ActivationFunction.java

/* *****************************************************************************
 *  Name:              Ada Lovelace
 *  Coursera User ID:  123456
 *  Last modified:     October 16, 1842
 **************************************************************************** */

public class ActivationFunction {

    // Returns the Heaviside function of x.
    public static double heaviside(double x) {
        if (Double.isNaN(x)) x = Double.NaN;
        else if (x < 0) x = 0;
        else if (x > 0) x = 1;
        else x = 0.5;
        return x;
    }

    // Returns the sigmoid function of x.
    public static double sigmoid(double x) {
        if (Double.isNaN(x)) return Double.NaN;
        else return 1 / (1 + (Math.exp(-x)));
    }

    // Returns the hyperbolic tangent of x.
    public static double tanh(double x) {
        if (Double.isNaN(x)) return Double.NaN;
        else if (x >= 20.0) return 1.0;
        else if (x <= -20.0) return -1.0;
        else return (Math.exp(x) - Math.exp(-x)) / (Math.exp(x) + Math.exp(-x));
    }

    // Returns the softsign function of x.
    public static double softsign(double x) {
        if (Double.isNaN(x))
            return Double.NaN;
        else if (x == Double.POSITIVE_INFINITY)
            return 1;
        else if (x == Double.NEGATIVE_INFINITY)
            return -1;
        else
            return x / (1 + Math.abs(x));
    }

    // Returns the square nonlinearity function of x.
    public static double sqnl(double x) {
        if (Double.isNaN(x)) x = Double.NaN;
        else if (x <= -2) x = -1;
        else if (x > -2 && x < 0) x = x + ((x * x) / 4);
        else if (x >= 0 && x < 2) x = x - ((x * x) / 4);
        else if (x >= 2) x = 1;
        return x;
    }

    // Takes a double command-line argument x and prints each activation

    public static void main(String[] args) {
        double x = Double.parseDouble(args[0]);
        System.out.println("heaviside(" + x + ") = " + heaviside(x));
        System.out.println("  sigmoid(" + x + ") = " + sigmoid(x));
        System.out.println("     tanh(" + x + ") = " + tanh(x));
        System.out.println(" softsign(" + x + ") = " + softsign(x));
        System.out.println("     sqnl(" + x + ") = " + sqnl(x));
    }
}