Get nearest centroid using Thrust library? (K-Means)

I already finished computing the distances and stored in a thrust vector, for instance, I have 2 centroids and 5 datapoints and the way I computed the distances was that for each centroid I computed the distances with the 5 datapoints first and stored in the array and later with the other centroid in a 1d array in distances, just like this:

for (int i = 0; i < centroids.size(); ++i)
{
    computeDistance(Data, distances, centroids[i], nDataPoints, nDimensions);
}

Resulting in a vector 1d, for instance:

DistancesValues = {10, 15, 20, 12, 10, 5, 17, 22, 8, 7}

DatapointsIndex = {1, 2,  3,   4,  5,  1,  2,  3, 4, 5}

Where the first 5 values represent centroid 1 and other 5 values centroid 2.

What I would like to know if there is a thrust function in which I can store the count in another array of the minimum values for each centroid?

Comparing the values of each index, Result should be:

Counts = {2, 3}

where:

CountOfCentroid 1 = 2       
CountOfCentroid 2 = 3

Solution

Here is one possible approach:

Create an additional centroid index vector:

DistancesValues = {10, 15, 20, 12, 10, 5, 17, 22,  8, 7}
DatapointsIndex = {1,   2,  3,  4,  5, 1,  2,  3,  4, 5}
CentroidIndex   = {1,   1,  1,  1,  1, 2,  2,  2,  2, 2}

Now do a sort_by_key, using DatapointsIndex as the keys, and the other two vectors zipped together as the values. This has the effect rearranging all 3 vectors so that the DatapointsIndex has like indices grouped together:
```
DatapointsIndex = {1, 1, 2, 2, 3, 3, 4, 4, 5, 5} 
```
(and the other 2 vectors are correspondingly rearranged).
Now do a reduce_by_key. If we choose the thrust::minimum operator, we get a reduction that effectively chooses the minimum value in a group (rather than summing the values in a group). reduce_by_key means that a reduction of this type is done on each consecutive group of like keys. So we will once again use the DatapointsIndex as our key vector, and the other two vectors zipped together as our value vector. Most of the output of reduce_by_key we don't care about, except for the results vector that emanates from the CentroidIndex vector. By counting centroid indices in this results vector, we can get the desired output.

Here is a fully worked example:

$ cat t428.cu
#include <thrust/host_vector.h>
#include <thrust/device_vector.h>
#include <thrust/sort.h>
#include <thrust/reduce.h>
#include <thrust/copy.h>
#include <thrust/iterator/zip_iterator.h>
#include <thrust/iterator/discard_iterator.h>
#include <stdio.h>
#define NUM_POINTS 5
#define NUM_CENTROID 2
#define DSIZE (NUM_POINTS*NUM_CENTROID)

int main(){

  int DistancesValues[DSIZE] = {10, 15, 20, 12, 10, 5, 17, 22, 8, 7};
  int DatapointsIndex[DSIZE] = {1, 2,  3,   4,  5,  1,  2,  3, 4, 5};
  int CentroidIndex[DSIZE]   = {1, 1, 1, 1, 1, 2, 2, 2, 2, 2};

  thrust::device_vector<int> DV(DistancesValues, DistancesValues + DSIZE);
  thrust::device_vector<int> DI(DatapointsIndex, DatapointsIndex + DSIZE);
  thrust::device_vector<int> CI(CentroidIndex, CentroidIndex + DSIZE);
  thrust::device_vector<int> Ra(NUM_POINTS);
  thrust::device_vector<int> Rb(NUM_POINTS);

  thrust::sort_by_key(DI.begin(), DI.end(), thrust::make_zip_iterator(thrust::make_tuple(DV.begin(), CI.begin())));
  thrust::reduce_by_key(DI.begin(), DI.end(), thrust::make_zip_iterator(thrust::make_tuple(DV.begin(), CI.begin())), thrust::make_discard_iterator(), thrust::make_zip_iterator(thrust::make_tuple(Ra.begin(), Rb.begin())), thrust::equal_to<int>(), thrust::minimum<thrust::tuple<int, int> >());
  printf("CountOfCentroid 1 = %d\n", thrust::count(Rb.begin(), Rb.end(), 1));
  printf("CountOfCentroid 2 = %d\n", thrust::count(Rb.begin(), Rb.end(), 2));

  return 0;
}

$ nvcc -arch=sm_20 -o t428 t428.cu
$ ./t428
CountOfCentroid 1 = 2
CountOfCentroid 2 = 3
$

As Eric points out in his answer here (your question is almost a duplicate of that one), the sort_by_key is probably unnecessary. The reordering of this data follows a regular pattern, so we don't need to harness the complexity of sort, and can therefore re-order the data with clever use of iterators. Under those circumstances, it may be possible to do the entire operation (approximately) with a single call to reduce_by_key.