将计算值存储到数组中

Question

In my program, the equation is solved for y given a range of x values. How do I then store those values into y and print them out as the array. I thought I was supposed to use the index method but I have an error. Multiple markers at this line - Syntax error on token "]", VariableDeclaratorId expected after this token - y cannot be resolved to a type

What do I need to modify?

import java.lang.Math;
import java.util.Arrays;
public class Standard {
    public static void main (String[] args) {
        double exponent,  x, pi, e, sqrtpart;
        double[] y;
        pi = 3.14159;
        e = 2.71828;
        x = -2.0;
        int count = 0;

        while (count < 20)
        {
        exponent = - ((x*x)/(2));

        sqrtpart = Math.sqrt(2*pi);

        y[] = (Math.pow(e,exponent))/sqrtpart;
        System.out.println(y[index]);

        x = x + 0.2;
        count++;
        }
       }
    }

Answer 1

Change

double[] y;

into

double[] y = new double[20];

And

    y[] = (Math.pow(e,exponent))/sqrtpart;
    System.out.println(y[index]);

into

    y[count] = (Math.pow(e,exponent))/sqrtpart;
    System.out.println(y[count]);

Answer 2

You can not put values into the y array as presently it does not have any size.

Try double y [] = new double [20];

Also

System.out.println(y[index]);

should be

System.out.println(y[count]);

Answer 3

you need to initialize y

Put this outside the while loop

y = new double[<size of your array>];

Then you can use the index method

y[index] = (Math.pow(e,exponent))/sqrtpart;

Answer 4

The previous answers help you fix the compiler problems. But there are other things you should consider:

1. You are calculating 1/sqrt(2*pi) in every single iteration in the while loop. This is not good from a performance point of view since this calculation will never change. So it is better to pull this calculation out of the while loop.

2. You don't need to define your own pi and e. Just use the static ones in the Math class: Math.PI, Math.E

3. You have hard-coded values in your code for the size of the array and the increments. That makes your code fragile. You could change one number and miss another.

So consider the following:

import java.lang.Math;

public class Standard {

    static int size = 20;
    static double width = 4.0;
    static double increments = width/size; 

    public static void main (String[] args) {

        double exponent, x, sqrtpart;

        double[] y = new double[size];

        x = - (width / 2);

        sqrtpart = Math.sqrt(2* Math.PI);

        int count = 0;
        while (count < size)
        {
            exponent = - ((x*x)/(2));

            y[count] = (Math.pow(Math.E,exponent))/sqrtpart;

            System.out.println(y[count]);

            x = x + increments;

            count++;
        }
    }
}