Q: Input a value for a scanf(), but nothing happens I'm writing some code as part of a few assignment exercises to learn C programming from absolute basics, and I've run into a problem, which is probably quite simple to solve, but I'm completely stuck! I'm writing a program to implement a basic Newton's method for differentiation. Whenever I input an initial value to the scanf(), the program simply stops, doesn't return anything, terminate or freeze. Any help would be great.
Here is my code to start with:
 #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>

//function to be used f(x) = cos(x)-x^3

//f'(x) = -sin(x)-3x^2

int main()
{
  double x, xold;
  printf("Enter an initial value between 0 and 1\n");
  scanf("%lf",&xold);
  double eps = 1e-12;
   x = xold - ((cos(xold)-pow(xold,3))/(-(sin(xold)-(3*pow(xold,2)))));
    while (fabs(x-xold)>eps)
    {
    x = xold - ((cos(xold)-pow(xold,3))/(-sin(xold)-(3*pow(xold,2))));
    }
    printf("The answer is %.12lf",x);
    return 0;
};


A: In your while loop:
x = xold - ((cos(xold)-pow(xold,3))/(-sin(xold)-(3*pow(xold,2))));

the value of the = right operand is always the same, how could you exit the loop once you enter it?

A: Actually the thing is you are not updating your xold variable. Try the below modified code for your problem, see if I have done correctly:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>


int main()
{
    double x, xold;
    printf("Enter an initial value between 0 and 1\n");
    scanf("%lf",&x);
    double eps = 1e-12;

    x = x - ((cos(x)-pow(x,3))/(-(sin(x)-(3*pow(x,2)))));
    while (fabs(x - xold)>eps)
    { 
        xold = x;
        x = x - ((cos(x)-pow(x,3))/(-sin(x)-(3*pow(x,2))));
    }
    printf("The answer is %.12lf\n",x);

    return 0;
}

