Re: [Boost-users] Interprenting nearest's neighbor's output

Subject: Re: [Boost-users] Interprenting nearest's neighbor's output
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-04-15 06:38:21

Georgios Samaras wrote:
> I changed the code to use points, but order the items are returned is 
> not understandable by me.
>
> I request the 5 NNs of the query point (0, 0), in a dataset of points 
> (0, 0), (1, 1), .., (9, 9).
>
> The output I receive is this:
> knn query result:
> POINT(4 4)
> POINT(3 3)
> POINT(2 2)
> POINT(0 0)
> POINT(1 1)
>
>
> If you need the code, let me know. I remember that I read that the 
> order is not specified, but does that mean that there is no logic in 
> the results? I mean, clearly the 1st NN is (0, 0).
>

If the kNN query using query() function is performed, the order of 
returned Values isn't specified. The function returns 5 NNs, not sorted 
5NNs. This way we save a call of sort() or sort_heap().
AFAIK the most typical use case would be to perform a kNN query and then 
to process all of those neighbours so it's not needed to sort them 
prematurely.

On the other hand in the case of query iterators ( 
http://www.boost.org/doc/libs/1_55_0/libs/geometry/doc/html/geometry/spatial_indexes/queries.html#geometry.spatial_indexes.queries.breaking_or_pausing_the_query 
)
you should get closer Values first.

Regards,
Adam