perlform(1) - perldoc - phpman

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 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
       @*   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 "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 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
          ~~    ^*
                $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 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 determined by the current LC_NUMERIC
    locale if "use locale" is in effect. 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 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 $| ($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 an 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;
        $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 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:

     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 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;

WARNINGS
    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 cutoff.

    Lexical variables (declared with "my") are not visible within a format unless the format is
    declared within the scope of the lexical variable.

    If a program's environment specifies an LC_NUMERIC locale and "use locale" is in effect when the
    format is declared, the locale is used to specify the decimal point character in formatted
    output. Formatted output cannot be controlled by "use locale" at the time when write() is
    called. 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.