Source

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    float course_1=-1,course_2=-1,result;
    while(1){
        printf("Please enter the score of the first course:");
        scanf("%f",&course_1);
            if(course_1 < 0 || course_1 > 100){
                    printf("Please enter the correct score!\n");
                    continue;
            }
        printf("\nPlease enter the score of the second course:");
        scanf("%f",&course_2);
            if(course_2 < 0 || course_2 > 100){
                    printf("Please enter the correct score!\n");
                    continue;
            }
            break;
    }       
    result = (course_1 + course_2)/2;
    printf("\nThe average score is:%.1f",result);
}

输出

如果输入的值为非字符时结果为:

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Please enter the score of the first course:20

Please enter the score of the second course:30

The average score is:25.0

如果输入的值为字符时结果为:

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Please enter the score of the first course:a

Please enter the score of the first course:Please enter the correct score!
Please enter the score of the first course:Please enter the correct score!
Please enter the score of the first course:Please enter the correct score!
...

原因

因为scanf里占位符是%d,也就是录入缓冲区中的整形数字. 当缓冲区有字符,scanf便会读取,但是不是整形就不会录入.

第一次输入了a,是非整形,scanf略过了但是又没有清空缓冲区,第二次便不会要求我输入 因为缓冲区里有字符,又略过,如此便是死循环

解决

可以使用fflush(stdin)来清空缓冲区.

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   float course_1=-1,course_2=-1,result;
    while(1){
        printf("Please enter the score of the first course:");
        scanf("%f",&course_1);
        fflush(stdin)
            if(course_1 < 0 || course_1 > 100){
                    printf("Please enter the correct score!\n");
                    continue;
            }
        printf("\nPlease enter the score of the second course:");
        scanf("%f",&course_2);
        fflush(stdin)
            if(course_2 < 0 || course_2 > 100){
                    printf("Please enter the correct score!\n");
                    continue;
            }
            break;
    }       
    result = (course_1 + course_2)/2;
    printf("\nThe average score is:%.1f",result);
}

这时候程序就正常了.