How to properly clear buffer for input (First Project) #rewritten code - c

Question

How do i proplerly clear input buffer since fflush(stdin) is undefined and only available in some compilers,
My plan is to clear input since after looping back it still use the buffered input rather than getting a new input.
Here is my code:
#include <stdio.h>
#include <stdbool.h>
//bool function
bool isInteger(double val)
{
int truncated = (int)val;
return (val == truncated);
}
int main()
{
//local variable
double result, a, b;
char oper, option = 'y';
//loop
while(option == 'y')
{
system("cls");
printf("Enter number and operator to solve:\n\nExapmple [a + b][a - b][a * b][a / b]\n");
scanf("%lf %c %lf", &a, &oper, &b);
// nested if
if(isInteger(a) && isInteger(b))
{
if(oper == '+')
{
result = a + b; //calculation
printf("%.1lf %c %.1lf = %.1lf\n\n", a, oper, b, result); //ouput result
}
else if(oper == '-')
{
result = a - b; //calculation
printf("%.1lf %c %.1lf = %.1lf\n\n", a, oper, b, result); //ouput result
}
else if(oper == '*')
{
result = a * b; //calculation
printf("%.1lf %c %.1lf = %.1lf\n\n", a, oper, b, result); //ouput result
}
else if(oper == '/')
{
result = a / b; //calculation
printf("%.1lf %c %.1lf = %.1lf\n\n", a, oper, b, result); //ouput result
}
else
{
//output error
printf("Invalid Operater!\n");
}
}
else
{
//output error
printf("Input is not a digit!\n");
}
//output selection
printf("Do you want to retry y/n?");
option = getch();
} // end loop
}
I only use scanf since we are still in scanf and printf.
Please help me out, this is our first project in college.