CODEWITHSUNDEEP
javarecursion
Raven Maddison
Raven Maddison

Reputation: 45

Using recursion in multiply

I have values x = 10 and y = 6. Then, i have this addxy function to calculate the sum of two numbers. I did my best in this add and now im working on how to construct it to get a product.... ive been wracking my brain for quite a day now but i still cant get it....

public static int addxy(int a, int b)
{
    if (b==0)
        return a;
    else
        if (b>0)
            return 1 + addxy(a,b-1);
        else
            if (a==0)
                return b;
            else
                return 1 + addxy(a-1,b);
}

any help will be appreciated

Upvotes: 0

Views: 800

Answers (3)

sarveshseri
sarveshseri

Reputation: 13985

public static int mulxy( int a, int b ) {
    if ( b == 0 || a == 0 ) {
        return 0;
    }

    // We need to solve the sign problem.         
    int signA = ( a > 0) ? 1 : -1;
    int signB = ( b > 0 )? 1 : -1;

    // now we know the sign of result.
    int signResult = signA * signB;
    // we don't need sign anymore
    int posA = a * signA;
    int posB = b * signB;

    return signResult * mulPosAB( posA, posB );   
}

public int multPosAB( int a, int b ) {
    // here we already know that a and b not -tive.

    if( a == 0 || b == 0 ) return 0;

    // return a + mulPosAB( a, b - 1 );
    // or using newAddXY
    return newAddXY( a, mulPosAB( a, b - 1 ) );

}

Also your addxy won't work for cases where both a and b are negative.

public int newAddXY( int a, int b ) {
    if( b == 0 ) return a;
    if( a == 0 ) return b;

    // We need to solve the sign problem.         
    int signA = ( a > 0) ? 1 : -1;
    int signB = ( b > 0 )? 1 : -1;

    //return signB + newAddXY( a, b - signB );  
    // I think we should ensure we re not adding numbers bigger than 1.
    // so this will be better
    return newAddXY( signB, newADDXY( a, b-signB ) );
}

Upvotes: 0

TheLostMind
TheLostMind

Reputation: 36304

If you want to have x*y then You have to recursively add x , y times like this :

public class Sample {
public static void main(String[] args) {
    System.out.println(add(5, 6));
    System.out.println(add(-5, 6));
    System.out.println(add(5, -6));
    System.out.println(add(-5, -6));
}

static int add(int x, int y) {

    if (x==0 ||y == 0) {
        return 0;
    }
    if (y > 0)
        return x + add(x, y - 1); //for positive numbers 
    else
        return -x + add(x, y + 1); // for negative numbers

}
}

O/P :

30
-30
-30
30

Upvotes: 2

ikh
ikh

Reputation: 10417

I guess you want something like this, right?

public static int multiply(int a, int b)
{
    if (a == 0 || b == 0)
        return 0;
    if (a < 0)
        return -multiply(-a, b);
    if (b < 0)
        return -multiply(a, -b);

    return multiply(a, b - 1) + a;
}

Upvotes: 3

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