请问，c中的这段代码，scanf（）读取数值后，缓冲区中剩下的\n到哪里去了？

Question

如这个代码中的2个while
#include

#include

double twice(double x);
double half(double x);
double thrice(double x);
void showmenu(void);
#define NUM 4
int main(void)
{
double(*pf[NUM])(double) = { twice,half,thrice,sqrt };
double val;
double ans;
int sel;

puts("enter a number (negative to quit):");
while (scanf("%lf", &val) == 1 && val >= 0)      //用户输入数值后，缓冲区剩个\n
{                                                                       //这中间我并没有加入读掉\n的代码
    showmenu();
    while (scanf("%d",&sel)&&sel>=0&&sel<=3) //然后用户再次输入，第一个读取的不应该是\n吗？为何程序完美调试？
    {
        ans = (*pf[sel])(val);
        printf("answer=%f\n", ans);
        ans = pf[sel](val);
        printf("to repeat,answer=%f\n", ans);
        showmenu();
    }
    puts("enter next number (negative to quit)");
}
puts("bye");
getchar();
getchar();
return 0;

}

double twice(double x)
{
return 2.0*x;
}

double half(double x)
{
return x/2.0;
}

double thrice(double x)
{
return 3.0*x;
}

void showmenu(void)
{
puts("enter one pf the following choices:");
puts("0)double the value 1)halve the value");
puts("2)triple the value 3)squareroot the value");
puts("4)next number");
}

Answer 1

\n是作为界符被scanf识别掉的，所以此时不存在\n了。