Help with derivative code

[rokujohaven]

I have to find the derivative of a polynomial with degree 5 with any given coefficient but it's been giving me nonsense answers
This is my rationale
The polynomial format is ax^5 + bx^4 + cx^3 + dx^2 +ex + f
where a,b,c,d,e,f are coefficients
So deriv should be 5ax^4 + 4bx^3 + 3cx^2 + 2dx + 1ex^0 + 0f
What am I doing wrong?

double Deriv (double array[], int arraySize, double xVal)
{
    double answer = 0; //clears out garbage values
    for (int i = 0; i < arraySize; i++) //i is the degree in descending order
    {
        if (i==0) //for the case of i=0 (a constant number)
        {
            answer = 0;
        }
        answer = answer + i*array[i]*pow(xVal,i-1); //i-1 to show that taking deriv decreases the degree exponent by 1
    }
    return answer;
}

[ne555]

> it's been giving me nonsense answers
Give an example input, with its expected output and what is giving you instead.
Also, show how you are using your function.