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), , fp);
     while (!feof(fp)) {
         linkedlist_insert(list, TRAVELDIR_BACKWARD, , word);
         fread(&word, sizeof(struct Word), , 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 = ; i < count; i++) {
         fwrite(linkedlist_get(list, TRAVELDIR_FORWARD, i + ),
                 sizeof(struct Word), , 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 == '\n' ?  : *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?\ny/n>");
     if (__fpurge(stdin), getchar() == 'y') {
         printf("Ok, what does it mean?\n>");
         mygets(word.chn);

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

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

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

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

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

 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))
             == -) {    // not found
         dict_add(eng);
     } else {            // found
         printf("%s\n", linkedlist_get(list, TRAVELDIR_FORWARD, location)->chn);
     }
 }

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

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

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

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

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 () {
         string str;
         printf("\n>");
         mygets(str);

         if (!strcmp(str, "quit;")) {
             dict_store();
             linkedlist_destory(&list);
             printf("Bye.\n");
             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！