使用然后删除 typedef 语句后 C 链表不再工作

Question

我这里有一个程序可以显示输入日期的下一天。这不是一个实用或高效的程序，我只是用它来测试我学到的概念。它工作了一段时间，但是在了解了 typedef 语句之后，我尝试将它们用于程序的结构“weekday”来声明该类型的变量，但随后删除了 typedef，因为它会产生错误(例如“不兼容的指针”和“不完整的定义”)。现在，即使我删除了 typedef 语句，文件仍会显示错误。最奇怪的是，如果我将所有代码复制并粘贴到一个新文件中，错误就不会出现。有谁知道可能导致错误的原因是什么？

#include <stdio.h>
#include <stdbool.h>

    struct weekday {
        char *ptrday;
        struct weekday *next;
    };

void matchtest(char *eday, struct weekday *head, struct weekday *cursor) {
    cursor=head;
    bool equalstring=true;

    while (cursor!=NULL) {
        char *p=cursor->ptrday;
        equalstring=true;

        while (*eday!='\0') {
            if (*eday!=*p) {
                equalstring=false;
                break;
            }
            ++eday; ++p;
        }
        if (equalstring==1) {
            printf("The next day of the week is %s\n", cursor->next->ptrday);
            break;
        }
        cursor=cursor->next;
    }
    if (equalstring==false)
        printf("The next day of the week is Sunday\n");
}

int main (void) {
    char enteredday[80], *ptreday=enteredday;
    struct weekday sunday, monday, tuesday, wednesday, thursday, friday, saturday;
    struct weekday *head=&sunday;
    struct weekday *cursor=NULL;

    sunday.ptrday="Sunday"; monday.ptrday="Monday"; tuesday.ptrday="Tuesday";
        wednesday.ptrday="Wednesday"; thursday.ptrday="Thursday";
        friday.ptrday="Friday"; saturday.ptrday="Saturday";

    sunday.next=&monday; monday.next=&tuesday; tuesday.next=&wednesday;
        wednesday.next=&thursday; thursday.next=&friday; friday.next=&saturday;
        saturday.next=NULL;


    printf("This is a test for the next day of the week.\n\n");
    printf("Enter a day ");

    scanf("%s", enteredday);
    matchtest(ptreday, head, cursor);

    return 0;
}

Answer 1

您遇到的一个问题是您修改了 eday在比较器函数中，但是如果比较失败，则无法将搜索设置为从字符串的开头开始，因为您不再知道字符串从哪里开始。这会影响“星期四”，它报告“下一天”是星期日。对于其他一些测试，这工作正常。由于困惑，周六后的第二天被报告为周日。

你还有一个问题，星期六后的那一天是未定义的。

这是有效的代码。我用 strcmp() (因此也是 <string.h>)进行比较。我转换了 while (cursor != NULL)循环到 do { ... } while (cursor != head);循环，使循环链表正常工作。我还打印读取的输入，并在正式输出中再次打印。这允许我使用 bash 进行测试的 Here String方便的符号 ( <<< string )。请注意，输入检查 EOF 并将输入字符串限制为 79 个字符，以防止缓冲区溢出(也称为堆栈溢出)。该函数的“游标”参数是不必要的；它在调用代码中设置为 null，但被调用函数立即将其设置为 head .因此该参数不再存在，但仍然需要该变量，因此它对函数来说是纯粹的本地变量。

代码

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

struct weekday
{
    char *ptrday;
    struct weekday *next;
};

static void matchtest(char *eday, struct weekday *head)
{
    struct weekday *cursor = head;

    do
    {
        if (strcmp(eday, cursor->ptrday) == 0)
        {
            printf("The next day of the week after %s is %s\n", eday, cursor->next->ptrday);
            return;
        }
        cursor = cursor->next;
    } while (cursor != head);

    printf("The 'day of the week' %s does not match any day of the week!\n", eday);
}

int main(void)
{
    char enteredday[80];
    struct weekday sunday, monday, tuesday, wednesday, thursday, friday, saturday;
    struct weekday *head = &sunday;

    sunday.ptrday = "Sunday";
    monday.ptrday = "Monday";
    tuesday.ptrday = "Tuesday";
    wednesday.ptrday = "Wednesday";
    thursday.ptrday = "Thursday";
    friday.ptrday = "Friday";
    saturday.ptrday = "Saturday";

    sunday.next = &monday;
    monday.next = &tuesday;
    tuesday.next = &wednesday;
    wednesday.next = &thursday;
    thursday.next = &friday;
    friday.next = &saturday;
    saturday.next = &sunday;

    printf("This is a test for the next day of the week.\n\n");
    printf("Enter a day ");

    if (scanf("%79s", enteredday) == 1)
    {
        printf("Got: [%s]\n", enteredday);
        matchtest(enteredday, head);
    }

    return 0;
}

示例运行

$ for d in Sunday Monday Tuesday Wednesday Thursday Friday Saturday Otherday; do ./nwd <<< $d; done
This is a test for the next day of the week.

Enter a day Got: [Sunday]
The next day of the week after Sunday is Monday
This is a test for the next day of the week.

Enter a day Got: [Monday]
The next day of the week after Monday is Tuesday
This is a test for the next day of the week.

Enter a day Got: [Tuesday]
The next day of the week after Tuesday is Wednesday
This is a test for the next day of the week.

Enter a day Got: [Wednesday]
The next day of the week after Wednesday is Thursday
This is a test for the next day of the week.

Enter a day Got: [Thursday]
The next day of the week after Thursday is Friday
This is a test for the next day of the week.

Enter a day Got: [Friday]
The next day of the week after Friday is Saturday
This is a test for the next day of the week.

Enter a day Got: [Saturday]
The next day of the week after Saturday is Sunday
This is a test for the next day of the week.

Enter a day Got: [Otherday]
The 'day of the week' Otherday does not match any day of the week!
$

如您所知，我不喜欢多次输入。

---

代码使用 typedef

也可以在数组中使用链表(尽管您也可以只使用日期名称数组)。

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

typedef struct Weekday Weekday;
struct Weekday
{
    char *ptrday;
    Weekday *next;
};

Weekday days_of_the_week[7] =
{
    { "Sunday",    &days_of_the_week[1] },
    { "Monday",    &days_of_the_week[2] },
    { "Tuesday",   &days_of_the_week[3] },
    { "Wednesday", &days_of_the_week[4] },
    { "Thursday",  &days_of_the_week[5] },
    { "Friday",    &days_of_the_week[6] },
    { "Saturday",  &days_of_the_week[0] },
};

static void matchtest(char *eday, Weekday *head)
{
    Weekday *cursor = head;

    do
    {
        if (strcmp(eday, cursor->ptrday) == 0)
        {
            printf("The next day of the week after %s is %s\n", eday, cursor->next->ptrday);
            return;
        }
        cursor = cursor->next;
    } while (cursor != head);

    printf("The 'day of the week' %s does not match any day of the week!\n", eday);
}

int main(void)
{
    char enteredday[80];

    printf("This is a test for the next day of the week.\n\n");
    printf("Enter a day ");

    if (scanf("%79s", enteredday) == 1)
    {
        printf("Got: [%s]\n", enteredday);
        matchtest(enteredday, days_of_the_week);
    }

    return 0;
}