All,
In this node, I pointed out that it is often a waste to sort a list if you were after the top value and that a watermark algorithm should be used instead. I am not sure this rule of thumb holds true when N != 1. The size of list X and N seem to be contributing factors.
Here is the code I used in that node:
sub top_x {
my ($list, $x) = @_;
$x--;
my @top;
$#top = $x;
for my $item ( @$list ) {
next if defined $top[ -1 ] && $item <= $top[ -1 ];
for my $id ( 0 .. $#top ) {
$top[ $id ] = $item and last if ! defined $top[ $id ];
if ( $item > $top[ $id ] ) {
@top[ $id .. $#top ] = ($item, @top[ $id .. $#top - 1]
+);
#splice(@top, $id, 0, $item), pop @top;
last;
}
}
}
return @top;
}
After writing it, I wondered:
- At what value of N does one of the 2 methods (splice vs array slice) beat the other?
- Does the length of the list X and the value of N have to be considered together when deciding?
- Under what circumstances is sorting better than watermarks?
- Is there a better way to keep track of the watermarks other than the one I have shown?
- Is there a better method, other than sorting and watermarks, for determining the top N of a list?
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