先看程序代码，下面这个程序代码axx+bx+c=0也就是x=(-b±√(b^2-4ac))/2a　的方程的解。
# include <stdio.h>
# include <math.h>

int main(void)
{
    double a, b, c;   
    double delta;
    double x1, x2;   
    char ch;
   
    do   
    {   
        printf("请输入一元二次方程的三个系数:\n");
        printf("a = ");
        scanf("%lf", &a);
        
        printf("b = ");
        scanf("%lf", &b);
        
        printf("c = ");
        scanf("%lf", &c);
        
        delta = b*b - 4*a*c;
        
        if (delta > 0)
        {
            x1 = (-b + sqrt(delta)) / (2*a);
            x2 = (-b - sqrt(delta)) / (2*a);
            printf("有两个解，x1 = %lf, x2 = %lf\n", x1, x2);
        }
        else if (0 == delta)
        {
            x1 = x2 = (-b) / (2*a);
            printf("有唯一解，x1 = x2 = %lf\n", x1, x2);
        }
        else
        {
            printf("无实数解!\n");
        }

        printf("您想继续么(Y/N): ");
        scanf(" %c", &ch);
    } while ('y'==ch || 'Y'==ch);
   
    return 0;
}

我的问题是：
下面语句
 printf("a = ");
scanf("%lf", &a);
当我输入数字时不会出现错误，但当我在输入a的值不是数字而是y或n时，为何同时（我认为是同时）执行了 scanf(" %c", &ch);？

谢谢！