what is the complexity of a program having only one loop, is it log n? can someone give me some ideas about estimating complexity of codes?
There is a field of study that attempts to quantify exactly that.
It's called cyclomatic complexity.
In this case, I believe your complexity rating would be p = 2, because the loop is conditional and that means there are two paths through the method.
If you are referring to time complexity, it's still just O(1) unless the loop iteration count is derived via a polynomial or exponential function, perhaps indirectly because the method is itself called in a higher loop, or because the loop counter is effectively multiplied by string operations or something at a lower level.