do..while loop not stopping

Question

I am trying to create interval semi diving method (numerical method) and in the program there is a do/while loop, which does not work correctly:

double Funkcija(double x){
    double result = -1/(1+abs(x));
    return result;
}

...

double x[20] = {};
x[0] = 1; //initial point
int k = 1;  // index (from 1 because 0 is giving (x0)
do
{
    cout << "x[" << k << "]= ";
    x[k] = x[k - 1] - pow(2, k - 1)*abs(d);
    cout << x[k] << "\t";
    cout << "f(x[" << k << "])= " << Funkcija(x[k]) << endl;
    k++;
} while (Funkcija(x[k]) < Funkcija(x[k-1]));

what I get is:

I need that loop stop before x[4] , because f(x[3]) > f(x[2]), maybe someone see my error?

Answer 1

It's an "off by one" error. You have incremented k before the loop test, so you are testing the wrong indices - compare the elements at k - 1 and k - 2 and everything should be fine.

Note that if you had single-stepped through the code in your debugger, inspecting variables as you go, then the mistake would have quickly become apparent. This is an important skill to work on when you are learning to program.