* 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.
Index: perlfaq4.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq4.pod,v
retrieving revision 1.67
diff -u -d -r1.67 perlfaq4.pod
--- perlfaq4.pod 10 Aug 2005 15:55:49 -0000 1.67
+++ perlfaq4.pod 10 Oct 2005 19:03:23 -0000
@@ -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 Anno 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<$n>.
-Please do not use
+These methods guarantee fast individual tests but require a
re-organization
+of the original list or array. They only pay off if you have to test
+multiple values against the same array.
- ($is_there) = grep $_ eq $whatever, @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
--
brian d foy, [EMAIL PROTECTED]
