fourier1.c

double fourier1(double x_in[], double n, int length) {
 double x_complex[2] = { 0, 0 };
 int i;
 for(i = 0; i < length; i++) {
   x_complex[0] += x_in[i] * cos(M_PI * 2 * i * n / (double) length);
   x_complex[1] += x_in[i] * sin(M_PI * 2 * i * n / (double) length);
 }
 return sqrt(x_complex[0]*x_complex[0] + x_complex[1]*x_complex[1]) / (double) length; 
}