C/C++ First Derivative Code

In calculus, the first derivative test uses the first derivative of a function to determine whether a given critical point of a function is a local maximum, a local minimum, or neither. The idea behind the first derivative test is to examine the monotonicity properties of a function just to the left and right of a given point in its domain. If the function “switches” from increasing to decreasing at the point, then close to that point, it will achieve a highest value at that point. Similarly, if the function “switches” from decreasing to increasing at the point, then close to that point, it will achieve a least value at that point. If the function fails to “switch”, and remains increasing or remains decreasing, then no highest or least value is achieved.

From this first derivative description, I create a C/C++ first derivative code.

void firstDerivative(float *data, float *firstderv, int nx)
{
	int i;
	float tmp1, tmp2;

	//compute derivative first index
	firstderv[0] = data[1]-data[0];

	for(i=1; i<nx-2; i++)  //compute first derivative
	{
		tmp1 = data[i]-data[i-1];
		tmp2 = data[i+1]-data[i];
		firstderv[i] = (tmp2+tmp1)/2;
	}

	//compute derivative last index
	firstderv[nx-1] = data[nx-1]-data[nx-2];
}

Output from this first derivative function is firstderv. firstderv have same size with input data (variable data). This is output from data processed with first derivative code :

Source :