/**
* Returns the weighted arithmetic mean of the entries in the specified portion of the input
* array, or <code>Double.NaN</code> if the designated subarray is empty.
*
* <p>Throws <code>IllegalArgumentException</code> if either array is null.
*
* <p>See {@link Mean} for details on the computing algorithm. The two-pass algorithm described
* above is used here, with weights applied in computing both the original estimate and the
* correction factor.
*
* <p>Throws <code>IllegalArgumentException</code> if any of the following are true:
*
* <ul>
* <li>the values array is null
* <li>the weights array is null
* <li>the weights array does not have the same length as the values array
* <li>the weights array contains one or more infinite values
* <li>the weights array contains one or more NaN values
* <li>the weights array contains negative values
* <li>the start and length arguments do not determine a valid array
* </ul>
*
* @param values the input array
* @param weights the weights array
* @param begin index of the first array element to include
* @param length the number of elements to include
* @return the mean of the values or Double.NaN if length = 0
* @throws MathIllegalArgumentException if the parameters are not valid
* @since 2.1
*/
public double evaluate(
final double[] values, final double[] weights, final int begin, final int length)
throws MathIllegalArgumentException {
if (test(values, weights, begin, length)) {
Sum sum = new Sum();
// Compute initial estimate using definitional formula
double sumw = sum.evaluate(weights, begin, length);
double xbarw = sum.evaluate(values, weights, begin, length) / sumw;
// Compute correction factor in second pass
double correction = 0;
for (int i = begin; i < begin + length; i++) {
correction += weights[i] * (values[i] - xbarw);
}
return xbarw + (correction / sumw);
}
return Double.NaN;
}
/**
* Returns the arithmetic mean of the entries in the specified portion of the input array, or
* <code>Double.NaN</code> if the designated subarray is empty.
*
* <p>Throws <code>IllegalArgumentException</code> if the array is null.
*
* <p>See {@link Mean} for details on the computing algorithm.
*
* @param values the input array
* @param begin index of the first array element to include
* @param length the number of elements to include
* @return the mean of the values or Double.NaN if length = 0
* @throws MathIllegalArgumentException if the array is null or the array index parameters are not
* valid
*/
@Override
public double evaluate(final double[] values, final int begin, final int length)
throws MathIllegalArgumentException {
if (test(values, begin, length)) {
Sum sum = new Sum();
double sampleSize = length;
// Compute initial estimate using definitional formula
double xbar = sum.evaluate(values, begin, length) / sampleSize;
// Compute correction factor in second pass
double correction = 0;
for (int i = begin; i < begin + length; i++) {
correction += values[i] - xbar;
}
return xbar + (correction / sampleSize);
}
return Double.NaN;
}
