=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.69 $, $Date: 2005/10/14 15:34:06 $)
+perlfaq4 - Data Manipulation ($Revision: 1.73 $, $Date: 2005/12/31 00:54:37 $)
=head1 DESCRIPTION
The localtime function returns the day of the year. Without an
argument localtime uses the current time.
- $day_of_year = (localtime)[7];
+ $day_of_year = (localtime)[7];
The POSIX module can also format a date as the day of the year or
week of the year.
give you the same time of day, only the day before.
use DateTime;
-
+
my $yesterday = DateTime->now->subtract( days => 1 );
-
+
print "Yesterday was $yesterday\n";
You can also use the C<Date::Calc> module using its 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";
-
+
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
most people, there are two days a year when they aren't: the switch to
This is documented in L<perlref>, and although it's not the easiest
thing to read, it does work. In each of these examples, we call the
-function inside the braces of used to dereference a reference. If we
+function inside the braces used to dereference a reference. If we
have a more than one return value, we can construct and dereference an
anonymous array. In this case, we call the function in list context.
- print "The time values are @{ [localtime] }.\n";
+ print "The time values are @{ [localtime] }.\n";
If we want to call the function in scalar context, we have to do a bit
more work. We can really have any code we like inside the braces, so
that is up to you, and you can use code inside the braces.
print "The time is ${\(scalar localtime)}.\n"
-
+
print "The time is ${ my $x = localtime; \$x }.\n";
-
+
If your function already returns a reference, you don't need to create
the reference yourself.
sub timestamp { my $t = localtime; \$t }
-
+
print "The time is ${ timestamp() }.\n";
-
-In most cases, it is probably easier to simply use string
-concatenation, which also forces scalar context.
+
+The C<Interpolation> module can also do a lot of magic for you. You can
+specify a variable name, in this case C<E>, to set up a tied hash that
+does the interpolation for you. It has several other methods to do this
+as well.
+
+ use Interpolation E => 'eval';
+ print "The time values are $E{localtime()}.\n";
+
+In most cases, it is probably easier to simply use string concatenation,
+which also forces scalar context.
print "The time is " . localtime . ".\n";
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 do I sort a hash (optionally by value instead of key)?
-Internally, hashes are stored in a way that prevents you from imposing
-an order on key-value pairs. Instead, you have to sort a list of the
-keys or values:
+(contributed by brian d foy)
+
+To sort a hash, start with the keys. In this example, we give the list of
+keys to the sort function which then compares them ASCIIbetically (which
+might be affected by your locale settings). The output list has the keys
+in ASCIIbetical order. Once we have the keys, we can go through them to
+create a report which lists the keys in ASCIIbetical order.
+
+ my @keys = sort { $a cmp $b } keys %hash;
+
+ foreach my $key ( @keys )
+ {
+ printf "%-20s %6d\n", $key, $hash{$value};
+ }
+
+We could get more fancy in the C<sort()> block though. Instead of
+comparing the keys, we can compute a value with them and use that
+value as the comparison.
+
+For instance, to make our report order case-insensitive, we use
+the C<\L> sequence in a double-quoted string to make everything
+lowercase. The C<sort()> block then compares the lowercased
+values to determine in which order to put the keys.
+
+ my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
+
+Note: if the computation is expensive or the hash has many elements,
+you may want to look at the Schwartzian Transform to cache the
+computation results.
+
+If we want to sort by the hash value instead, we use the hash key
+to look it up. We still get out a list of keys, but this time they
+are ordered by their value.
- @keys = sort keys %hash; # sorted by key
- @keys = sort {
- $hash{$a} cmp $hash{$b}
- } keys %hash; # and by value
+ my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
-Here we'll do a reverse numeric sort by value, and if two keys are
-identical, sort by length of key, or if that fails, by straight ASCII
-comparison of the keys (well, possibly modified by your locale--see
-L<perllocale>).
+From there we can get more complex. If the hash values are the same,
+we can provide a secondary sort on the hash key.
- @keys = sort {
- $hash{$b} <=> $hash{$a}
- ||
- length($b) <=> length($a)
- ||
- $a cmp $b
- } keys %hash;
+ my @keys = sort {
+ $hash{$a} <=> $hash{$b}
+ or
+ "\L$a" cmp "\L$b"
+ } keys %hash;
=head2 How can I always keep my hash sorted?
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
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