C: 'else' if 条件满足时执行 - 结构

Question

我目前正在从一本书上学习 C，一个问题是编写一个程序来查找用户输入 2 次之间经过的时间，我已经使用 24 小时格式完成了。

问题是 main 函数中的 while 循环没有在应该执行的时候执行。即用户输入的时间超出范围（小于 0、大于 24 小时等）

while(error == 1) {
    printf("Enter time 1 in the format hours:mins:sec\n");
    scanf("%i:%i:%i", &time1.hours, &time1.minutes, &time1.seconds);

while 循环应该继续执行直到用户输入有效时间，但它只是被跳过。

我想原因可能是因为检查时间并将全局变量'error'赋值等于1或0的函数总是执行else语句（设置error = 0），即使如果陈述为真。

void checkTime (struct time a)
{   

if(a.hours < 0 || a.hours > 23) {
    printf("Please enter a valid figure for hours in the range 1 to 23\n");
    error = 1;
}

if(a.minutes < 0 || a.minutes > 59) {
    printf("Please enter a valid figure for minutes in the range of 1 to 59\n");
    error = 1;
}

if(a.seconds < 0 || a.seconds > 59) {
    printf("Please enter a valid figure for seconds in the range of 1 to 59\n");
    error = 1;

}

else {
    error = 0;
}
}

这是一个与结构工作方式有关的逻辑问题吗？

完整代码如下：

#include <stdio.h>

//Program to find the time elapsed between 2 times entered by user

int error = 0;

struct time 
{
int hours;
int minutes;
int seconds;
};

//Function to find elapsed time between the 2 arguments
struct time elapsed_time (struct time time1, struct time time2)
{

struct time eTime;


if (time2.hours > time1.hours)
    eTime.hours = time2.hours - time1.hours;
else
    eTime.hours = (24 - time1.hours) + time2.hours;


if (time2.minutes > time1.minutes)
    eTime.minutes = time2.minutes - time1.minutes;
else
    eTime.minutes = (60 - time1.minutes) + time2.minutes;


if (time2.seconds > time1.seconds)
    eTime.seconds = time2.seconds - time1.seconds;
else
    eTime.seconds = (60 - time1.seconds) + time2.seconds;


return eTime;
}

//Function to check if inputed times are within valid parameters
void checkTime (struct time a)
{   

if(a.hours < 0 || a.hours > 23) {
    printf("Please enter a valid figure for hours in the range 1 to 23\n");
    error = 1;
}

if(a.minutes < 0 || a.minutes > 59) {
    printf("Please enter a valid figure for minutes in the range of 1 to 59\n");
    error = 1;
}

if(a.seconds < 0 || a.seconds > 59) {
    printf("Please enter a valid figure for seconds in the range of 1 to 59\n");
    error = 1;

}

else {
    error = 0;
}

}


int main (void)
{   
struct time time1;
struct time time2;
struct time main_eTime;

printf("Enter time 1 in the format hours:mins:sec\n");
scanf("%i:%i:%i", &time1.hours, &time1.minutes, &time1.seconds);

checkTime(time1);

while(error == 1) {
    printf("Enter time 1 in the format hours:mins:sec\n");
    scanf("%i:%i:%i", &time1.hours, &time1.minutes, &time1.seconds);
}

printf("Enter time 2 in the format hours:mins:sec\n");
scanf("%i:%i:%i", &time2.hours, &time2.minutes, &time2.seconds);

checkTime(time2);

while(error == 1) {
    printf("Enter time 2 in the format hours:mins:sec\n");
    scanf("%i:%i:%i", &time2.hours, &time2.minutes, &time2.seconds);
}

main_eTime = elapsed_time(time1, time2);

printf("Elapsed time between time 1 and time 2 is %i:%i:%i\n", main_eTime.hours, 
        main_eTime.minutes, main_eTime.seconds);

return 0;

}

Answer 1

我认为您的分支条件有问题，尤其是在 checkTime 函数中。如果秒有效，最后的else条件将执行，它与您输入的小时和分钟是否正确无关。

改为：

void checkTime (struct time a)
{   
    if(a.hours < 0 || a.hours > 23) {
        printf("Please enter a valid figure for hours in the range 1 to 23\n");
        error = 1;
    }

    else if(a.minutes < 0 || a.minutes > 59) {
        printf("Please enter a valid figure for minutes in the range of 1 to 59\n");
        error = 1;
    }

    else if(a.seconds < 0 || a.seconds > 59) {
        printf("Please enter a valid figure for seconds in the range of 1 to 59\n");
        error = 1;
    }

    else {
        error = 0;
    }
}

只有当所有三个变量（小时、分钟和秒都有效）时，以上才会运行 else 阻塞。

Answer 2

您的代码结构是：

if (COND_1) {
    ...
}

if (COND_2) {
    ...
}

if (COND_3) {
    ...
}
else {
    ...
}

如果 COND_1、COND_2 或 COND_3 的 none 为真，您似乎认为 else 应该运行。但这不是它的工作原理：else 附加到前面的 if 语句，仅此而已。 else 块是否运行唯一重要的部分是 COND_3 条件。

你应该做的是：

if (COND_1) {
    ...
}
else if (COND_2) {
    ...
}
else if (COND_3) {
    ...
}
else {
    ...
}

通过以这种方式构建代码，我们确保只执行一个块。

... 或：

error = 0;

if(a.hours < 0 || a.hours > 23) {
    printf("Please enter a valid figure for hours in the range 1 to 23\n");
    error = 1;
}

if(a.minutes < 0 || a.minutes > 59) {
    printf("Please enter a valid figure for minutes in the range of 1 to 59\n");
    error = 1;
}

if(a.seconds < 0 || a.seconds > 59) {
    printf("Please enter a valid figure for seconds in the range of 1 to 59\n");
    error = 1;
}

通过在开始时设置 error = 0，您根本不需要 else 块。

C: 'else' if 条件满足时执行 - 结构

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