双向循环链表（C语言描述）（四）

　　下面以一个电子英汉词典程序（以下简称电子词典）为例，应用双向循环链表。分离数据结构，可以使逻辑代码独立于数据结构操作代码，程序结构更清晰，代码更简洁；电子词典的增、删、查、改操作分别对应于链表的插入、删除、查找、查找和获取链表元素操作。

　　在程序初始化时，除了初始化链表，还要将保存在文件中的词库加载到链表中：

void dict_init() {
    list = linkedlist_new();

    dict_load();
    printf("Welcome.");
}

函数dict_load()实现如下：

static void dict_load() {
    FILE * fp;
    struct Word word;

    while (!(fp = fopen(PATH, "rb"))) {
        fp = fopen(PATH, "wb");
        fclose(fp);
    }
    assert(fp);

    fread(&word, sizeof(struct Word), 1, fp);
    while (!feof(fp)) {
        linkedlist_insert(list, TRAVELDIR_BACKWARD, 1, word);
        fread(&word, sizeof(struct Word), 1, fp);
    }

    fclose(fp);
}

函数feof()应先读后判断，所以在进入循环之前应先读一次。

　　当然，在程序结束前，也要将链表中的词组保存到文件中，函数dict_store()实现如下：

static void dict_store() {
    FILE * fp;
    const int count = linkedlist_length(list);

    assert(fp = fopen(PATH, "wb"));
    for (int i = 0; i < count; i++) {
        fwrite(linkedlist_get(list, TRAVELDIR_FORWARD, i + 1),
                sizeof(struct Word), 1, fp);
    }

    fclose(fp);
}

 　　一个英汉词组包含一个英文，一个中文，可以把它定义为一个结构体，并让它作为链表节点数据域的数据类型；修改linkedlist.h中LinkedlistData的相关定义：

typedef struct Word {
    string eng;
    string chn;
} LinkedListData;

C语言中没有string类型，可以使用定长的字符数组来定义它；再定义一个函数mygets()用来代替scanf()函数，以确保输入的内容不会超过定义的字符串长度：

#define MAX_STR_LEN 8
typedef char string[MAX_STR_LEN];

void mygets(char * s)
{
    __fpurge(stdin);
    fgets(s, MAX_STR_LEN, stdin);
    while (*s++) {
        *s = *s == '
' ? 0 : *s;
    }
}

在Linux下，使用__fpurge()函数来代替Windows下的fflush()函数清空输入流。

 　　接下来就是电子词典的增、删、查、改操作：

void dict_add(const char * eng) {
    struct Word word;
    strcpy(word.eng, eng);

    printf("The word does not exist, add it?
y/n>");
    if (__fpurge(stdin), getchar() == 'y') {
        printf("Ok, what does it mean?
>");
        mygets(word.chn);

        linkedlist_insert(list, TRAVELDIR_BACKWARD, 1, word);
        printf("The word is existed now.
");
    }
}

void dict_delete() {
    int location;
    struct Word word;

    printf("What word do you wanna delete?
>");
    mygets(word.eng);

    if ((location = linkedlist_locate(list, TRAVELDIR_FORWARD, word, dict_cmp))
            != -1) {    // found
        struct Word * pWord = linkedlist_get(list, TRAVELDIR_FORWARD, location);

        printf("Delete: %s %s
Are you sure?
y/n>", pWord->eng, pWord->chn);
        if (__fpurge(stdin), getchar() == 'y') {
            linkedlist_delete(list, TRAVELDIR_FORWARD, location);
            printf("The word is deleted now.
");
        }
    } else {            // not found
        printf("The word does not exist.
");
    }
}

void dict_search(const char * eng) {
    int location;
    struct Word word;
    strcpy(word.eng, eng);

    if ((location = linkedlist_locate(list, TRAVELDIR_FORWARD, word, dict_cmp))
            == -1) {    // not found
        dict_add(eng);
    } else {            // found
        printf("%s
", linkedlist_get(list, TRAVELDIR_FORWARD, location)->chn);
    }
}

void dict_modify() {
    int location;
    struct Word word;

    printf("What word do you wanna modify?
>");
    mygets(word.eng);

    if ((location = linkedlist_locate(list, TRAVELDIR_FORWARD, word, dict_cmp))
            != -1) {    // found
        struct Word * pWord = linkedlist_get(list, TRAVELDIR_FORWARD, location);

        printf("Ok, what does it mean?
>");
        mygets(pWord->chn);
        printf("The word is modified now.
");
    } else {            // not found
        printf("The word does not exist.
");
    }
}

dict_cmp()函数作为参数传递给linkedlist_locate()函数，用以比较词组是否相同，相同则返回0，它的实现如下：

int dict_cmp(const void * s1, const void * s2) {
    return strcmp(((LinkedListData *) s1)->eng, ((LinkedListData *) s2)->eng);
}

还需要一个函数来组织这些子函数的调用：

void dict_show() {
    while (1) {
        string str;
        printf("
>");
        mygets(str);

        if (!strcmp(str, "quit;")) {
            dict_store();
            linkedlist_destory(&list);
            printf("Bye.
");
            return;
        } else if (!strcmp(str, "delete;")) {
            dict_delete();
        } else if (!strcmp(str, "modify;")) {
            dict_modify();
        } else {
            dict_search(str);
        }
    }
}

　　最后，编写主函数，电子词典就大功告成了！