IRIX Base Documentation 2002 November/SGI IRIX Base Documentation 2002 November.iso/usr/share/catman/u_man/cat1/perlform.z/perlform

Text File | 2002-10-03 | 17.6 KB | 463 lines

PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



NNNNAAAAMMMMEEEE
     perlform - Perl formats

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     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: _f_o_r_m_a_t() to declare and _w_r_i_t_e()
     to execute; see their entries in the _p_e_r_l_f_u_n_c manpage.  Fortunately, the
     layout is much more legible, more like BASIC's PRINT USING statement.
     Think of it as a poor man's _n_r_o_f_f(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 name is omitted, format "STDOUT" is defined.  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 are printed exactly as they look, except for certain fields
     that substitute values into the line.  Each field in a picture line
     starts with either "@" (at) or "^" (caret).  These lines do not undergo
     any kind of variable interpolation.  The at field (not to be confused
     with the array marker @) is the normal kind of field; the other kind,
     caret fields, are used to do rudimentary multi-line text block filling.
     The length of the field is supplied by padding out the field with
     multiple "<", ">", or "|" characters to specify, respectively, left
     justification, right justification, or centering.  If the variable would
     exceed the width specified, it is truncated.






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PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



     As an alternate form of right justification, you may also use "#"
     characters (with an optional ".") to specify a numeric field.  This way
     you can line up the decimal points.  If any value supplied for these
     fields contains a newline, only the text up to the newline is printed.
     Finally, the special field "@*" can be used for printing multi-line,
     nontruncated values; it should appear by itself on a line.

     The values are specified on the following line in the same order as the
     picture fields.  The expressions providing the values should be separated
     by commas.  The expressions 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 wwwwiiiitttthhhhoooouuuutttt an embedded "."), the character used for the decimal
     point is aaaallllwwwwaaaayyyyssss 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 the
     _p_e_r_l_l_o_c_a_l_e manpage and the section on _W_A_R_N_I_N_G_S for more information.

     Picture fields that begin with ^ rather than @ are treated specially.
     With a # field, the field is blanked out if the value is undefined.  For
     other field types, the caret enables a kind of fill mode.  Instead of an
     arbitrary expression, the value supplied must be a scalar variable name
     that contains a text string.  Perl puts as much text as it can 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 _w_r_i_t_e()
     call, and is not returned.)  Normally you would use a sequence of fields
     in a vertical stack 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.  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.

     Using caret fields can produce variable length records.  If the text to
     be formatted is short, you can suppress blank lines by putting a "~"
     (tilde) character anywhere in the line.  The tilde will be translated to
     a space upon output.  If you put a second tilde contiguous to the first,
     the line will be repeated until all the fields on the line are exhausted.
     (If you use a field of the at variety, the expression you supply had
     better not give the same value every time forever!)

     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 the write entry in the _p_e_r_l_f_u_n_c
     manpage.





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PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



     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 _p_r_i_n_t()s with _w_r_i_t_e()s on the same output
     channel, but you'll have to handle $- ($FORMAT_LINES_LEFT) yourself.

     FFFFoooorrrrmmmmaaaatttt VVVVaaaarrrriiiiaaaabbbblllleeeessss

     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



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PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



     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 _s_e_l_e_c_t() 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;
         $ofh = select(OUTF);
         $FORMAT_NAME     = "My_Other_Format";
         $FORMAT_TOP_NAME = "My_Top_Format";
         select($ofh);

     But you still have those funny _s_e_l_e_c_t()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!

NNNNOOOOTTTTEEEESSSS
     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 _s_p_r_i_n_t_f() or one of your own.  For example:

         format Ident =
             @<<<<<<<<<<<<<<<
             &commify($n)
         .

     To get a real at or caret into the field, do this:





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PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



         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 _e_v_a_l() 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 _f_m_t(1):

      format =
      ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
      $_

      .

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






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PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



     FFFFooooooootttteeeerrrrssss

     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 _w_r_i_t_e() and print the
     footer yourself if necessary.

     Here's another strategy: Open a pipe to yourself, using open(MYSELF, "|-
     ") (see the open() entry in the _p_e_r_l_f_u_n_c manpage) and always _w_r_i_t_e() 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.

     AAAAcccccccceeeessssssssiiiinnnngggg FFFFoooorrrrmmmmaaaattttttttiiiinnnngggg IIIInnnntttteeeerrrrnnnnaaaallllssss

     For low-level access to the formatting mechanism.  you may use _f_o_r_m_l_i_n_e()
     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 _s_w_r_i_t_e() subroutine, which is to _w_r_i_t_e() what _s_p_r_i_n_t_f() is
     to _p_r_i_n_t_f(), 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;








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PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))                                                        PPPPEEEERRRRLLLLFFFFOOOORRRRMMMM((((1111))))



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.
     (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 the _p_e_r_l_l_o_c_a_l_e manpage for
     further discussion of locale handling.

     Inside of an expression, the whitespace characters \n, \t and \f are
     considered to be equivalent to a single space.  Thus, you could think of
     this filter being applied to each value in the format:

      $value =~ tr/\n\t\f/ /;

     The remaining whitespace character, \r, forces the printing of a new line
     if allowed by the picture line.


























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