// *********************************
// ** Example of use of malloc()  **
// **      PJM CompII Lab. 2      **       
// *********************************

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
 
void Calculate_Derivative(float *, float, int);

int main(void)
       {
     
         float theta=0.0, delta_theta, *sin_values;
         int n;
         int size=513;
     
         sin_values=(float *) malloc(size*4); // Allocate memory for array
                  
         delta_theta=(4*M_PI)/(size-1);         

            for (n=0;n<size;n++)               // sin_values[] holds sines of angles
                {                              // between 0 and 4 PI   
                  sin_values[n]=sin(theta);                          
                  theta=theta+delta_theta;
                }
                
        Calculate_Derivative(sin_values,delta_theta,size);              
        free(sin_values); // Free up memory
       }
       
void Calculate_Derivative(float *sin_array, float delta, int size)
       {
        FILE *fp;
        int index;
        float *deriv_sin_values,angle;
        
        deriv_sin_values=(float *) malloc(size*4); // Allocate memory for array
        deriv_sin_values[0]=(sin_array[1]-sin_array[0])/delta; // Forward difference
   
           for (index=1;index<size-1;index++) // Centred differences
             deriv_sin_values[index]=(sin_array[index+1]-sin_array[index-1])/(2.0*delta);               

        deriv_sin_values[size-1]=(*(sin_array+size-1)-*(sin_array+size-2))/delta;  
        // Above line evaluates backward difference
       
        // Save values to file
        
        fp=fopen("dsin.dat","w"); 
        
         for (index=0;index<size;index++)
             {
             angle=(float)(index*(4.0/(size-1)));
             fprintf(fp,"%f,%f \n",angle,deriv_sin_values[index]);
             }

        fclose(fp);
        
        free(deriv_sin_values); // Free up memory
       }