do not undergo any kind of variable interpolation. The
@
field (not to be confused with the array marker
@
) is the normal kind of field; the other kind, the ^
field, is used to do rudimentary multiline 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
exceeds the width specified, it is truncated.
As an alternate form of right justification, you may also
use #
characters (after an initial
@
or ^
) to specify a numeric
field. You can insert a . in place of one of the #
characters to 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 multiline, nontruncated values; it should generally appear
on a picture line by itself.
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). This lets you line up the values under their respective format fields for easier reading.
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, a comma
will be used instead of a period. See the
perllocale manpage for more information.
Inside an expression, the whitespace characters
,
, and f
are all considered equivalent to a single space. Thus, you could think
of this filter as being applied to each value in the format:
$value =~ tr/ f/ /;
The remaining whitespace character,
, forces
the printing of a new line if the picture line allows it.
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 preserved. Use
a scratch variable if you want to preserve the original value.) Normally
you would use a sequence of fields lined up vertically 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 (or after) 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 ^
fields can produce
variable-length records. If the text to be formatted is short, just
repeat the format line with the ^
field in it a few
times. If you just do this for short data you'd end up getting a few
blank lines. To suppress lines that would end up blank, put a
~
(tilde) character anywhere in the line. (The tilde
itself will be translated to a space upon output.) If you put a second
tilde next to the first, the line will be repeated until all the text in
the fields on that line are exhausted. (This works because the
^
fields chew up the strings they print. But if you
use a field of the @
variety in conjunction with two
tildes, the expression you supply had better not give the same value
every time forever! Use a shift
, or some other
operator with a side effect that exhausts the set of values.)
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 Chapter 29.
Here are some 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 .
Lexical variables are not visible within a format unless the format is declared within the scope of the lexical variable.
It is possible to intermix print
s with
write
s on the same output channel, but you'll have to
handle the $-
special variable
($FORMAT_LINES_LEFT
if you're using the
English
module) yourself.
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 flush the
output buffer on this handle automatically is stored in
$|
($OUTPUT_AUTOFLUSH
). The
string to be 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
the
filehandle associated with a format in order to affect its format
variables:
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:
$ofh = select(OUTF); $~ = "My_Other_Format"; $^ = "My_Top_Format"; select($ofh);
This is a much better approach in general because not
only does legibility improve, but you now have an intermediary
statement in the code to stop on when you're single-stepping in the
debugger. 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 calls to
select
. If you want to avoid them, use the
FileHandle
module bundled with Perl. Now you can
access these special variables using lowercase method names
instead:
use FileHandle; OUTF->format_name("My_Other_Format"); OUTF->format_top_name("My_Top_Format");
Much better!
Since the values line following your picture line may
contain arbitrary expressions (for @
fields, not
^
fields), you can farm out more sophisticated
processing to other functions, like sprintf
or one
of your own. For example, to insert commas into a number:
format Ident = @<<<<<<<<<<<<<<< commify($n) .
To get a real @
, ~
, or
^
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" .
The >
field-length indicator ensures that
the text will be right-justified within the field, but the field as a
whole occurs exactly where you show it occurring. There is no built-in
way to say "float this field to the right-hand side of the page,
however wide it is." You have to specify where it goes relative to the
left margin. The truly desperate can generate their own format on the
fly, based on the current number of columns (not supplied), and then
eval
it:
$format = "format STDOUT = " . '^' . '<' x $cols . " " . '$entry' . " " . " ^" . "<" x ($cols-8) . "~~ " . '$entry' . " " . ". "; print $format if $Debugging; eval $format; die $@ if $@;
The most important line there is probably the
print
. What the print
would
print out looks something like this:
format STDOUT = ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< $entry ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~ $entry .
Here's a little program that behaves like the fmt (1) Unix utility:
format = ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ $_ . $/ = ""; while (<>) { s/s* s*/ /g; write; }
[1] Even those fields maintain the integrity of the columns you put them in, however. There is nothing in a picture line that can cause fields to grow or shrink or shift back and forth. The columns you see are sacred in a WYSIWYG sense--assuming you're using a fixed-width font. Even control characters are assumed to have a width of one.