C语言测验游戏程序BUG排查：正确输入仍提示错误

开发的测验游戏程序，输入正确答案时始终输出“Wrong!”，无论答案对错都显示错误。以下是第一道题的运行示例：

QUIZ GAME !
------------------------------------
.1 What year did the C language debut ? :
A. 1969
B. 1972
C. 1965
D. 1999
guess: b
Wrong!

对应的代码：

#include <stdio.h>
#include <ctype.h>

int main()
{
    char questions[][100] = {".1 What year did the C language debut ? : ",
                             ".2 Who is credited with creating C ? : ",
                             ".3 what is the predecessor of C ? : "};

    char options[][100] = {"A. 1969", "B. 1972", "C. 1965", "D. 1999",
                           "A. Dennis Ritchie", "B. Nikola Teala", "C. John Carmack", "D. Doc Brown",
                           "A. Objective C", "B. B", "C. C++", "D. C#"};

    char answers[3] = {'B', 'A', 'B'};
    int numberOFquestions = sizeof(questions) / sizeof(questions[0]);

    char guess;
    int score;
    score = 0;

    printf("QUIZ GAME !\n");

    for (int i = 0; i < numberOFquestions; i++)
    {
        printf("------------------------------------\n");
        printf("%s\n", questions[i]);

        for (int j = (i * 4); j < (i * 4) + 4; j++)
        {
            printf("%s\n", options[j]);
        }

        printf("guess: ");
        scanf("%c", &guess);
        scanf("%c"); // second scanf helps clear \n from the input buffer

        // accept lowercase answers
        guess = toupper(guess);

        if (guess == answers[i])
        {
            printf("Correct!\n");
            score++;
        }
        else
        {
            printf("Wrong!\n");
        }
    }

    printf("**********************\n");
    printf("FINAL SCORE: %d/%d\n", score, numberOFquestions);

    return 0;
}

问题根源 #

错误不在你怀疑的for循环中，而是输入处理的scanf调用存在问题：

• 第二行scanf("%c");缺少指针参数，属于未定义行为，无法正确清除输入缓冲区的换行符。
• 第一行scanf("%c", &guess)会读取输入缓冲区中的任意字符（包括换行符），如果缓冲区残留换行符，会导致guess被赋值为换行符，经toupper转换后仍与答案字符不匹配，最终输出“Wrong!”。

修复方案 #

方案1：优化scanf格式字符串（推荐） #

删除错误的第二行scanf，在第一行scanf的%c前添加一个空格，让scanf自动跳过所有空白字符（包括换行、空格、制表符）：

printf("guess: ");
scanf(" %c", &guess); // 开头的空格跳过空白字符

方案2：正确使用第二个scanf #

定义一个临时字符变量，用于接收换行符，确保scanf调用参数合法：

printf("guess: ");
char dummy;
scanf("%c", &guess);
scanf("%c", &dummy); // 读取输入后的换行符

修复后的完整代码 #

#include <stdio.h>
#include <ctype.h>

int main()
{
    char questions[][100] = {".1 What year did the C language debut ? : ",
                             ".2 Who is credited with creating C ? : ",
                             ".3 what is the predecessor of C ? : "};

    char options[][100] = {"A. 1969", "B. 1972", "C. 1965", "D. 1999",
                           "A. Dennis Ritchie", "B. Nikola Teala", "C. John Carmack", "D. Doc Brown",
                           "A. Objective C", "B. B", "C. C++", "D. C#"};

    char answers[3] = {'B', 'A', 'B'};
    int numberOFquestions = sizeof(questions) / sizeof(questions[0]);

    char guess;
    int score = 0;

    printf("QUIZ GAME !\n");

    for (int i = 0; i < numberOFquestions; i++)
    {
        printf("------------------------------------\n");
        printf("%s\n", questions[i]);

        for (int j = (i * 4); j < (i * 4) + 4; j++)
        {
            printf("%s\n", options[j]);
        }

        printf("guess: ");
        scanf(" %c", &guess);

        guess = toupper(guess);

        if (guess == answers[i])
        {
            printf("Correct!\n");
            score++;
        }
        else
        {
            printf("Wrong!\n");
        }
    }

    printf("**********************\n");
    printf("FINAL SCORE: %d/%d\n", score, numberOFquestions);

    return 0;
}

内容的提问来源于stack exchange，提问作者AmineDELTA