get Fibonacci number index, recursively

Question

I need a recursive function which can get me the index of a Fibonacci number passed to it.

For example, the mentioned function should return 6 when we pass 8; or it should return 8 when we pass 21.

Any help would be appreciated. Thanks !

Answer 1

This is my solution !
It works fine and return what I expect.
Make sure that a and b have their own default value in function declaration.

int getTermIndex(int, int&, int=1, int=1);

function implementation:

int getTermIndex(int number, int& i, int a, int b) {

    if(number==0) return i;
    else if(number==1){
        i++;
        return i;
    }

    int nextFib = a+b;

    if(nextFib==number || nextFib>number) {
        i += 3;
        return i;
    }
    else {
        i++;
        getTermIndex(number, i, b, nextFib);
    }

}

function usage:

int number;
cin >> number;

int nth=0;
getTermIndex(number, nth);