为什么我的程序不会标记字符？

Question

我正在使用 switch 语句制作这个程序，该语句将根据用户是否输入数字 0 - 10 来分配字母等级。如果用户输入的数字不是 0-10，则程序会输出一条错误消息并显示用户重新输入。但是，如果用户输入一个字符，程序将在默认情况下循环。我希望它从默认情况下输出一次错误消息，如果他们输入一个字符，让他们重新输入。我不确定为什么在输入字符时它会循环默认大小写。

#include <stdio.h>

int main()
{
    int grade;
    int r;
    while((r = scanf("%i", &grade)) != EOF)
    {
        switch(grade)
            {
                case 10:
                case 9:
                    printf("Your grade is an A\n");
                    break;
                case 8:
                    printf("Your grade is a B\n");
                    break;
                case 7:
                    printf("Your grade is a C\n");
                    break;
                case 6:
                    printf("Your grade is a D\n");
                    break;
                case 5:
                case 4:
                case 3:
                case 2:
                case 1:
                case 0:
                    printf("Your grade is an F\n");
                    break;
                default:
                    printf("Invalid score, please re-enter\n");
            }
    }

    return 0;
}

Answer 1

尝试类似：

#include <stdio.h>

int main()
{
    int grade;
    int r=0;
    while(r != 1)
    {
    scanf("%i", &grade);
        switch(grade)
            {
                case 10:
                case 9:
                    printf("Your grade is an A\n");
                    r=1
                    break;
                case 8:
                    printf("Your grade is a B\n");
                    r=1
                    break;
                case 7:
                    printf("Your grade is a C\n");
                    r=1
                    break;
                case 6:
                    printf("Your grade is a D\n");
                    r=1
                    break;
                case 5:
                case 4:
                case 3:
                case 2:
                case 1:
                case 0:
                    printf("Your grade is an F\n");
                    r=1
                    break;
                default:
                    printf("Invalid score, please re-enter\n");
                    break;
            }
    }

    return 0;
}

Answer 2

这将清除无效输入的输入缓冲区并允许重试。

#include <stdio.h>

int main()
{
    int grade;
    int r;
    while((r = scanf("%i", &grade)) != EOF)
    {
        if ( r != 1) {//r == 1 is successful input of integer
            grade = -1;//reset grade on invalid input
        }
        switch(grade)
        {
            case 10:
            case 9:
                printf("Your grade is an A\n");
                break;
            case 8:
                printf("Your grade is a B\n");
                break;
            case 7:
                printf("Your grade is a C\n");
                break;
            case 6:
                printf("Your grade is a D\n");
                break;
            case 5:
            case 4:
            case 3:
            case 2:
            case 1:
            case 0:
                printf("Your grade is an F\n");
                break;
            default:
                printf("Invalid score, please re-enter\n");
                while ( getchar() != '\n');//clear input buffer
        }
    }
    return 0;
}

Answer 3

你的代码总是循环的原因是因为除了杀死程序之外没有办法退出你的while。请记住，break 只会跳出最里面的switch 或循环。

打破多个级别的最简洁方法是使用标志。一种做你想做的事情的方法是这样的：

bool valid_grade = false;
while(!valid_grade && (r = scanf("%i", &grade)) != EOF)
{
    valid_grade = true;
    switch(grade)
        {
            case 10:

             // unchanged from your code

            default:
                valid_grade = false;
                printf("Invalid score, please re-enter\n");
        }
}