Question

I am trying to locate the point of rotation in a sorted array through a modified binary search.

Consider this array int values[9]={7, 8, 9, 1, 2, 3, 4, 5, 6};
The point of rotation here is at index = 3 i.e at 9.

I wrote this function for the above operation.

void FindRotationPoint(int values[], int numvalues)
{
    int first =0;
    int last = numvalues-1;
    int middle;
    bool moreTosearch= (first<=last);
    while(first<=last)
    {
        middle = (first+last)/2;
        if(values[0]>values[middle]) //Keep looking right if the middle value in array is greater than first
        {
            last = middle-1;
        }
        if (values[0]<values[middle]) //Keep looking left if the middle value in array is less than first
        {
            first = middle+1;
        }
    }
    cout<<middle+1<<endl;
    cout<<values[middle];
}

If the elements are
int values[9]={7, 8, 9, 1, 2, 3, 4, 5, 6}; Output: 4, 1 (Wrong)

int values[9]={7, 8, 9, 10, 2, 3, 4, 5, 6}; Output: 4, 10 (Correct)

The point of rotation which is at an even place is found correct whereas in the other case, it finds the succeeding element. What am I missing in the above code?

Answer 1

This works:

void FindRotationPoint(int values[], int numvalues)
{
    int first =0;
    int last = numvalues-1;
    int middle=0;
    bool moreTosearch= (first<=last);
    while(first<last)
    {
        middle = (first+last)/2;
        if(values[first]>=values[middle]) //Keep looking right if the middle value in array is greater than first
        {
            last = middle;
            cout<<"first>middle: "<<first<<" "<<middle<<" "<<last<<"\n";
        }
        else if (values[middle]>=values[last]) //Keep looking left if the middle value in array is less than first
        {
            first = middle;
            cout<<"middle<last: "<<first<<" "<<middle<<" "<<last<<"\n";
        }
    }
    cout<<middle+1<<endl;
    cout<<values[middle];
}

int main()
{
int values[9]={7, 8, 9, 1, 2, 3, 4, 5, 6};

FindRotationPoint(values, 9);
return 0;
}