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
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 justification
| pad character for centering
> pad character for right justification
# 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
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 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 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 "\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 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 determined by the current LC_NUMERIC
locale if "use locale" is in effect. 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 carriage
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 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
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 $|
($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 an 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 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
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(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
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;
WARNINGS
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.
If a program's environment specifies an LC_NUMERIC locale and "use
locale" is in effect when the format is declared, the locale is used to
specify the decimal point character in formatted output. Formatted
output cannot be controlled by "use locale" at the time when write() is
called. 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 v5.26.3 2018-03-23 PERLFORM(1)
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