Bad operand type for binary operator '^'

Question

Trying to create a recursive method that raises a double value to an int power for a java course. The instructions say "However, write the code so that when n is even the method will return (x ^ (n/2))^2."

This is what I have thus far:

     public static double powerFaster(double x, int n) {
         if (n == 0) {
            return 1;
         } 
         else if ((n % 2) == 0) {
           return ((x ^ (n / 2.0) ^ 2.0));  //Error occurs here.
         } else {
           return x * powerFaster(x, (n - 1));
         }
     }

Answer 1

^ is a XOR operator, not power. Use Math.pow() for power.

That said, I think you missed the point of the exercise.

You should return powerFaster(x, n/2) * powerFaster(x, n/2); when n is even (actually make one recursive call, store its result in a variable and multiply it by itself).

public static double powerFaster(double x, int n) {
     if (n == 0) {
        return 1;
     } 
     else if ((n % 2) == 0) {
       double pow = powerFaster(x, n/2);
       return pow * pow;
     } else {
       return x * powerFaster(x, (n - 1));
     }
 }

Answer 2

If you are doing this for speed, you want to avoid using Math.pow as this is the function you are trying to replace.

To get a square of a value you can do d * d

public static void main(String[] args) {
    System.out.println(powerOf(2, 9));
}

public static double powerOf(double d, int n) {
    if (n < 0) return 1 / powerOf(d, -n);

    double ret = 1;
    if (n > 1)
        ret = powerOf(d * d, n / 2);
    if (n % 2 != 0)
        ret *= d;
    return ret;
}

prints

512.0

Answer 3

^ is bitwise XOR, used with integers:

int a = 6; //00000110
int b = 5; //00000101
int c = a ^ b; //gives you 3 = 00000011, not 6^5

The operation is in the binary level:

00000110 //a
00000101 //b
--------- XOR
00000011

To perform power, use Math.pow():

Math.pow(2.0, 1.0) //gives you 2.0

Answer 4

In Java you should use java.lang.Math.pow(double x, double n) for raising value x to a power of n

^ is a XOR operator - see this SO question for more details