perlform



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 state-
       ment.  Think of it as a poor man’s nroff(1).

       Formats, like packages and subroutines, are declared rather than exe-
       cuted, 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 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 we’ll 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.
       Let’s 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 $: (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 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.)  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 $│ ($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 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):

           $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
        .

       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 form-
       line() 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 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 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 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 v5.8.6                       2004-11-05                       PERLFORM(1)

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