What you probably mean is a parameterized circle equation: Let variable t run from 0 to 2*PI in small increments and then calculate:
double x, y;
x = cos (t) * r;
y = sin (t) * r;
This will give you a set of coordinated for a circle around the origin with radius r.
If you want to print the circle in the way you showed in your question then consider that the space taken up by each print character is different in x and y direction. Hence you have to compensate this by applying a correction factor in the x or y calculation. Otherwise the circle will appear as an ellipse on your chart.