CODEWITHSUNDEEP
javarecursion
DarkLeafyGreen
DarkLeafyGreen

Reputation: 70406

Calculate the power of any exponent (negative or positive)

I want to calculate the result, given any exponent (negative or positive) and a base of type integer. I am using recursion:

public static double hoch(double basis, int exponent) {
    if (exponent > 0) {
        return (basis * hoch(basis, exponent - 1));
    } else if (exponent < 0) {
        return ((1 / (basis * hoch(basis, exponent + 1))));
    } else {
        return 1;
    }
}

If exponent is negative 1.0 is returned but that is wrong. For e.g. hoch(2,-2) it should be 0.25. Any ideas what could be wrong?

Upvotes: 1

Views: 22030

Answers (5)

Bogdan Butnaru
Bogdan Butnaru

Reputation: 21

Working code for raising BASE to a pos or neg BASE:

FUNC Raise_To_Power

LPARAMETERS pnBase, pnPow

DO CASE

  CASE pnPow = 0
    RETURN 1  
  CASE pnPow > 0
    RETURN pnBase * Raise_To_Power(pnBase, pnPow-1)  
  CASE pnPow < 0
    RETURN 1 / (pnBase * Raise_To_Power(pnBase, -(pnPow+1)))

ENDCASE

ENDFUNC

Upvotes: 0

The Archetypal Paul
The Archetypal Paul

Reputation: 41749

 }else if(exponent < 0){
         return ((1/(basis*hoch(basis, exponent+1))))

should be

 }else if(exponent < 0){
        return (1/hoch(basis, -exponent));

Upvotes: 5

Andreas Dolk
Andreas Dolk

Reputation: 114767

With hoch(2,-2) you actually calculate

     1 / (-2 * (1 / (-1 * (1 / 1)))
<=>  1 / (-2 * (1 / (-1))
<=>  1 / (-2 * -1)
<=>  1/2

Upvotes: 1

dspyz
dspyz

Reputation: 5490

public static double hoch(double basis, int exponent){
    if(exponent > 0){
        return basis*hoch(basis, exponent-1);
    }else if(exponent < 0){
        return hoch(basis, exponent+1)/basis;
    }else{
        return 1;
    }
}

although the more efficient (recursive) solution is

public static double hoch(double basis, int exponent){
    if(exponent == 0)
        return 1;
    else{
        double r = hoch(basis, exponent/2);
        if(exponent % 2 < 0)
            return r * r / basis;
        else if(exponent % 2 > 0)
            return r * r * basis;
        else
            return r * r;
    }
}

Upvotes: 2

Karl Knechtel
Karl Knechtel

Reputation: 61508

Your parentheses are the wrong way around. You want to be multiplying by the result of the recursive call, not dividing by it; and you want the thing you multiply by to be 1/basis (which "peels off" one negative exponent).

Upvotes: 1

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