void RefinedStart::Cluster(const MatType& data,
const size_t clusters,
arma::mat& centroids) const
{
// This will hold the sampled datasets.
const size_t numPoints = size_t(percentage * data.n_cols);
MatType sampledData(data.n_rows, numPoints);
// vector<bool> is packed so each bool is 1 bit.
std::vector<bool> pointsUsed(data.n_cols, false);
arma::mat sampledCentroids(data.n_rows, samplings * clusters);
for (size_t i = 0; i < samplings; ++i)
{
// First, assemble the sampled dataset.
size_t curSample = 0;
while (curSample < numPoints)
{
// Pick a random point in [0, numPoints).
size_t sample = (size_t) math::RandInt(data.n_cols);
if (!pointsUsed[sample])
{
// This point isn't used yet. So we'll put it in our sample.
pointsUsed[sample] = true;
sampledData.col(curSample) = data.col(sample);
++curSample;
}
}
// Now, using the sampled dataset, run k-means. In the case of an empty
// cluster, we re-initialize that cluster as the point furthest away from
// the cluster with maximum variance. This is not *exactly* what the paper
// implements, but it is quite similar, and we'll call it "good enough".
KMeans<> kmeans;
kmeans.Cluster(sampledData, clusters, centroids);
// Store the sampled centroids.
sampledCentroids.cols(i * clusters, (i + 1) * clusters - 1) = centroids;
pointsUsed.assign(data.n_cols, false);
}
// Now, we run k-means on the sampled centroids to get our final clusters.
KMeans<> kmeans;
kmeans.Cluster(sampledCentroids, clusters, centroids);
}
void RefinedStart::Cluster(const MatType& data,
const size_t clusters,
arma::Col<size_t>& assignments) const
{
math::RandomSeed(std::time(NULL));
// This will hold the sampled datasets.
const size_t numPoints = size_t(percentage * data.n_cols);
MatType sampledData(data.n_rows, numPoints);
// vector<bool> is packed so each bool is 1 bit.
std::vector<bool> pointsUsed(data.n_cols, false);
arma::mat sampledCentroids(data.n_rows, samplings * clusters);
// We will use these objects repeatedly for clustering.
arma::Col<size_t> sampledAssignments;
arma::mat centroids;
KMeans<> kmeans;
for (size_t i = 0; i < samplings; ++i)
{
// First, assemble the sampled dataset.
size_t curSample = 0;
while (curSample < numPoints)
{
// Pick a random point in [0, numPoints).
size_t sample = (size_t) math::RandInt(data.n_cols);
if (!pointsUsed[sample])
{
// This point isn't used yet. So we'll put it in our sample.
pointsUsed[sample] = true;
sampledData.col(curSample) = data.col(sample);
++curSample;
}
}
// Now, using the sampled dataset, run k-means. In the case of an empty
// cluster, we re-initialize that cluster as the point furthest away from
// the cluster with maximum variance. This is not *exactly* what the paper
// implements, but it is quite similar, and we'll call it "good enough".
kmeans.Cluster(sampledData, clusters, sampledAssignments, centroids);
// Store the sampled centroids.
sampledCentroids.cols(i * clusters, (i + 1) * clusters - 1) = centroids;
pointsUsed.assign(data.n_cols, false);
}
// Now, we run k-means on the sampled centroids to get our final clusters.
kmeans.Cluster(sampledCentroids, clusters, sampledAssignments, centroids);
// Turn the final centroids into assignments.
assignments.set_size(data.n_cols);
for (size_t i = 0; i < data.n_cols; ++i)
{
// Find the closest centroid to this point.
double minDistance = std::numeric_limits<double>::infinity();
size_t closestCluster = clusters;
for (size_t j = 0; j < clusters; ++j)
{
const double distance = kmeans.Metric().Evaluate(data.col(i),
centroids.col(j));
if (distance < minDistance)
{
minDistance = distance;
closestCluster = j;
}
}
// Assign the point to its closest cluster.
assignments[i] = closestCluster;
}
}
