// **************************************
// ** Computing II Lab. session 3 (PJM)**
// ** Solution of coupled differential **
// ** equations using Euler's method   **
// **************************************

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

float d_omega_dt(float); 
                
int main(void)
       {
         FILE *fp;
         float omega[2000], theta[2000],time=0.0,time_step=.04;
         int t;
          
         theta[0]=M_PI/4.0;             // Starting angle 
         omega[0]=0.00;                 // Starting velocity is 0 rad/s
                           
         fp=fopen("pendulum.dat","w");
         
             for (t=0;t<1999;t++)
                 {                  
                  omega[t+1]=omega[t]+d_omega_dt(theta[t])*time_step; 
                  theta[t+1]=theta[t]+(time_step*omega[t]);                                                                                                                                          
                  fprintf(fp, "%f,%.2f \n",time,theta[t]*180/M_PI); 
                  time+=time_step;
                 }
                 
         fclose(fp);            
 
    }
         
// Function to calculate dv/dt using
// formula dv/dt = -(g/l)*sin(theta)

float d_omega_dt(float angle)
        {         
          float g=9.8;
          float l=9.8; 
          float dveldt;       

          dveldt=-(g/l)*sin(angle);                    

          return(dveldt);
         }