=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.64 $, $Date: 2005/04/27 00:18:04 $)
+perlfaq4 - Data Manipulation ($Revision: 1.69 $, $Date: 2005/10/14 15:34:06 $)
=head1 DESCRIPTION
my $week_of_year = strftime "%W",
localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
-The Date::Calc module provides two functions for to calculate these.
+The Date::Calc module provides two functions to calculate these.
use Date::Calc;
my $day_of_year = Day_of_Year( 1987, 12, 18 );
=head2 How can I compare two dates and find the difference?
-If you're storing your dates as epoch seconds then simply subtract one
-from the other. If you've got a structured date (distinct year, day,
-month, hour, minute, seconds values), then for reasons of accessibility,
-simplicity, and efficiency, merely use either timelocal or timegm (from
-the Time::Local module in the standard distribution) to reduce structured
-dates to epoch seconds. However, if you don't know the precise format of
-your dates, then you should probably use either of the Date::Manip and
-Date::Calc modules from CPAN before you go hacking up your own parsing
-routine to handle arbitrary date formats.
+(contributed by brian d foy)
+
+You could just store all your dates as a number and then subtract. Life
+isn't always that simple though. If you want to work with formatted
+dates, the Date::Manip, Date::Calc, or DateTime modules can help you.
+
=head2 How can I take a string and turn it into epoch seconds?
=head2 How can I tell whether a certain element is contained in a list or array?
+(portions of this answer contributed by Anno Siegel)
+
Hearing the word "in" is an I<in>dication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
designed to answer this question quickly and efficiently. Arrays aren't.
Now check whether C<vec($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 uses 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
=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