perlform
PERLFORM(1) Perl Programmers Reference Guide 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 for-
mat 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 pic-
ture line.
Picture lines contain output field definitions, intermingled with lit-
eral text. These lines do not undergo any kind of variable interpola-
tion. Field definitions are made up from a set of characters, for
starting and extending a field to its desired width. This is the com-
plete 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
The length of the field is supplied by padding out the field with mul-
tiple "<", ">", 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 multi-line text block filling; see "Using Fill Mode"
for details.
Example:
format STDOUT =
@<<<<<< @││││││ @>>>>>>
"left", "middle", "right"
.
Output:
left middle right
Numeric Fields
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 num-
ber 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 val-
ues; 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 "\n") 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 1\nline 2\nline 3";
format STDOUT =
Text: ^*
$text
~~ ^*
$text
.
Output:
Text: line 1
line 2
line 3
Specifying Values
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 pro-
duce 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
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 algorithm. You may use the car-
riage return character ("\r") to force a line break. 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 mod-
ule) to a list of the desired characters.
Normally you would use a sequence of fields in a vertical stack asso-
ciated 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
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
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
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 sur-
prised 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 inter-
mediary 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 pro-
cessing 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(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 mes-
sage 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 vari-
able. (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. Format-
ted output cannot be controlled by "use locale" because the pragma is
tied to the block structure of the program, and, for historical rea-
sons, 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 substituted by a space. (But remember the
special meaning of "\r" when using fill mode.) This is done to avoid
misalignment when control characters "disappear" on some output media.
perl v5.8.8 2006-01-07 PERLFORM(1)
