C语言中fgets输入字符串后strcmp匹配失败的问题求助

我正在开发一款单位转换器，编写了Distance()、time()等不同类型的转换函数。在Distance函数中，我使用fgets()获取cm、mm等多字符单位输入，期望通过strcmp()匹配不同的单位组合来执行对应的转换计算，但程序始终跳转到提示“Please enter valid conversion units”，无法执行正确的计算逻辑。我是编程初学者，可能是细节错误，请求帮助。以下是我的代码：

int Distance() {
    char from[40], to[40];
    double val;
    double result;

    printf("Enter what you want to convert from: ");
    fflush(stdin);
    fgets(from, 40, stdin);

    printf("Enter what you want to convert to: \n");
    fflush(stdin);
    fgets(to, 40, stdin);

    printf("Enter the numerical value of conversion: \n");
    scanf("%lf", &val);

    (strcmp(from, "cm") == 0 && strcmp(to, "mm") == 0) ? printf("Answer is %lf MilliMeters", val / 10) : 
    (strcmp(from, "mm") == 0 && strcmp(to, "cm") == 0) ? printf("Answer is %lf CentiMeters", val * 10) :
    (strcmp(from, "cm") == 0 && strcmp(to, "km") == 0) ? printf("Answer if %lf KiloMeters", val * 100000) : 
    (strcmp(from, "km") == 0 && strcmp(to, "cm") == 0) ? printf("Answer is %lf CentiMeters", val / 100000) : 
    (strcmp(from, "mm") == 0 && strcmp(to, "km") == 0) ? printf("Anser is %lf KiloMeters", val  * 1000000) : 
    (strcmp(from, "km") == 0 && strcmp(to, "mm") == 0) ? printf("Answer is %lf  MilliMeters", val / 1000000) : 
    printf("Please enter valid conversion units");
}

1. 核心问题：fgets()读取的字符串包含换行符 #

fgets()会把用户输入时按下的回车键（\n）也读取到字符串末尾，比如输入cm后按回车，from数组里实际存储的是"cm\n"，而strcmp(from, "cm")对比的是不带换行的字符串，自然不相等，导致所有匹配条件失效。

修复方法：读取后去除字符串末尾的换行符：

// 处理from的换行符
size_t len = strlen(from);
if (len > 0 && from[len-1] == '\n') {
    from[len-1] = '\0';
}
// 同理处理to的换行符
len = strlen(to);
if (len > 0 && to[len-1] == '\n') {
    to[len-1] = '\0';
}

2. 禁用fflush(stdin) #

fflush(stdin)是未定义行为，标准C仅允许对输出流使用fflush，对输入流使用会引发不可预测的问题。若要清除输入缓冲区残留字符，改用循环读取：

// 替代fflush(stdin)的正确写法
int c;
while ((c = getchar()) != '\n' && c != EOF);

3. 修正转换公式与拼写错误 #

代码里存在多处错误：

• 单位转换公式写反：cm转mm应是val * 10（1cm=10mm），你写的是val / 10；cm转km应是val / 100000，你写的是val * 100000；mm转km应是val / 1000000，你写的是val * 1000000
• 拼写错误：Answer if应为Answer is，Anser应为Answer

修正后的完整代码 #

#include <stdio.h>
#include <string.h>

int Distance() {
    char from[40], to[40];
    double val;

    printf("Enter what you want to convert from: ");
    // 清除输入缓冲区残留
    int c;
    while ((c = getchar()) != '\n' && c != EOF);
    fgets(from, 40, stdin);
    // 去掉换行符
    size_t len = strlen(from);
    if (len > 0 && from[len-1] == '\n') {
        from[len-1] = '\0';
    }

    printf("Enter what you want to convert to: ");
    while ((c = getchar()) != '\n' && c != EOF);
    fgets(to, 40, stdin);
    len = strlen(to);
    if (len > 0 && to[len-1] == '\n') {
        to[len-1] = '\0';
    }

    printf("Enter the numerical value of conversion: ");
    scanf("%lf", &val);

    if (strcmp(from, "cm") == 0 && strcmp(to, "mm") == 0) {
        printf("Answer is %.2lf MilliMeters\n", val * 10);
    } else if (strcmp(from, "mm") == 0 && strcmp(to, "cm") == 0) {
        printf("Answer is %.2lf CentiMeters\n", val / 10);
    } else if (strcmp(from, "cm") == 0 && strcmp(to, "km") == 0) {
        printf("Answer is %.6lf KiloMeters\n", val / 100000.0);
    } else if (strcmp(from, "km") == 0 && strcmp(to, "cm") == 0) {
        printf("Answer is %.0lf CentiMeters\n", val * 100000.0);
    } else if (strcmp(from, "mm") == 0 && strcmp(to, "km") == 0) {
        printf("Answer is %.9lf KiloMeters\n", val / 1000000.0);
    } else if (strcmp(from, "km") == 0 && strcmp(to, "mm") == 0) {
        printf("Answer is %.0lf MilliMeters\n", val * 1000000.0);
    } else {
        printf("Please enter valid conversion units\n");
    }
    return 0;
}

内容的提问来源于stack exchange，提问作者sumiicode.exe