cvs commit: perlfaq perlfaq2.pod perlfaq3.pod perlfaq4.pod perlfaq7.pod perlfaq9.pod
[email protected] (brian d foy) 13 Oct 2005 19:43:14 -0000
| Newsgroups | perl.cvs.perlfaq |
|---|---|
| Message-ID | <[email protected]> |
cvsuser 05/10/13 12:43:13
Modified: . perlfaq2.pod perlfaq3.pod perlfaq4.pod perlfaq7.pod
perlfaq9.pod
Log:
* perlfaq2: URL updates and winnowing out ancient books from the list
* perlfaq3: URL updates and formatting fixes
* perlfaq4:
* How can I tell whether a certain element is contained in a list or
array?
+ Anno updated this answer with pointers to List::Util
* How can I use a reference as a hash key?
+ I updated the answer to explain why you shouldn't.
+ If people have other favorite modules to handle this sort
of thing, let me know so I can add them to the answer.
* perlfaq7:
* I updated the answer to note that we've fixed this bug. I kept
the old answer around although I added example output to clarify
the symptoms.
* This might be a candidate for removal.
*perlfaq9:
* I fixed a syntax error involving a missing close quote
* I changed the double quotes to single quotes since I'm not
interpolating anything.
Revision Changes Path
1.35 +16 -46 perlfaq/perlfaq2.pod
Index: perlfaq2.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq2.pod,v
retrieving revision 1.34
retrieving revision 1.35
diff -u -r1.34 -r1.35
--- perlfaq2.pod 10 Aug 2005 15:56:54 -0000 1.34
+++ perlfaq2.pod 13 Oct 2005 19:43:13 -0000 1.35
@@ -149,9 +149,8 @@
have a proper man command, or it's been misinstalled. If that doesn't
work, try looking in /usr/local/lib/perl5/pod for documentation.
-If all else fails, consult http://perldoc.cpan.org/ or
-http://www.perldoc.com/ both offer the complete documentation
-in html format.
+If all else fails, consult http://perldoc.perl.org/ which has the
+complete documentation in HTML and PDF format.
Many good books have been written about Perl--see the section below
for more details.
@@ -162,11 +161,9 @@
L<perlreftut> for managing references, L<perlretut> for regular
expressions, L<perlthrtut> for threads, L<perldebtut> for debugging,
and L<perlxstut> for linking C and Perl together. There may be more
-by the time you read this. The following URLs might also be of
-assistance:
+by the time you read this. These URLs might also be useful:
- http://perldoc.cpan.org/
- http://www.perldoc.com/
+ http://perldoc.perl.org/
http://bookmarks.cpan.org/search.cgi?cat=Training%2FTutorials
=head2 What are the Perl newsgroups on Usenet? Where do I post questions?
@@ -221,10 +218,11 @@
A number of books on Perl and/or CGI programming are available. A few
of these are good, some are OK, but many aren't worth your money.
There is a list of these books, some with extensive reviews, at
-http://books.perl.org/ .
+http://books.perl.org/ . If you don't see your book listed here, you
+can write to [email protected] .
The incontestably definitive reference book on Perl, written by
-the creator of Perl, is now (Sept 2004) in its third edition:
+the creator of Perl, is Programming Perl:
Programming Perl (the "Camel Book"):
by Larry Wall, Tom Christiansen, and Jon Orwant
@@ -242,13 +240,13 @@
http://www.oreilly.com/catalog/perlckbk2/
If you're already a seasoned programmer, then the Camel Book might
-suffice for you to learn Perl from. If you're not, check out the
+suffice for you to learn Perl. If you're not, check out the
Llama book:
- Learning Perl (the "Llama Book")
- by Randal L. Schwartz and Tom Phoenix
- ISBN 0-596-00132-0 [3rd edition July 2001]
- http://www.oreilly.com/catalog/lperl3/
+ Learning Perl
+ by Randal L. Schwartz, Tom Phoenix, and brian d foy
+ ISBN 0-596-10105-8 [4th edition July 2005]
+ http://www.oreilly.com/catalog/learnperl4/
And for more advanced information on writing larger programs,
presented in the same style as the Llama book, continue your education
@@ -306,11 +304,6 @@
ISBN 0-596-00032-4 [3rd edition May 2000]
http://www.oreilly.com/catalog/perlpr3/
- Perl in a Nutshell
- by Ellen Siever, Stephan Spainhour, and Nathan Patwardhan
- ISBN 1-56592-286-7 [1st edition December 1998]
- http://www.oreilly.com/catalog/perlnut/
-
=item Tutorials
Beginning Perl
@@ -324,38 +317,15 @@
http://www.manning.com/Johnson/
Learning Perl
- by Randal L. Schwartz and Tom Phoenix
- ISBN 0-596-00132-0 [3rd edition July 2001]
- http://www.oreilly.com/catalog/lperl3/
+ by Randal L. Schwartz, Tom Phoenix, and brian d foy
+ ISBN 0-596-10105-8 [4th edition July 2005]
+ http://www.oreilly.com/catalog/learnperl4/
Learning Perl Objects, References, and Modules
by Randal L. Schwartz, with Tom Phoenix (foreword by Damian Conway)
ISBN 0-596-00478-8 [1st edition June 2003]
http://www.oreilly.com/catalog/lrnperlorm/
- Learning Perl on Win32 Systems
- by Randal L. Schwartz, Erik Olson, and Tom Christiansen,
- with foreword by Larry Wall
- ISBN 1-56592-324-3 [1st edition August 1997]
- http://www.oreilly.com/catalog/lperlwin/
-
- Perl: The Programmer's Companion
- by Nigel Chapman
- ISBN 0-471-97563-X [1997, 3rd printing Spring 1998]
- http://www.wiley.com/compbooks/catalog/97563-X.htm
- http://www.wiley.com/compbooks/chapman/perl/perltpc.html (errata etc)
-
- Cross-Platform Perl
- by Eric Foster-Johnson
- ISBN 1-55851-483-X [2nd edition September 2000]
- http://www.pconline.com/~erc/perlbook.htm
-
- MacPerl: Power and Ease
- by Vicki Brown and Chris Nandor,
- with foreword by Matthias Neeracher
- ISBN 1-881957-32-2 [1st edition May 1998]
- http://www.macperl.com/ptf_book/
-
=item Task-Oriented
Writing Perl Modules for CPAN
1.52 +23 -28 perlfaq/perlfaq3.pod
Index: perlfaq3.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq3.pod,v
retrieving revision 1.51
retrieving revision 1.52
diff -u -r1.51 -r1.52
--- perlfaq3.pod 10 Aug 2005 15:56:39 -0000 1.51
+++ perlfaq3.pod 13 Oct 2005 19:43:13 -0000 1.52
@@ -47,7 +47,7 @@
power of Perl. The goal is a full featured shell that behaves as
expected for normal shell activity and uses Perl syntax and
functionality for control-flow statements and other things.
-You can get psh at http://www.focusresearch.com/gregor/psh/ .
+You can get psh at http://sourceforge.net/projects/psh/ .
Zoidberg is a similar project and provides a shell written in perl,
configured in perl and operated in perl. It is intended as a login shell
@@ -772,30 +772,26 @@
solutions simply bundle a Perl interpreter in the final product
(but see L<How can I make my Perl program run faster?>).
-The Perl Archive Toolkit (http://par.perl.org/index.cgi) is
-Perl's analog to Java's JAR. It's freely available and on
-CPAN (http://search.cpan.org/dist/PAR/).
-
-The B::* namespace, often called "the Perl compiler", but is really a
-way for Perl programs to peek at its innards rather than create
-pre-compiled versions of your program. However. the B::Bytecode
-module can turn your script into a bytecode format that could be
-loaded later by the ByteLoader module and executed as a regular Perl
-script.
-
-There are also some commercial products that may work for
-you, although you have to buy a license for them.
-
-The Perl Dev Kit
-(http://www.activestate.com/Products/Perl_Dev_Kit/) from
-ActiveState can "Turn your Perl programs into ready-to-run
-executables for HP-UX, Linux, Solaris and Windows."
+The Perl Archive Toolkit ( http://par.perl.org/index.cgi ) is Perl's
+analog to Java's JAR. It's freely available and on CPAN (
+http://search.cpan.org/dist/PAR/ ).
+
+The B::* namespace, often called "the Perl compiler", but is really a way
+for Perl programs to peek at its innards rather than create pre-compiled
+versions of your program. However. the B::Bytecode module can turn your
+script into a bytecode format that could be loaded later by the
+ByteLoader module and executed as a regular Perl script.
+
+There are also some commercial products that may work for you, although
+you have to buy a license for them.
-Perl2Exe (http://www.indigostar.com/perl2exe.htm) is a
-command line program for converting perl scripts to
-executable files. It targets both Windows and unix
-platforms.
+The Perl Dev Kit ( http://www.activestate.com/Products/Perl_Dev_Kit/ )
+from ActiveState can "Turn your Perl programs into ready-to-run
+executables for HP-UX, Linux, Solaris and Windows."
+Perl2Exe ( http://www.indigostar.com/perl2exe.htm ) is a command line
+program for converting perl scripts to executable files. It targets both
+Windows and unix platforms.
=head2 How can I compile Perl into Java?
@@ -923,12 +919,11 @@
A good place to start is L<perltoot>, and you can use L<perlobj>,
L<perlboot>, L<perltoot>, L<perltooc>, and L<perlbot> for reference.
-(If you are using really old Perl, you may not have all of these,
-try http://www.perldoc.com/ , but consider upgrading your perl.)
A good book on OO on Perl is the "Object-Oriented Perl"
-by Damian Conway from Manning Publications,
-http://www.manning.com/Conway/index.html
+by Damian Conway from Manning Publications, or "Learning Perl
+References, Objects, & Modules" by Randal Schwartz and Tom
+Phoenix from O'Reilly Media.
=head2 Where can I learn about linking C with Perl?
1.68 +39 -20 perlfaq/perlfaq4.pod
Index: perlfaq4.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq4.pod,v
retrieving revision 1.67
retrieving revision 1.68
diff -u -r1.67 -r1.68
--- perlfaq4.pod 10 Aug 2005 15:55:49 -0000 1.67
+++ perlfaq4.pod 13 Oct 2005 19:43:13 -0000 1.68
@@ -1212,6 +1212,8 @@
=head2 How can I tell whether a certain element is contained in a list or array?
+(portions of this answer contributed by Annoo Siegel)
+
Hearing the word "in" is an I<in>dication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
designed to answer this question quickly and efficiently. Arrays aren't.
@@ -1247,28 +1249,35 @@
Now check whether C<vec($read,$n,1)> is true for some C<$n>.
-Please do not use
-
- ($is_there) = grep $_ eq $whatever, @array;
+These methods guarantee fast individual tests but require a re-organization
+of the original list or array. They only pay off if you have to test
+multiple values against the same array.
+
+If you are testing only once, the standard module List::Util exports
+the function C<first> for this purpose. It works by stopping once it
+finds the element. It's written in C for speed, and its Perl equivalant
+looks like this subroutine:
-or worse yet
+ sub first (&@) {
+ my $code = shift;
+ foreach (@_) {
+ return $_ if &{$code}();
+ }
+ undef;
+ }
- ($is_there) = grep /$whatever/, @array;
+If speed is of little concern, the common idiom use grep in scalar context
+(which returns the number of items that passed its condition) to traverse the
+entire list. This does have the benefit of telling you how many matches it
+found, though.
-These are slow (checks every element even if the first matches),
-inefficient (same reason), and potentially buggy (what if there are
-regex characters in $whatever?). If you're only testing once, then
-use:
+ my $is_there = grep $_ eq $whatever, @array;
- $is_there = 0;
- foreach $elt (@array) {
- if ($elt eq $elt_to_find) {
- $is_there = 1;
- last;
- }
- }
- if ($is_there) { ... }
+If you want to actually extract the matching elements, simply use grep in
+list context.
+ my @matches = grep $_ eq $whatever, @array;
+
=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes that
@@ -1982,8 +1991,18 @@
=head2 How can I use a reference as a hash key?
-You can't do this directly, but you could use the standard Tie::RefHash
-module distributed with Perl.
+(contributed by brian d foy)
+
+Hash keys are strings, so you can't really use a reference as the key.
+When you try to do that, perl turns the reference into its stringified
+form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back
+the reference from the stringified form, at least without doing some
+extra work on your own. Also remember that hash keys must be unique, but
+two different variables can store the same reference (and those variables
+can change later).
+
+The Tie::RefHash module, which is distributed with perl, might be what
+you want. It handles that extra work.
=head1 Data: Misc
1.26 +28 -12 perlfaq/perlfaq7.pod
Index: perlfaq7.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq7.pod,v
retrieving revision 1.25
retrieving revision 1.26
diff -u -r1.25 -r1.26
--- perlfaq7.pod 8 Aug 2005 02:38:25 -0000 1.25
+++ perlfaq7.pod 13 Oct 2005 19:43:13 -0000 1.26
@@ -271,24 +271,40 @@
=head2 What is variable suicide and how can I prevent it?
-Variable suicide is when you (temporarily or permanently) lose the
-value of a variable. It is caused by scoping through my() and local()
-interacting with either closures or aliased foreach() iterator
-variables and subroutine arguments. It used to be easy to
-inadvertently lose a variable's value this way, but now it's much
-harder. Take this code:
+This problem was fixed in perl 5.004_05, so preventing it means upgrading
+your version of perl. ;)
- my $f = "foo";
+Variable suicide is when you (temporarily or permanently) lose the value
+of a variable. It is caused by scoping through my() and local()
+interacting with either closures or aliased foreach() iterator variables
+and subroutine arguments. It used to be easy to inadvertently lose a
+variable's value this way, but now it's much harder. Take this code:
+
+ my $f = 'foo';
sub T {
- while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
+ while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" }
}
T;
print "Finally $f\n";
+If you are experiencing variable suicide, that C<my $f> in the subroutine
+doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
+shows that inside the subroutine the value of C<$f> leaks through when it
+shouldn't, as in this output:
+
+ foobar
+ foobarbar
+ foobarbarbar
+ Finally foo
+
The $f that has "bar" added to it three times should be a new C<$f>
-(C<my $f> should create a new local variable each time through the loop).
-It isn't, however. This was a bug, now fixed in the latest releases
-(tested against 5.004_05, 5.005_03, and 5.005_56).
+C<my $f> should create a new lexical variable each time through the loop.
+The expected output is:
+
+ foobar
+ foobar
+ foobar
+ Finally foo
=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1.24 +5 -5 perlfaq/perlfaq9.pod
Index: perlfaq9.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq9.pod,v
retrieving revision 1.23
retrieving revision 1.24
diff -u -r1.23 -r1.24
--- perlfaq9.pod 10 Aug 2005 15:54:54 -0000 1.23
+++ perlfaq9.pod 13 Oct 2005 19:43:13 -0000 1.24
@@ -366,9 +366,9 @@
use CGI qw(:standard);
- my $total = param( "price" ) + param( "shipping" );
+ my $total = param( 'price' ) + param( 'shipping' );
- my @items = param( "item ); # multiple values, same field name
+ my @items = param( 'item' ); # multiple values, same field name
If you want an object-oriented approach, CGI.pm can do that too.
@@ -376,9 +376,9 @@
my $cgi = CGI->new();
- my $total = $cgi->param( "price" ) + $cgi->param( "shipping" );
+ my $total = $cgi->param( 'price' ) + $cgi->param( 'shipping' );
- my @items = $cgi->param( "item" );
+ my @items = $cgi->param( 'item' );
You might also try CGI::Minimal which is a lightweight version
of the same thing. Other CGI::* modules on CPAN might work better