Thread: Program to compute powers

Hi there folks.

I'm trying to create a program that takes 2 numbers from command line arguments and use the second value as a power so for e.g. java Power 10 2 should give 100. But i want to do this without using the Math.pow method, and using a for loop. I have the following so far but i'm not sure what the problem is and it;s only seem to be doinng square numbers (i.e. to the power of 2)

public class Power
{
  public static void main(String [] args)
  {
    int mantissa = Integer.parseInt(args[0]);
    int exponent = Integer.parseInt(args[1]);
    int answer;
 
 
    for(int i=0; (mantissa*mantissa) < exponent; i++)
      exponent=exponent + i;
 
    answer = mantissa*mantissa;
    System.out.println("Result is " + answer);
 
 
  }
}

public power(int num, int power)
{
int num2;
if (power == 0 && num!=0)
{
num2 = 1;
}
else if (power == 1)
{
num2 = num;
}
 
else if (1 < power)
{
num2 = num * power(num, power -1);
}
 
else if(0 > power && num!=0)
{
num2 = 1/power(num, - power);
}
}

Well...that may not quite work, haven't tried it out yet, but it's on the right track.

You're algorithm doesn't compute x^y.

Get out a piece of paper and "execute" the code as if you were the computer (pick some sample inputs):

ex. mantissa = 4, exponent = 3

4^3 = 64

    for(int i=0; (mantissa*mantissa) < exponent; i++)
      exponent=exponent + i;

1. initialize i = 0
2. 4 * 4 = 16
3. exponent = 3
4. 16 < 3?
5. false, exit the for loop

answer = mantissa*mantissa;
System.out.println("Result is " + answer);

6. answer = 4 * 4 = 16
7. Print out "result is 16"

Re-visit your pseudo-code and make sure that that is correct.

As far as I can tell, your code at best will give you mantissa^2, and at worst will keep you stuck indefinately inside the for-loop (well, not really indefinately, integers wrap around at 2^31 so eventually your code will stop and give you mantissa^2)

edit:

public power(int num, int power)
{
 
if (power == 0 && num!=0)
{
num2 = 1;
}
else if (power == 1)
{
num2 = num;
}
 
else if (1 < power)
{
num2 = num * power(num, power -1);
}
 
else if(0 > power)
{
num2 = 1/power(num, - power);
}
}

This isn't quite correct (for various reasons).

1. This isn't proper Java syntax (no return type, num2 is never declared).
2. 0^0 = 1. In your code, as far as I can work out it's un-determined
3. Try running power(2,-1). You'll get a rather interesting result (correct result = 0.5). I actually can't think of a case off the top of my head where this code would give you correct results.
4. No non-integer inputs for power (or number, but this is easier to fix than non-integer powers), i.e. power(2.5, -1.346)
5. (minor issue) Assuming this code did actually work, this gives an O(n) solution at best (I didn't perform a thourough analysis, but it can only be worse). The best solution for computing x^y is O(1) (granted, there is a bit of "cheating" by using logarithm tables)
6. We're here to help people find the answers to their problems, not give the answer to them