Sigmoid

#include <stdio.h>
#include <math.h>



double fx(double x) //Sigmoid function
{
    return 1 / (1 + exp(-(x)));
}


double fd(double x) //Sigmoid derivative
{
   return x * (1 - x);
}


int _tmain(int argc, _TCHAR* argv[])
{

double f_out = 0;  //output
double input=0.50;
double weight=0.10;
double learn=0.1; //learning-rate
double desired_output=0.12;
double requested_data=0.65;
double err;  //error value calculation, the error function 2(out-desired) is the derivative of the cost function C = (out-desired)^2
int i; // iteration

for (i = 0; i <= 30000; i++) {

                            // 1 single node, sigmoid backprop.
                            f_out = fx(input * weight);
                            err = 2*( f_out - desired_output );
                            weight = weight - learn * err *fd(f_out) * input;     // weight minus learning_rate * error * derivative_output * input
                                                                                  //all numbers must be between -1 and 1.

                            printf("\n%d. desired output is %.2lf",i, desired_output);
                            printf("\nweight %.9lf  error %.9lf", weight, err);
                            printf("\nOutput for the current input (0.50) is %.9lf", f_out);
                            printf("\nOutput for value 0.65 as new requested data would be %.9lf\n", fx(requested_data*weight));

                          }



    scanf("%lf", &input);

    return 0;
}