compute_wcss.hpp

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#ifndef KMEANS_COMPUTE_WCSS_HPP
#define KMEANS_COMPUTE_WCSS_HPP
 
#include <algorithm>
#include "Matrix.hpp"
 
namespace kmeans {
 
template<class Matrix_, typename Cluster_, typename Float_>
void compute_wcss(const Matrix_& data, Cluster_ ncenters, const Float_* centers, const Cluster_* clusters, Float_* wcss) {
    auto nobs = data.num_observations();
    size_t ndim = data.num_dimensions();
    std::fill_n(wcss, ncenters, 0);
 
    auto work = data.new_extractor(static_cast<Index<Matrix_> >(0), nobs);
    for (Index<Matrix_> obs = 0; obs < nobs; ++obs) {
        auto curdata = work->get_observation();
        auto cen = clusters[obs];
        auto curcenter = centers + static_cast<size_t>(cen) * ndim; // cast to size_t to avoid overflow.
 
        Float_& curwcss = wcss[cen];
        for (size_t d = 0; d < ndim; ++d) {
            Float_ delta = static_cast<Float_>(curdata[d]) - curcenter[d]; // cast for consistent precision regardless of Data_.
            curwcss += delta * delta;
        }
    }
}
 
}
 
#endif