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matlabcluster-analysisk-means

Getting the index of closest data point to the centriods in Kmeans clustering in MATLAB

I am doing some clustering using K-means in MATLAB. As you might know the usage is as below:

[IDX,C] = kmeans(X,k)

where IDX gives the cluster number for each data point in X, and C gives the centroids for each cluster.I need to get the index(row number in the actual data set X) of the closest datapoint to the centroid. Does anyone know how I can do that? Thanks

Solution

  • The "brute-force approach", as mentioned by @Dima would go as follows

    %# loop through all clusters
for iCluster = 1:max(IDX)
    %# find the points that are part of the current cluster
    currentPointIdx = find(IDX==iCluster);
    %# find the index (among points in the cluster)
    %# of the point that has the smallest Euclidean distance from the centroid
    %# bsxfun subtracts coordinates, then you sum the squares of
    %# the distance vectors, then you take the minimum
    [~,minIdx] = min(sum(bsxfun(@minus,X(currentPointIdx,:),C(iCluster,:)).^2,2));
    %# store the index into X (among all the points)
    closestIdx(iCluster) = currentPointIdx(minIdx);
end

    To get the coordinates of the point that is closest to the cluster center k, use

    X(closestIdx(k),:)