Program to return power of 2 without using any power function.

Question

I want a program that takes an int x as parameter and returns 2^x. The implementation is not allowed to use power of.

public int double2 (int x) {
    int r = 2;
    int y = 1;
    for (int i=0; i < x; i++){    
        y = r* y;
    }
    return y;
}

Do you think this is a right solution?

Answer 1

How about this:

int power = someNumberHere;
int result = 1;
while (power-- > 0) result *= 2;

And I think that is what you want...I think. Are you trying to find a power of two? Maybe expand the question, scope and reasons behind what you want a little.

Answer 2

y = x*x;

or

y = x*2;

or

y = 42; // ;)

Depending in what do you mean by the "double of 2".

EDIT

naive implementation

public int 2powerOf( int n ) {
    int r = 2;
    for( int i = 1 ; i < n ; i++ ) {
        r = r * 2;
    }
    return r;
 }

It is almost as the one you posted, this might not handle negative values though.

If you want to know if yours works, then run it and find out for your self.

Answer 3

Does your implementation passes the following JUnit test?

import junit.framework.TestCase;

public class YourClassTest extends TestCase {

    private YourClass sut = new YourClass();

    public void testPowersOfTwo() {
     assertEquals(1, sut.double2(0));
     assertEquals(2, sut.double2(1));
     assertEquals(4, sut.double2(2));
     assertEquals(8, sut.double2(3));
     assertEquals(16, sut.double2(4));
    }

}

If it does, it is a working solution (but maybe not optimal).

Answer 4

The solution you posted using the for loop produces the right result, but you should look into a more efficient solution (bit shifting) as Adamski first mentioned.

Answer 5

Just in case if you are needing such kind of explanatory code....(its in c though)

#include <stdio.h>
  int main(void)
 {

   int base, power, index;
   long answer;
   base = 0;
   power = 0;
   answer = 1.00;

   printf(" Enter a base number: ");
   scanf("%d", &base);
   printf(" Enter a power to raise the base to: ");
   scanf("%d", &power);

   for(index = 1; index <= power; index++)
       answer = answer * base;

   printf("%d raised to the power of %d is %ld.", base, power, answer);
   getchar();
   getchar();
   return 0;

}