Query regarding k-means clustering in MATLAB

Question

I have a very large amount of data in the form of matrix.I have already clustered it using k-means clustering in MATLAB R2013a. I want the exact coordinates of the centroid

Answer 1

The centroid is simply evaluated as the average value of all the points' coordinates that are assigned to that cluster.

If you have the assignments {point;cluster} you can easily evaluate the centroid: let's say you have a given cluster with n points assigned to it and these points are a1,a2,...,an. You can evaluate the centroid for such cluster by using:

centroid=(a1+a2+...+an)/n

Obviously you can run this process in a loop, depending on how your data structure (i.e. the assignment point/centroid) is organized.

Answer 2

In MATLAB, use

[idx,C] = kmeans(..) 

instead of

idx = kmeans(..) 

As per the documentation:

[idx,C] = kmeans(..) returns the k cluster centroid locations in the k-by-p matrix C.