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PERLFORM(1)            Perl Programmers Reference Guide            PERLFORM(1)

       perlform - Perl formats

       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 =

       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 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, respec-
       tively, 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

       The length of the field is supplied by padding out the field with multiple "<", ">", or "|" characters to spec-
       ify a non-numeric field with, respectively, left justification, right justification, or centering.  For a regu-
       lar 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.

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

             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 "\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.

             $text = "line 1\nline 2\nline 3";
             format STDOUT =
             Text: ^*
             ~~    ^*
             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 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.


        # 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: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        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) 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 dif-
       ferent one to affect them:

                   $~ = "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

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

       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";

       But you still have those funny select()s.  So just use the FileHandle module.  Now, you can access these spe-
       cial variables using lowercase method names instead:

           use FileHandle;
           format_name     OUTF "My_Other_Format";
           format_top_name OUTF "My_Top_Format";

       Much better!

       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 = \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 =

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

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


        $/ = '';
        while (<>) {
            s/\s*\n\s*/ /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

       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;
           @<<<  @|||  @>>>

           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 = "";
               return $^A;

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

       The lone dot that ends a format can also prematurely end a mail message passing through a misconfigured Inter-
       net 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 cutoff.

       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 format-
       ted 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 v5.8.8                       2006-01-07                       PERLFORM(1)