#include <stdio.h>
int a = 10;

printf("a = %d" "  a is visible here, before main!\n", a);

/* function prototype */

double product(double x, double y);

int main()
{
  double var1 = 3.0, var2 = 5.0;
  double ans;

printf("a = %d" "  a is visible here, in main!\n", a);    
    
  ans = product(var1, var2);
  printf("var1 = %.2f\n"
         "var2 = %.2f\n", var1, var2);
  printf(" var1*var2 = %.2f\n", ans);
    
printf("a = %d" "  a is still visible at the end of main, but got changed in the function!\n", a);  

  return(0);
}

/* function definition */ 
double product(double x, double y)

 {
   double result;

printf("a = %d" "  a is visible here, in function product()!\n", a);      
     
   result = x * y;
   a = 5;           /* changing a! */
   return result;
 }