#include <stdio.h>
#define QUIT 5
#define HOTEL1 80.00
#define HOTEL2 125.00
#define HOTEL3 155.00
#define HOTEL4 200.00
#define DISCOUNT 0.95
#define STARS "***************************"

int menu();
int getnights();
void showprice(double, int);
int main()
{
    int nights;
    double hotel_rate;
    int code;
   
    while((code = menu()) != QUIT)
    {
        switch(code)
        {
            case 1: hotel_rate = HOTEL1;
                break;
            case 2: hotel_rate = HOTEL2;
                break;
            case 3: hotel_rate = HOTEL3;
                break;
            case 4: hotel_rate = HOTEL4;
                break;
            default: hotel_rate = 0.0;
                printf("Oops!\n");
                break;
        }
        nights = getnights();
        showprice(hotel_rate, nights);
    }
    printf("Thank you and goodbye. ");
    return 0;
}

int menu()
{
    int code, status;
   
    printf("\n%s%s\n", STARS, STARS);
    printf("Enter the number of the desired hotel: \n");
    printf("1) Fairfield Arms    2) Hotel Olympic\n");
    printf("3) Chertworthy Plaza 4) The Stockton\n");
    printf("5) quit\n");
    printf("%s%s\n", STARS, STARS);
    while((status = scanf("%d", &code)) != 1 ||
          (code < 1 || code > 5))
    {
        if(status != 1)
            scanf("%*s");
        printf("Enter an integer from 1 to 5, please.\n");
    }
    return code;
}

int getnights()
{
    int nights;
   
    printf("How mant nights are needed?  ");
    while(scanf("%d", &nights) != 1)
    {
        scanf("%*s");
        printf("Please enter an integer, such as 2.\n");
    }
    return nights;
}

void showprice(double rate, int nights)
{
    int n;
    double total = 0.0;
    double factor = 1.0;
    for(n = 1; n <= nights; n++, factor *= DISCOUNT)
        total += rate * factor;
    printf("The total cost will be $%0.2f. \n", total);
   
}

输入4.1 结果是
4.1
How mant nights are needed?  Please enter an integer, such as 2.
因为这个模块上的while语句里已经限定了输入值必须是整数才会输出how many nights are needed?但不知怎么的输入浮点数之后循环和外部同时进行，百思不得其解。求教

int getnights()
{
    int nights;
   
    printf("How mant nights are needed?  ");
    while(scanf("%d", &nights) != 1)
    {
        scanf("%*s");
        printf("Please enter an integer, such as 2.\n");
    }
    return nights;
}
就是这一段

你在while(scanf("%d", &nights) != 1)这句之前加一句fflush(stdin)(头文件stdlib.h)
这是因为你输入小数，实际上int menu()函数里的code是被成功赋值，例如输入“4.1”，code被成功赋值“4”，这样输入流里就遗留“.1”。
由于遗留的“.1”对接下来的nights赋值，很明显，赋值失败。
这就是为什么你输入 4.1会直接跳到int getnights()这个函数里的while循环里了。

我试过了可还是不行啊，输入4.1还是跟原来一样的问题

怎么可能，你是不是没加对地方
int getnights()
{
    int nights;
   
    printf("How mant nights are needed?  ");
    fflush(stdin);   //头文件stdlib.h
    while(scanf("%d", &nights) != 1)
    {
        scanf("%*s");
        printf("Please enter an integer, such as 2.\n");
    }
    return nights;
}

肯定又是我的软件问题－－哎 xcode出了不少硬伤了

将fflush(stdin)换成scanf("%*[^\n]")可能有用。毕竟scanf("%*[^\n]");的移植性比fflush更好。
scanf("%*[^\n]");来清空缓冲区，这里要注意的是它会保留下“\n”。再加上scanf("%*c");这句，读取保留下来的\n。
如果有用而fflush没用的话，你以后可以用这两句来清空输入流。