=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.17 $, $Date: 2002/05/23 19:33:50 $)
+perlfaq5 - Files and Formats ($Revision: 1.26 $, $Date: 2002/09/21 21:04:17 $)
=head1 DESCRIPTION
This assumes no funny games with newline translations.
+=head2 How can I use Perl's C<-i> option from within a program?
+
+C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
+the behavior of C<< <> >>; see L<perlrun> for more details. By
+modifying the appropriate variables directly, you can get the same
+behavior within a larger program. For example:
+
+ # ...
+ {
+ local($^I, @ARGV) = ('.orig', glob("*.c"));
+ while (<>) {
+ if ($. == 1) {
+ print "This line should appear at the top of each file\n";
+ }
+ s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
+ print;
+ close ARGV if eof; # Reset $.
+ }
+ }
+ # $^I and @ARGV return to their old values here
+
+This block modifies all the C<.c> files in the current directory,
+leaving a backup of the original data from each file in a new
+C<.c.orig> file.
+
=head2 How do I make a temporary file name?
Use the File::Temp module, see L<File::Temp> for more information.
=head2 How can I output my numbers with commas added?
-This one from Benjamin Goldberg will do it for you:
+This subroutine will add commas to your number:
+
+ sub commify {
+ local $_ = shift;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
+ return $_;
+ }
+
+This regex from Benjamin Goldberg will add commas to numbers:
s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
-or written verbosely:
+It is easier to see with comments:
s/(
^[-+]? # beginning of number.
=head2 How can I reliably rename a file?
-If your operating system supports a proper mv(1) utility or its functional
-equivalent, this works:
+If your operating system supports a proper mv(1) utility or its
+functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
=head2 How do I print to more than one file at once?
-If you only have to do this once, you can do this:
-
- for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
-
-To connect up to one filehandle to several output filehandles, it's
-easiest to use the tee(1) program if you have it, and let it take care
-of the multiplexing:
+To connect one filehandle to several output filehandles,
+you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
- open (FH, "| tee file1 file2 file3");
+If you only have to do this once, you can print individually
+to each filehandle.
-Or even:
+ for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
- # make STDOUT go to three files, plus original STDOUT
- open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
- print "whatever\n" or die "Writing: $!\n";
- close(STDOUT) or die "Closing: $!\n";
+=head2 How can I read in an entire file all at once?
-Otherwise you'll have to write your own multiplexing print
-function--or your own tee program--or use Tom Christiansen's,
-at http://www.cpan.org/authors/id/TOMC/scripts/tct.gz , which is
-written in Perl and offers much greater functionality
-than the stock version.
+You can use the File::Slurp module to do it in one step.
-=head2 How can I read in an entire file all at once?
+ use File::Slurp;
+
+ $all_of_it = read_file($filename); # entire file in scalar
+ @all_lines = read_file($filename); # one line perl element
The customary Perl approach for processing all the lines in a file is to
do so one line at a time:
This is elaborately and painstakingly described in the
F<file-dir-perms> article in the "Far More Than You Ever Wanted To
-Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz .
+Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in that file.
srand;
rand($.) < 1 && ($line = $_) while <>;
-This has a significant advantage in space over reading the whole
-file in. A simple proof by induction is available upon
-request if you doubt the algorithm's correctness.
+This has a significant advantage in space over reading the whole file
+in. You can find a proof of this method in I<The Art of Computer
+Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
+
+You can use the File::Random module which provides a function
+for that algorithm:
+
+ use File::Random qw/random_line/;
+ my $line = random_line($filename);
+
+Another way is to use the Tie::File module, which treats the entire
+file as an array. Simply access a random array element.
=head2 Why do I get weird spaces when I print an array of lines?
projects / p5sagit/p5-mst-13.2.git / blobdiff