Computing Gini Index of an image (measure of Impurity)

Using my previous posts Class file and this reference:
http://people.revoledu.com/kardi/tutorial/DecisionTree/how-to-measure-impurity.htm

float computeGiniIndex(Mat r, Mat g, Mat b)

    {

        float giniIndex = 0.0;

        float frequency = getFrequencyOfBin(r);

        for( int i = 1; i < histSize; i++ )

        {

            float Hc = abs(getHistogramBinValue(r,i));

            giniIndex += Hc*Hc;

        }

        frequency = getFrequencyOfBin(g);

        for( int i = 1; i < histSize; i++ )

        {

            float Hc = abs(getHistogramBinValue(g,i));

            giniIndex += Hc*Hc;

        }

        frequency = getFrequencyOfBin(b);

        for( int i = 1; i < histSize; i++ )

        {

            float Hc = abs(getHistogramBinValue(b,i));

            giniIndex += Hc*Hc;

        }

        giniIndex = 1 - (giniIndex);

        //cout << giniIndex <<endl;

        return giniIndex;

    }