On 1/27/12 5:27 PM, Derek wrote:
On Sat, 28 Jan 2012 06:36:49 +1100, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
Here's a multikey sorting problem that's bothering me; I think it's
well researched but I can't find the right keywords.
Say we have a collection of people with height and hair length
information. We want to get people who are "tall and long-haired".
More precisely, we want to order people by rank(p.height) +
rank(p.hairlength), where rank(p.k) is the function that returns the
rank of person p if ordered by key k.
The brute force approach would essentially compute the two ranks and
then sort by their sum. That's three sorts and at least one temporary
array. But I think things can be done considerably better. Any ideas?
Just a further clarification ...
If a person is tall and has short hair, would they be included in the
sort? In other words, are you ONLY looking for tall AND long-haired
people or are you looking for relative ranking of tallness and hairiness?
Everybody is included in the sort: the size of the array before and
after the sort is the same. Then, of course, the top K problem is very
interesting too.
Andrei
