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 its printed. It can keep track of things like
how many lines are on a page, what page youre 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 BASICs PRINT USING state
ment. Think of it as a poor mans nroff(1).
Formats, like packages and subroutines, are declared rather than exe
cuted, so they may occur at any point in your program. (Usually its
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
arent.
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 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 well call, respectively, a "regular" or "spe
cial" field. The choice of pad characters determines whether a field
is textual or numeric. The tilde operators are not part of a field.
Lets 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 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 over
flow 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 produce multi
ple 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 deci
mal part should appear in the output (that is, any picture except mul
tiple "#" 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 carriage
return character ("\r") to force a line break. You can change which
characters are legal to break on by changing the variable $: (thats
$FORMAT_LINE_BREAK_CHARACTERS if youre using the English module) to a
list of the desired characters.
Normally you would use a sequence of fields in a vertical stack associ
ated 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.) Dont
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. Its
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
~ ^<<<<<<<<<<<<<<<<<<<<<<<...
$description
.
It is possible to intermix print()s with write()s on the same output
channel, but youll have to handle "$-" ($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 youll need to select() into a different one
to affect them:
select((select(OUTF),
$~ = "My_Other_Format",
$^ = "My_Top_Format"
)[0]);
Pretty ugly, eh? Its a common idiom though, so dont 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!
NOTES
Because the values line may contain arbitrary expressions (for at
fields, not caret fields), you can farm out more sophisticated process
ing to other functions, like sprintf() or one of your own. For exam
ple:
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
.
Heres a little program thats 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. Its on the TODO list.
Heres 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.
Heres 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
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
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 experi
ence, such misconfiguration is the rule, not the exception). So when
sending format code through mail, you should indent it so that the for
mat-ending dot is not on the left margin; this will prevent SMTP cut
off.
Lexical variables (declared with "my") are not visible within a format
unless the format is declared within the scope of the lexical variable.
(They werent visible at all before version 5.001.)
Formats are the only part of Perl that unconditionally use information
from a programs locale; if a programs environment specifies an
LC_NUMERIC locale, it is always used to specify the decimal point char
acter 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 discus
sion 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 2008-04-25 PERLFORM(1)
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