/* program free_fall C version */
#include <stdio.h>

void initial(double *y,double *v,double *a,double *g,double *t,
             double *dt);
void Euler(double *y,double *v,double *a,double g,double *t,
           double dt);
void print_table(double y,double v,double a,double t,int *nshow);

main()
/* no air resistance */
 {
     double y,v,a,g,t,dt;
     int counter,nshow;
/*   initial conditions and parameters */
     initial(&y,&v,&a,&g,&t,&dt);
/*   print initial conditions */
     print_table(y,v,a,t,&nshow);
     counter = 0;
     while (y >= 0)
       {
         Euler(&y,&v,&a,g,&t,dt);
/* next 2 lines usually written as  if (++counter % nshow == 0)   */
         counter = counter + 1;
         if (counter % nshow == 0)  /* % is modulus operator */
            print_table(y,v,a,t,&nshow);
        }
/*   print values at surface */
     print_table(y,v,a,t,&nshow);
  }

void initial(double *y,double *v,double *a,double *g,double *t,
             double *dt)
  {
     *t = 0;       /* initial time (sec)  */
     *y = 10;      /* initial height (m)  */
     *v = 0;       /* initial velocity    */
     *g = 9.8;     /* (magnitude) of accel due to gravity */
     *a = -(*g);
     printf("time step dt = ");
     scanf("%lf",dt);
  }

void Euler(double *y,double *v,double *a,double g,double *t,double dt)
  {
/*   use velocity at beginning of interval */     
     *y = *y + (*v)*dt;         
/*   reminder that acceleration is constant */
/*   y positive upward  */   
     *a = -g;              
     *v = *v + (*a)*dt;
     *t = *t + dt;
   }

void print_table(double y,double v,double a,double t,int *nshow)
   {
    if (t == 0.0)
      {
       printf("number of time steps between output = ");
       scanf("%d",nshow);
       printf(" \n");
       printf(" time (s)    y (m)   velocity (m/s)  accel (m/s^2) \n");
       printf(" \n");
      }
    printf("%10.3f,%10.3f,%10.3f,%12.3f\n",t,y,v,a);
    }