Calculating the slope of a series of values

Question

I have 2 arrays of equal length. The following function attempts to calculate the slope using these arrays. It returns the average of the slope between each points. For the foll

Answer 1

Edit: use Apache Commons Math class SimpleRegression if that's an option. Else, here's a method that calculates slope and also intercept, should yield the same results as excel and apache:

private static double intercept(List yList, List xList) {
    if (yList.size() != xList.size())
        throw new IllegalArgumentException("Number of y and x must be the same");
    if (yList.size() < 2)
        throw new IllegalArgumentException("Need at least 2 y, x");

    double yAvg = average(yList);
    double xAvg = average(xList);

    double sumNumerator = 0d;
    double sumDenominator = 0d;
    for (int i = 0; i < yList.size(); i++) {
        double y = yList.get(i);
        double x = xList.get(i);
        double yDiff = y - yAvg;
        double xDiff = x - xAvg;
        double numerator = xDiff * yDiff;
        double denominator = xDiff * xDiff;
        sumNumerator += numerator;
        sumDenominator += denominator;
    }

    double slope = sumNumerator / sumDenominator;
    double intercept = yAvg - (slope * xAvg);
    return intercept;
}

private static double average(Collection doubles) {
    return doubles.stream().collect(Collectors.averagingDouble(d -> d));
}

Sources: Excel doc for SLOPE Excel doc for INTERCEPT