How to write a function that can calculate power in Java. No loops

Question

I've been trying to write a simple function in Java that can calculate a number to the nth power without using loops.
I then found the Math.pow(a, b) class... or method still can't distinguish the two am not so good with theory. So i wrote this..

public static void main(String[] args) {

    int a = 2;

    int b = 31;

    System.out.println(Math.pow(a, b));

    }

Then i wanted to make my own Math.pow without using loops i wanted it to look more simple than loops, like using some type of Repeat I made a lot of research till i came across the commons-lang3 package i tried using StringUtils.repeat
So far I think this is the Syntax:-

public static String repeat(String str, int repeat)
    StringUtils.repeat("ab", 2);

The problem i've been facing the past 24hrs or more is that StringUtils.repeat(String str, int 2); repeats strings not out puts or numbers or calculations.
Is there anything i can do to overcome this or is there any other better approach to creating a function that calculates powers?
without using loops or Math.pow

This might be funny but it took me while to figure out that StringUtils.repeat only repeats strings this is how i tried to overcome it. incase it helps

 public static int repeat(int cal, int repeat){
     cal = 2+2;
     int result = StringUtils.repeat(cal,2);
     return result;
}  

can i not use recursion maybe some thing like this

public static RepeatThis(String a)
{
     System.out.println(a);
     RepeatThis(a);
} 

just trying to understand java in dept thanks for all your comments even if there were syntax errors as long as the logic was understood that was good for me :)

Answer 1

Another implementation with O(Log(n)) complexity

public static long pow(long base, long exp){        
    if(exp ==0){
        return 1;
    }
    if(exp ==1){
        return base;
    }

    if(exp % 2 == 0){
        long half = pow(base, exp/2);
        return half * half;
    }else{
        long half = pow(base, (exp -1)/2);
        return base * half * half;
    }       
}

Answer 2

Try with recursion:

int pow(int base, int power){
    if(power == 0) return 1;
    return base * pow(base, --power);
}

Answer 3

Function to handle +/- exponents with O(log(n)) complexity.

double power(double x, int n){
 if(n==0)
  return 1;

  if(n<0){
      x = 1.0/x;
      n = -n;
  }
 double ret = power(x,n/2);
 ret = ret * ret;
 if(n%2!=0)
   ret = ret * x;
 return ret;

}

Answer 4

I think in Production recursion just does not provide high end performance.

double power(double num, int exponent)
{

double value=1;
int Originalexpn=exponent;
double OriginalNumber=num;

if(exponent==0)
    return value;

if(exponent<0)
{
    num=1/num;
    exponent=abs(exponent);
}

while(exponent>0)
{
    value*=num;
    --exponent;
}

cout << OriginalNumber << " Raised to  " << Originalexpn << " is " << value << endl;
return value;

}

Answer 5

Sure, create your own recursive function:

public static int repeat(int base, int exp) {
 if (exp == 1) {
  return base;
 }

 return base * repeat(base, exp - 1);
}

Math.pow(a, b)

Math is the class, pow is the method, a and b are the parameters.

Answer 6

Use this code.

public int mypow(int a, int e){
    if(e == 1) return a;
    return a * mypow(a,e-1);
}

Answer 7

This one handles negative exponential:

public static double pow(double base, int e) {
    int inc;
    if(e <= 0) {
        base = 1.0 / base;
        inc = 1;
    }
    else {
        inc = -1;
    }
    return doPow(base, e, inc);
}

private static double doPow(double base, int e, int inc) {
    if(e == 0) {
        return 1;
    }
    return base * doPow(base, e + inc, inc);
}

Answer 8

Here is a O(log(n)) code that calculates the power of a number. Algorithmic technique used is divide and conquer. It also accepts negative powers ie, x^(-y)

import java.util.Scanner;

public class PowerOfANumber{
        public static void main(String args[]){
                float result=0, base;
                int power;
                PowerOfANumber calcPower = new PowerOfANumber();
                /* Get the user input for the base and power */
                Scanner input = new Scanner(System.in);
                System.out.println("Enter the base");   
                base=input.nextFloat();
                System.out.println("Enter the power");
                power=input.nextInt();
                result = calcPower.calculatePower(base,power);
                System.out.println(base + "^" + power + " is " +result);
        }   
        private float calculatePower(float x, int y){ 
                float temporary;
                /* Termination condition for recursion */    
                if(y==0)
                        return 1;
                temporary=calculatePower(x,y/2);
                /* Check if the power is even */
                if(y%2==0)
                        return (temporary * temporary);
                else{
                        if(y>0)
                                return (x * temporary * temporary);
                        else
                                return (temporary*temporary)/x;
                }    
        }
}

Answer 9

Remembering the definition of the logarithm , this can be done with ln and exp if these functions are allowed. Works for any positive base and any real exponent (not necessarily integer):

x = 6.7^4.4
ln(x) = 4.4 * ln(6.7) = about 8.36
x = exp(8.36) = about 4312.5

You can read more here and also here . Java provides both ln and exp .

Answer 10

A recursive method would be the easiest for this :

int power(int base, int exp) {
    if (exp != 1) {
        return (base * power(base, exp - 1));
    } else {
        return base;
    }
}

where base is the number and exp is the exponenet