perlform


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

       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
             ~~    ^*

       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 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 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 intermediary
       stage in the expression to single-step the debugger through):
           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:


        .

        $/ = '';
        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;

       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.  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
       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 v5.10.1                      2009-02-26                       PERLFORM(1)
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