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NAME    [Toc]    [Back]

       perlform - Perl formats

DESCRIPTION    [Toc]    [Back]

       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: for-
       mat() 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 =

       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

       2.  A "picture" line giving the format for one output

       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 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
          @*   variable width field for a multi-line value
          ^*   variable width field for next line of a multi-line
          ~    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    [Toc]    [Back]

       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 multiline
 text block filling; see "Using Fill Mode" for

             format STDOUT =
             @<<<<<<   @||||||   @>>>>>>
             "left",   "middle", "right"
             left      middle    right

       Numeric Fields    [Toc]    [Back]

       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.
             format STDOUT =
             @###   @.###   @##.###  @###   @###   ^####
              42,   3.1415,  undef,    0, 10000,   undef
               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 "0) 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.

             $text = "line 10ine 20ine 3";
             format STDOUT =
             Text: ^*
             ~~    ^*
             Text: line 1
                   line 2
                   line 3

       Specifying Values    [Toc]    [Back]

       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 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

       Using Fill Mode    [Toc]    [Back]

       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")ltorforce aolineybreak.hYouacanage
       return character ("
       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

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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.

        # 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 =
        Index:                      @<<<<<<<<<<<<<<<<<<<<<<<<<<<<
               $index,                       $description
        Priority:        @<<<<<<<<<<        Date:        @<<<<<<<
                  $priority,        $date,   $description
        From:                      @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
              $from,                         $description
        Assigned           to:            @<<<<<<<<<<<<<<<<<<<<<<
                     $programmer,            $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)

       Format Variables    [Toc]    [Back]

       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

                   $~ = "My_Other_Format",
                   $^ = "My_Top_Format"

       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):

           $ofh = select(OUTF);
           $~ = "My_Other_Format";
           $^ = "My_Top_Format";

       If you use the English module, you can even read the variable

           use English '-no_match_vars';
           $ofh = select(OUTF);
           $FORMAT_NAME     = "My_Other_Format";
           $FORMAT_TOP_NAME = "My_Top_Format";

       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    [Toc]    [Back]

       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 =
       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 = 0
                    . '^' . '<' x $cols . "0
                    . '$entry' . "0
                    . "^" . "<" x ($cols-8) . "~~0
                    . '$entry' . "0
                    . ".0;
           print $format if $Debugging;
           eval $format;
           die $@ if $@;

       Which would generate a format looking something like this:

        format STDOUT =

       Here's a little program that's somewhat like fmt(1):

        format =
        ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~


        $/ = '';
        while (<>) {
            s/0s*/ /g;

       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    [Toc]    [Back]

       For low-level access to the formatting mechanism.  you may
       use formline() and access $^A (the $ACCUMULATOR variable)

       For example:

           $str = formline <<'END', 1,2,3;
           @<<<  @|||  @>>>

           print "Wow, I just stored `$^A' in the accumulator!0;

       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 = "";
               return $^A;

           $string = swrite(<<'END', 1, 2, 3);
        Check me out
        @<<<  @|||  @>>>
           print $string;

WARNINGS    [Toc]    [Back]

       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

       Lexical variables (declared with "my") are not visible
       within a format unless the format is declared within the
       scope of the lexical variable.  (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.  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 substit"twheny a
       space. (But remember the special meaning of "
       using fill mode.) This is done to avoid misalignment when
       control characters "disappear" on some output media.

perl v5.8.5                 2002-11-06                          9
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