=head1 NAME
-perlfaq1 - General Questions About Perl ($Revision: 7997 $)
+perlfaq1 - General Questions About Perl ($Revision: 8539 $)
=head1 DESCRIPTION
Ponie stands for "Perl On the New Internal Engine", started by Arthur
Bergman from Fotango in 2003, and subsequently run as a project of The
Perl Foundation. It was abandoned in 2006
-L<http://www.nntp.perl.org/group/perl.ponie.dev/487> .
+(http://www.nntp.perl.org/group/perl.ponie.dev/487).
Instead of using the current Perl internals, Ponie aimed to create a
new one that would provide a translation path from Perl 5 to Perl 6
of programming experience, an understanding of regular expressions, and
the ability to understand other people's code. If there's something you
need to do, then it's probably already been done, and a working example is
-usually available for free. Don't forget the new perl modules, either.
+usually available for free. Don't forget Perl modules, either.
They're discussed in Part 3 of this FAQ, along with CPAN, which is
discussed in Part 2.
=head1 REVISION
-Revision: $Revision: 7997 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-11-01 09:29:17 +0100 (mer, 01 nov 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq2 - Obtaining and Learning about Perl ($Revision: 7996 $)
+perlfaq2 - Obtaining and Learning about Perl ($Revision: 8539 $)
=head1 DESCRIPTION
first. Consult the Usenet FAQs for your operating system for
information on where to get such a binary version.
+You might look around the net for a pre-built binary of Perl (or a
+C compiler!) that meets your needs, though:
+
+For Windows, Vanilla Perl (http://vanillaperl.com/) comes with a
+bundled C compiler. ActivePerl is a pre-compiled version of Perl
+ready-to-use.
+
+For Sun systems, SunFreeware.com provides binaries of most popular
+applications, including compilers and Perl.
+
=head2 I copied the perl binary from one machine to another, but scripts don't work.
That's probably because you forgot libraries, or library paths differ.
=head1 REVISION
-Revision: $Revision: 7996 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-11-01 09:24:38 +0100 (mer, 01 nov 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq3 - Programming Tools ($Revision: 7875 $)
+perlfaq3 - Programming Tools ($Revision: 8539 $)
=head1 DESCRIPTION
=head1 REVISION
-Revision: $Revision: 7875 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-10-04 22:39:26 +0200 (mer, 04 oct 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 7996 $)
+perlfaq4 - Data Manipulation ($Revision: 8539 $)
=head1 DESCRIPTION
problem with how computers store numbers and affects all computer
languages, not just Perl.
-L<perlnumber> show the gory details of number representations and
+L<perlnumber> shows the gory details of number representations and
conversions.
To limit the number of decimal places in your numbers, you can use the
=head2 How do I get a random number between X and Y?
-To get a random number between two values, you can use the
-C<rand()> builtin to get a random number between 0 and
+To get a random number between two values, you can use the C<rand()>
+builtin to get a random number between 0 and 1. From there, you shift
+that into the range that you want.
-C<rand($x)> returns a number such that
-C<< 0 <= rand($x) < $x >>. Thus what you want to have perl
-figure out is a random number in the range from 0 to the
-difference between your I<X> and I<Y>.
+C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus
+what you want to have perl figure out is a random number in the range
+from 0 to the difference between your I<X> and I<Y>.
-That is, to get a number between 10 and 15, inclusive, you
-want a random number between 0 and 5 that you can then add
-to 10.
+That is, to get a number between 10 and 15, inclusive, you want a
+random number between 0 and 5 that you can then add to 10.
my $number = 10 + int rand( 15-10+1 );
print "Yesterday was $yesterday\n";
-You can also use the C<Date::Calc> module using its Today_and_Now
+You can also use the C<Date::Calc> module using its C<Today_and_Now>
function.
use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
- print "@date\n";
+ print "@date_time\n";
Most people try to use the time rather than the calendar to figure out
dates, but that assumes that days are twenty-four hours each. For
=head2 How do I process an entire hash?
-Use the each() function (see L<perlfunc/each>) if you don't care
-whether it's sorted:
+(contributed by brian d foy)
+
+There are a couple of ways that you can process an entire hash. You
+can get a list of keys, then go through each key, or grab a one
+key-value pair at a time.
- while ( ($key, $value) = each %hash) {
- print "$key = $value\n";
+To go through all of the keys, use the C<keys> function. This extracts
+all of the keys of the hash and gives them back to you as a list. You
+can then get the value through the particular key you're processing:
+
+ foreach my $key ( keys %hash ) {
+ my $value = $hash{$key}
+ ...
}
-If you want it sorted, you'll have to use foreach() on the result of
-sorting the keys as shown in an earlier question.
+Once you have the list of keys, you can process that list before you
+process the hashh elements. For instance, you can sort the keys so you
+can process them in lexical order:
+
+ foreach my $key ( sort keys %hash ) {
+ my $value = $hash{$key}
+ ...
+ }
+
+Or, you might want to only process some of the items. If you only want
+to deal with the keys that start with C<text:>, you can select just
+those using C<grep>:
+
+ foreach my $key ( grep /^text:/, keys %hash ) {
+ my $value = $hash{$key}
+ ...
+ }
+
+If the hash is very large, you might not want to create a long list of
+keys. To save some memory, you can grab on key-value pair at a time using
+C<each()>, which returns a pair you haven't seen yet:
+
+ while( my( $key, $value ) = each( %hash ) ) {
+ ...
+ }
+
+The C<each> operator returns the pairs in apparently random order, so if
+ordering matters to you, you'll have to stick with the C<keys> method.
+
+The C<each()> operator can be a bit tricky though. You can't add or
+delete keys of the hash while you're using it without possibly
+skipping or re-processing some pairs after Perl internally rehashes
+all of the elements. Additionally, a hash has only one iterator, so if
+you use C<keys>, C<values>, or C<each> on the same hash, you can reset
+the iterator and mess up your processing. See the C<each> entry in
+L<perlfunc> for more details.
=head2 What happens if I add or remove keys from a hash while iterating over it?
=head1 REVISION
-Revision: $Revision: 7996 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-11-01 09:24:38 +0100 (mer, 01 nov 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 8075 $)
+perlfaq5 - Files and Formats ($Revision: 8579 $)
=head1 DESCRIPTION
close $out;
To change only a particular line, the input line number, C<$.>, is
-useful. Use C<next> to skip all lines up to line 5, make a change and
-print the result, then stop further processing with C<last>.
+useful. First read and print the lines up to the one you want to
+change. Next, read the single line you want to change, change it, and
+print it. After that, read the rest of the lines and print those:
- while( <$in> )
+ while( <$in> ) # print the lines before the change
{
- next unless $. == 5;
- s/\b(perl)\b/Perl/g;
print $out $_;
- last;
+ last if $. == 4; # line number before change
}
+ my $line = <$in>;
+ $line =~ s/\b(perl)\b/Perl/g;
+ print $out $line;
+
+ while( <$in> ) # print the rest of the lines
+ {
+ print $out $_;
+ }
+
To skip lines, use the looping controls. The C<next> in this example
skips comment lines, and the C<last> stops all processing once it
encounters either C<__END__> or C<__DATA__>.
Error checking, as always, has been left as an exercise for the reader.
=head2 How do I close a file descriptor by number?
-X<file, closing file descriptors>
+X<file, closing file descriptors> X<POSIX> X<close>
+
+If, for some reason, you have a file descriptor instead of a
+filehandle (perhaps you used C<POSIX::open>), you can use the
+C<close()> function from the C<POSIX> module:
-This should rarely be necessary, as the Perl close() function is to be
+ use POSIX ();
+
+ POSIX::close( $fd );
+
+This should rarely be necessary, as the Perl Cclose()> function is to be
used for things that Perl opened itself, even if it was a dup of a
numeric descriptor as with MHCONTEXT above. But if you really have
to, you may be able to do this:
$rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
-Or, just use the fdopen(3S) feature of open():
+Or, just use the fdopen(3S) feature of C<open()>:
{
- local *F;
- open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
- close F;
+ open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
+ close $fh;
}
=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
=head1 REVISION
-Revision: $Revision: 8075 $
+Revision: $Revision: 8579 $
-Date: $Date: 2006-11-15 02:26:49 +0100 (mer, 15 nov 2006) $
+Date: $Date: 2007-01-14 19:28:09 +0100 (dim, 14 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 7910 $)
+perlfaq6 - Regular Expressions ($Revision: 8539 $)
=head1 DESCRIPTION
regular character, so that C<P.> matches a C<P> followed by a dot.
=head2 What is C</o> really for?
-X</o>
+X</o, regular expressions> X<compile, regular expressions>
-Using a variable in a regular expression match forces a re-evaluation
-(and perhaps recompilation) each time the regular expression is
-encountered. The C</o> modifier locks in the regex the first time
-it's used. This always happens in a constant regular expression, and
-in fact, the pattern was compiled into the internal format at the same
-time your entire program was.
+(contributed by brian d foy)
-Use of C</o> is irrelevant unless variable interpolation is used in
-the pattern, and if so, the regex engine will neither know nor care
-whether the variables change after the pattern is evaluated the I<very
-first> time.
+The C</o> option for regular expressions (documented in L<perlop> and
+L<perlreref>) tells Perl to compile the regular expression only once.
+This is only useful when the pattern contains a variable. Perls 5.6
+and later handle this automatically if the pattern does not change.
-C</o> is often used to gain an extra measure of efficiency by not
-performing subsequent evaluations when you know it won't matter
-(because you know the variables won't change), or more rarely, when
-you don't want the regex to notice if they do.
+Since the match operator C<m//>, the substitution operator C<s///>,
+and the regular expression quoting operator C<qr//> are double-quotish
+constructs, you can interpolate variables into the pattern. See the
+answer to "How can I quote a variable to use in a regex?" for more
+details.
-For example, here's a "paragrep" program:
+This example takes a regular expression from the argument list and
+prints the lines of input that match it:
- $/ = ''; # paragraph mode
- $pat = shift;
- while (<>) {
- print if /$pat/o;
- }
+ my $pattern = shift @ARGV;
+
+ while( <> ) {
+ print if m/$pattern/;
+ }
+
+Versions of Perl prior to 5.6 would recompile the regular expression
+for each iteration, even if C<$pattern> had not changed. The C</o>
+would prevent this by telling Perl to compile the pattern the first
+time, then reuse that for subsequent iterations:
+
+ my $pattern = shift @ARGV;
+
+ while( <> ) {
+ print if m/$pattern/o; # useful for Perl < 5.6
+ }
+
+In versions 5.6 and later, Perl won't recompile the regular expression
+if the variable hasn't changed, so you probably don't need the C</o>
+option. It doesn't hurt, but it doesn't help either. If you want any
+version of Perl to compile the regular expression only once even if
+the variable changes (thus, only using its initial value), you still
+need the C</o>.
+
+You can watch Perl's regular expression engine at work to verify for
+yourself if Perl is recompiling a regular expression. The C<use re
+'debug'> pragma (comes with Perl 5.005 and later) shows the details.
+With Perls before 5.6, you should see C<re> reporting that its
+compiling the regular expression on each iteration. With Perl 5.6 or
+later, you should only see C<re> report that for the first iteration.
+
+ use re 'debug';
+
+ $regex = 'Perl';
+ foreach ( qw(Perl Java Ruby Python) ) {
+ print STDERR "-" x 73, "\n";
+ print STDERR "Trying $_...\n";
+ print STDERR "\t$_ is good!\n" if m/$regex/;
+ }
=head2 How do I use a regular expression to strip C style comments from a file?
expression engine cannot skip over any characters to find
the next match with this anchor, so C<\G> is similar to the
beginning of string anchor, C<^>. The C<\G> anchor is typically
-used with the C<g> flag. It uses the value of pos()
+used with the C<g> flag. It uses the value of C<pos()>
as the position to start the next match. As the match
-operator makes successive matches, it updates pos() with the
+operator makes successive matches, it updates C<pos()> with the
position of the next character past the last match (or the
first character of the next match, depending on how you like
-to look at it). Each string has its own pos() value.
+to look at it). Each string has its own C<pos()> value.
-Suppose you want to match all of consective pairs of digits
+Suppose you want to match all of consecutive pairs of digits
in a string like "1122a44" and stop matching when you
encounter non-digits. You want to match C<11> and C<22> but
the letter <a> shows up between C<22> and C<44> and you want
$_ = "1122a44";
my @pairs = m/(\d\d)/g; # qw( 11 22 44 )
-If you use the \G anchor, you force the match after C<22> to
+If you use the C<\G> anchor, you force the match after C<22> to
start with the C<a>. The regular expression cannot match
there since it does not find a digit, so the next match
fails and the match operator returns the pairs it already
print "Found $1\n";
}
-After the match fails at the letter C<a>, perl resets pos()
+After the match fails at the letter C<a>, perl resets C<pos()>
and the next match on the same string starts at the beginning.
$_ = "1122a44";
print "Found $1 after while" if m/(\d\d)/g; # finds "11"
-You can disable pos() resets on fail with the C<c> flag.
-Subsequent matches start where the last successful match
-ended (the value of pos()) even if a match on the same
-string as failed in the meantime. In this case, the match
-after the while() loop starts at the C<a> (where the last
-match stopped), and since it does not use any anchor it can
-skip over the C<a> to find "44".
+You can disable C<pos()> resets on fail with the C<c> flag, documented
+in L<perlop> and L<perlreref>. Subsequent matches start where the last
+successful match ended (the value of C<pos()>) even if a match on the
+same string has failed in the meantime. In this case, the match after
+the C<while()> loop starts at the C<a> (where the last match stopped),
+and since it does not use any anchor it can skip over the C<a> to find
+C<44>.
$_ = "1122a44";
while( m/\G(\d\d)/gc )
}
}
-For each line, the PARSER loop first tries to match a series
+For each line, the C<PARSER> loop first tries to match a series
of digits followed by a word boundary. This match has to
start at the place the last match left off (or the beginning
of the string on the first match). Since C<m/ \G( \d+\b
=head1 REVISION
-Revision: $Revision: 7910 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-10-07 22:38:54 +0200 (sam, 07 oct 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq7 - General Perl Language Issues ($Revision: 7998 $)
+perlfaq7 - General Perl Language Issues ($Revision: 8539 $)
=head1 DESCRIPTION
sub counter { $count++ }
}
- my $start = count();
+ my $start = counter();
- .... # code that calls count();
+ .... # code that calls counter();
- my $end = count();
+ my $end = counter();
In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
=head1 REVISION
-Revision: $Revision: 7998 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-11-01 09:56:34 +0100 (mer, 01 nov 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq8 - System Interaction ($Revision: 6628 $)
+perlfaq8 - System Interaction ($Revision: 8539 $)
=head1 DESCRIPTION
system("cat /etc/termcap") == 0
or die "cat program failed!";
-which will get the output quickly (as it is generated, instead of only
-at the end) and also check the return value.
+which will echo the cat command's output as it is generated, instead
+of waiting until the program has completed to print it out. It also
+checks the return value.
C<system> also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.
=head1 REVISION
-Revision: $Revision: 6628 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-07-09 14:46:14 +0200 (dim, 09 jui 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
=head1 NAME
-perlfaq9 - Networking ($Revision: 8076 $)
+perlfaq9 - Networking ($Revision: 8539 $)
=head1 DESCRIPTION
=head1 REVISION
-Revision: $Revision: 8076 $
+Revision: $Revision: 8539 $
-Date: $Date: 2006-11-15 14:53:23 +0100 (mer, 15 nov 2006) $
+Date: $Date: 2007-01-11 00:07:14 +0100 (jeu, 11 jan 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
projects / platform / upstream / perl.git / commitdiff