CODEWITHSUNDEEP
javadifferential-equations
Rashid Ali
Rashid Ali

Reputation: 11

Coding for derivative approximate in java. no values being outputted

I have created a program that calculates the Newton Raphson method. I am trying to edit it so it will calculate Newton Rapshon but instead of using f'(x) I am using the Finite difference formula which is:

f(x-h)-f(h)/h where h is always more than 0.

I have the program working but when the user inputs a value the prgram does not calculate it. It keeps asking for a value. In previous cases the value would not output. Apologies about the long explanation.

public class NewtonRaphsonDevAp {

    public static double func(double x) {

        return (5 * Math.pow(x, 2) - 6 * Math.pow(x, 3) + 2 * x - 3);
    }

    public static double derivative_approx(double x, double h) {

        return ((func(x - h) - func(h)) / h);
    }

    public static double NewtonRaphsonDevAp() {
        boolean cont = true;
        double x, h, x1;
        Scanner scan = new Scanner(System. in );

        x = scan.nextDouble();
        h = 0.01;
        x1 = 0;

        while (cont) {
            x1 = x - (func(x) / derivative_approx(x, h));

            if (func(x1) <= 0.000001) {
                cont = false;

                System.out.println("f(x)= " + func(x));
                System.out.println("g(x)= " + derivative_approx(x, h));
                System.out.println("The root is " + x1);

            }
        }
        return x1;

    }

    public static void main(String[] args) {


        NewtonRaphsonDevAp();

    }
}

Upvotes: 1

Views: 146

Answers (1)

Dobz
Dobz

Reputation: 1213

while(cont) {
    x1 = x - (func(x) / derivative_approx(x,h));
    System.out.printf("x1 - %.7f\nfunc(x1) - %.7f\n", x1, func(x1));

    if (func(x1) <= 0.000001) {
        cont = false;
        System.out.println("f(x)= " + func(x));
        System.out.println("g(x)= " + derivative_approx(x,h));
        System.out.println("The root is " + x1);
    }
}

When I input 3 as the number this is my output.

x1 - 2.9895287
func(x1) - 193.9408206
x1 - 2.9895287
func(x1) - 193.9408206
x1 - 2.9895287
func(x1) - 193.9408206

x1 is never being set to another number. So basically your checking if(func(x1) <= 0.000001) on the same x1 an infinite number of times. I'd say you may be looking for.

Let x be set to x-(func(x) / derivative_approx(x,h)); each turn, so your x1 also gets smaller on each loop.

while (cont){
    x1 = x-(func(x) / derivative_approx(x,h));

    if (func(x1) <= 0.000001) {
        cont = false;
        System.out.println("f(x)= " + func(x));
        System.out.println("g(x)= " + derivative_approx(x,h));
        System.out.println("The root is " + x1);
    } else {
        x = x1;
    }
}

or ask for a new number.

while (cont){
    x1 = x-(func(x) / derivative_approx(x,h));

    if (func(x1) <= 0.000001){
        cont = false;
        System.out.println("f(x)= " + func(x));
        System.out.println("g(x)= " + derivative_approx(x,h));
        System.out.println("The root is " + x1);
    } else {
        x = scan.nextDouble();
    }
}

Upvotes: 2

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