[jira] [Created] (MATH-1165) Rare case for updateMembershipMatrix() in FuzzyKMeansClusterer

["Pashutan Modaresi (JIRA)" <ji...@apache.org>]

Pashutan Modaresi created MATH-1165:
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             Summary: Rare case for updateMembershipMatrix() in FuzzyKMeansClusterer
                 Key: MATH-1165
                 URL: https://issues.apache.org/jira/browse/MATH-1165
             Project: Commons Math
          Issue Type: Bug
    Affects Versions: 3.3
            Reporter: Pashutan Modaresi
            Priority: Minor


The function updateMembershipMatrix() in FuzzyKMeansClusterer assigns the points to the cluster with the highest membership. Consider the following case:

If the distance between a point and the cluster center is zero, then we will have a cluster membership of one, and all other membership values will be zero.

So the if condition:
if (membershipMatrix[i][j] > maxMembership) {
                    maxMembership = membershipMatrix[i][j];
                    newCluster = j;
}
will never be true during the for loop and newCluster will remain -1. This will throw an exception because of the line:
clusters.get(newCluster)
                    .addPoint(point);

Adding the following condition can solve the problem:
double d;
if (sum == 0)
d = 1;
else
d = 1.0/sum;



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