347 lines
13 KiB
Plaintext
347 lines
13 KiB
Plaintext
=head1 NAME
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perlform - Perl formats
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=head1 DESCRIPTION
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Perl has a mechanism to help you generate simple reports and charts. To
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facilitate this, Perl helps you code up your output page close to how it
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will look when it's printed. It can keep track of things like how many
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lines are on a page, what page you're on, when to print page headers,
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etc. Keywords are borrowed from FORTRAN: format() to declare and write()
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to execute; see their entries in L<perlfunc>. Fortunately, the layout is
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much more legible, more like BASIC's PRINT USING statement. Think of it
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as a poor man's nroff(1).
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Formats, like packages and subroutines, are declared rather than
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executed, so they may occur at any point in your program. (Usually it's
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best to keep them all together though.) They have their own namespace
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apart from all the other "types" in Perl. This means that if you have a
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function named "Foo", it is not the same thing as having a format named
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"Foo". However, the default name for the format associated with a given
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filehandle is the same as the name of the filehandle. Thus, the default
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format for STDOUT is named "STDOUT", and the default format for filehandle
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TEMP is named "TEMP". They just look the same. They aren't.
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Output record formats are declared as follows:
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format NAME =
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FORMLIST
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.
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If name is omitted, format "STDOUT" is defined. FORMLIST consists of
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a sequence of lines, each of which may be one of three types:
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=over 4
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=item 1.
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A comment, indicated by putting a '#' in the first column.
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=item 2.
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A "picture" line giving the format for one output line.
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=item 3.
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An argument line supplying values to plug into the previous picture line.
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=back
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Picture lines are printed exactly as they look, except for certain fields
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that substitute values into the line. Each field in a picture line starts
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with either "@" (at) or "^" (caret). These lines do not undergo any kind
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of variable interpolation. The at field (not to be confused with the array
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marker @) is the normal kind of field; the other kind, caret fields, are used
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to do rudimentary multi-line text block filling. The length of the field
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is supplied by padding out the field with multiple "E<lt>", "E<gt>", or "|"
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characters to specify, respectively, left justification, right
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justification, or centering. If the variable would exceed the width
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specified, it is truncated.
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As an alternate form of right justification, you may also use "#"
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characters (with an optional ".") to specify a numeric field. This way
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you can line up the decimal points. If any value supplied for these
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fields contains a newline, only the text up to the newline is printed.
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Finally, the special field "@*" can be used for printing multi-line,
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nontruncated values; it should appear by itself on a line.
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The values are specified on the following line in the same order as
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the picture fields. The expressions providing the values should be
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separated by commas. The expressions are all evaluated in a list context
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before the line is processed, so a single list expression could produce
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multiple list elements. The expressions may be spread out to more than
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one line if enclosed in braces. If so, the opening brace must be the first
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token on the first line. If an expression evaluates to a number with a
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decimal part, and if the corresponding picture specifies that the decimal
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part should appear in the output (that is, any picture except multiple "#"
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characters B<without> an embedded "."), the character used for the decimal
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point is B<always> determined by the current LC_NUMERIC locale. This
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means that, if, for example, the run-time environment happens to specify a
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German locale, "," will be used instead of the default ".". See
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L<perllocale> and L<"WARNINGS"> for more information.
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Picture fields that begin with ^ rather than @ are treated specially.
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With a # field, the field is blanked out if the value is undefined. For
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other field types, the caret enables a kind of fill mode. Instead of an
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arbitrary expression, the value supplied must be a scalar variable name
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that contains a text string. Perl puts as much text as it can into the
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field, and then chops off the front of the string so that the next time
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the variable is referenced, more of the text can be printed. (Yes, this
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means that the variable itself is altered during execution of the write()
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call, and is not returned.) Normally you would use a sequence of fields
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in a vertical stack to print out a block of text. You might wish to end
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the final field with the text "...", which will appear in the output if
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the text was too long to appear in its entirety. You can change which
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characters are legal to break on by changing the variable C<$:> (that's
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$FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a
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list of the desired characters.
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Using caret fields can produce variable length records. If the text
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to be formatted is short, you can suppress blank lines by putting a
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"~" (tilde) character anywhere in the line. The tilde will be translated
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to a space upon output. If you put a second tilde contiguous to the
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first, the line will be repeated until all the fields on the line are
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exhausted. (If you use a field of the at variety, the expression you
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supply had better not give the same value every time forever!)
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Top-of-form processing is by default handled by a format with the
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same name as the current filehandle with "_TOP" concatenated to it.
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It's triggered at the top of each page. See L<perlfunc/write>.
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Examples:
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# a report on the /etc/passwd file
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format STDOUT_TOP =
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Passwd File
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Name Login Office Uid Gid Home
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------------------------------------------------------------------
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.
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format STDOUT =
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@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
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$name, $login, $office,$uid,$gid, $home
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.
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# a report from a bug report form
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format STDOUT_TOP =
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Bug Reports
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@<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>>
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$system, $%, $date
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------------------------------------------------------------------
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.
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format STDOUT =
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Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$subject
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Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$index, $description
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Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$priority, $date, $description
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From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$from, $description
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Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$programmer, $description
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~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$description
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~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$description
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~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$description
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~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$description
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~ ^<<<<<<<<<<<<<<<<<<<<<<<...
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$description
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.
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It is possible to intermix print()s with write()s on the same output
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channel, but you'll have to handle C<$-> (C<$FORMAT_LINES_LEFT>)
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yourself.
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=head2 Format Variables
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The current format name is stored in the variable C<$~> (C<$FORMAT_NAME>),
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and the current top of form format name is in C<$^> (C<$FORMAT_TOP_NAME>).
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The current output page number is stored in C<$%> (C<$FORMAT_PAGE_NUMBER>),
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and the number of lines on the page is in C<$=> (C<$FORMAT_LINES_PER_PAGE>).
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Whether to autoflush output on this handle is stored in C<$|>
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(C<$OUTPUT_AUTOFLUSH>). The string output before each top of page (except
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the first) is stored in C<$^L> (C<$FORMAT_FORMFEED>). These variables are
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set on a per-filehandle basis, so you'll need to select() into a different
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one to affect them:
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select((select(OUTF),
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$~ = "My_Other_Format",
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$^ = "My_Top_Format"
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)[0]);
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Pretty ugly, eh? It's a common idiom though, so don't be too surprised
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when you see it. You can at least use a temporary variable to hold
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the previous filehandle: (this is a much better approach in general,
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because not only does legibility improve, you now have intermediary
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stage in the expression to single-step the debugger through):
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$ofh = select(OUTF);
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$~ = "My_Other_Format";
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$^ = "My_Top_Format";
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select($ofh);
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If you use the English module, you can even read the variable names:
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use English;
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$ofh = select(OUTF);
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$FORMAT_NAME = "My_Other_Format";
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$FORMAT_TOP_NAME = "My_Top_Format";
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select($ofh);
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But you still have those funny select()s. So just use the FileHandle
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module. Now, you can access these special variables using lowercase
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method names instead:
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use FileHandle;
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format_name OUTF "My_Other_Format";
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format_top_name OUTF "My_Top_Format";
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Much better!
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=head1 NOTES
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Because the values line may contain arbitrary expressions (for at fields,
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not caret fields), you can farm out more sophisticated processing
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to other functions, like sprintf() or one of your own. For example:
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format Ident =
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@<<<<<<<<<<<<<<<
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&commify($n)
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.
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To get a real at or caret into the field, do this:
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format Ident =
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I have an @ here.
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"@"
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.
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To center a whole line of text, do something like this:
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format Ident =
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@|||||||||||||||||||||||||||||||||||||||||||||||
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"Some text line"
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.
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There is no builtin way to say "float this to the right hand side
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of the page, however wide it is." You have to specify where it goes.
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The truly desperate can generate their own format on the fly, based
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on the current number of columns, and then eval() it:
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$format = "format STDOUT = \n"
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. '^' . '<' x $cols . "\n"
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. '$entry' . "\n"
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. "\t^" . "<" x ($cols-8) . "~~\n"
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. '$entry' . "\n"
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. ".\n";
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print $format if $Debugging;
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eval $format;
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die $@ if $@;
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Which would generate a format looking something like this:
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format STDOUT =
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^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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$entry
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^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
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$entry
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.
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Here's a little program that's somewhat like fmt(1):
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format =
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^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
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$_
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.
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$/ = '';
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while (<>) {
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s/\s*\n\s*/ /g;
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write;
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}
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=head2 Footers
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While $FORMAT_TOP_NAME contains the name of the current header format,
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there is no corresponding mechanism to automatically do the same thing
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for a footer. Not knowing how big a format is going to be until you
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evaluate it is one of the major problems. It's on the TODO list.
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Here's one strategy: If you have a fixed-size footer, you can get footers
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by checking $FORMAT_LINES_LEFT before each write() and print the footer
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yourself if necessary.
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Here's another strategy: Open a pipe to yourself, using C<open(MYSELF, "|-")>
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(see L<perlfunc/open()>) and always write() to MYSELF instead of STDOUT.
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Have your child process massage its STDIN to rearrange headers and footers
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however you like. Not very convenient, but doable.
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=head2 Accessing Formatting Internals
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For low-level access to the formatting mechanism. you may use formline()
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and access C<$^A> (the $ACCUMULATOR variable) directly.
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For example:
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$str = formline <<'END', 1,2,3;
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@<<< @||| @>>>
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END
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print "Wow, I just stored `$^A' in the accumulator!\n";
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Or to make an swrite() subroutine, which is to write() what sprintf()
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is to printf(), do this:
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use Carp;
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sub swrite {
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croak "usage: swrite PICTURE ARGS" unless @_;
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my $format = shift;
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$^A = "";
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formline($format,@_);
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return $^A;
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}
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$string = swrite(<<'END', 1, 2, 3);
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Check me out
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@<<< @||| @>>>
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END
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print $string;
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=head1 WARNINGS
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The lone dot that ends a format can also prematurely end a mail
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message passing through a misconfigured Internet mailer (and based on
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experience, such misconfiguration is the rule, not the exception). So
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when sending format code through mail, you should indent it so that
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the format-ending dot is not on the left margin; this will prevent
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SMTP cutoff.
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Lexical variables (declared with "my") are not visible within a
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format unless the format is declared within the scope of the lexical
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variable. (They weren't visible at all before version 5.001.)
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Formats are the only part of Perl that unconditionally use information
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from a program's locale; if a program's environment specifies an
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LC_NUMERIC locale, it is always used to specify the decimal point
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character in formatted output. Perl ignores all other aspects of locale
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handling unless the C<use locale> pragma is in effect. Formatted output
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cannot be controlled by C<use locale> because the pragma is tied to the
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block structure of the program, and, for historical reasons, formats
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exist outside that block structure. See L<perllocale> for further
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discussion of locale handling.
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Inside of an expression, the whitespace characters \n, \t and \f are
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considered to be equivalent to a single space. Thus, you could think
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of this filter being applied to each value in the format:
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$value =~ tr/\n\t\f/ /;
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The remaining whitespace character, \r, forces the printing of a new
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line if allowed by the picture line.
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