Java Entropy Calculate computeEntropy(int pos, int neg)

Description

Computes the entropy of two numbers.

License

Open Source License

Parameter

Parameter	Description
pos	number of "positive" instances
neg	number of "negative" instances

Return

the entropy as defined as -P(pos) * log2(P(pos)) - P(neg) * log2(P(neg))

Declaration

public static double computeEntropy(int pos, int neg) 

Method Source Code

//package com.java2s;
/*//from  w w w. ja  v a  2  s . c o m
Copyright 2009 Semantic Discovery, Inc. (www.semanticdiscovery.com)
    
This file is part of the Semantic Discovery Toolkit.
    
The Semantic Discovery Toolkit is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
    
The Semantic Discovery Toolkit is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.
    
You should have received a copy of the GNU Lesser General Public License
along with The Semantic Discovery Toolkit.  If not, see <http://www.gnu.org/licenses/>.
*/

public class Main {
    /**
     * Computes the entropy of two numbers.
     * @param pos number of "positive" instances
     * @param neg number of "negative" instances
     * @return the entropy as defined as <code>-P(pos) * log2(P(pos)) - P(neg) * log2(P(neg))</code>
     */
    public static double computeEntropy(int pos, int neg) {
        if (pos == 0 || neg == 0)
            return 0d;

        final double total = (double) pos + neg;
        final double posRatio = pos / total;
        final double negRatio = neg / total;
        final double entropy = -posRatio * log2(posRatio) - negRatio * log2(negRatio);

        return entropy;
    }

    /**
     * Computes the entropy of <code>N</code> numbers.
     * @param values numbers from which to compute entropy
     * @return the entropy as defined as <code>Sum(-P(values[i])) * log2(P(values[i])))</code>
     */
    public static double computeEntropy(int[] values) {
        double total = 0d;
        for (int i = 0; i < values.length; i++) {
            total += values[i];
        }

        double entropy = 0d;
        for (int i = 0; i < values.length; i++) {
            final double ratio = values[i] / total;
            entropy += ratio == 0 ? 0d : -ratio * log2(ratio);
        }

        return entropy;
    }

    /** 
     * Compute the Logarithm base 2 of the given value.
     */
    public static final double log2(double d) {
        return Math.log(d) / Math.log(2.0);
    }
}

Related

1. calcEntropyOfDiscreteProbDist(Double[] probDist)
2. discreteEntropy(int[] sequence, int numStates)
3. discretiseMaxEntropy(double data[], int numBins)
4. entropy(byte[] f)
5. entropy(double[] distribution)