下面以一個電子英漢詞典程序（以下簡稱電子詞典）為例，應用雙向循環鏈表。分離數據結構，可以使邏輯代碼獨立於數據結構操作代碼，程序結構更清晰，代碼更簡潔；電子詞典的增、刪、查、改操作分別對應於鏈表的插入、刪除、查找、查找和獲取鏈表元素操作。

　　在程序初始化時，除了初始化鏈表，還要將保存在文件中的詞庫加載到鏈表中：

1 void dict_init() {
2     list = linkedlist_new();
3 
4     dict_load();
5     printf("Welcome.");
6 }

函數dict_load()實現如下：

 1 static void dict_load() {
 
 2     FILE * fp;
 3     struct Word word;
 4 
 5     while (!(fp = fopen(PATH, "rb"))) {
 6         fp = fopen(PATH, "wb");
 7         fclose(fp);
 8     }
 9     assert(fp);
10 
11     fread(&word, sizeof(struct Word), 1, fp);
12     while (!feof(fp)) {
13         linkedlist_insert(list, TRAVELDIR_BACKWARD, 1
 
, word);
14         fread(&word, sizeof(struct Word), 1, fp);
15     }
16 
17     fclose(fp);
18 }

函數feof()應先讀後判斷，所以在進入循環之前應先讀一次。

　　當然，在程序結束前，也要將鏈表中的詞組保存到文件中，函數dict_store()實現如下：

 1 static void dict_store() {
 2     FILE * fp;
 3     const int count = linkedlist_length(list);
 4 
 5     assert(fp = fopen(PATH, "
 
wb"));
 6     for (int i = 0; i < count; i++) {
 7         fwrite(linkedlist_get(list, TRAVELDIR_FORWARD, i + 1),
 8                 sizeof(struct Word), 1, fp);
 9     }
10 
11     fclose(fp);
12 }

　　一個英漢詞組包含一個英文，一個中文，可以把它定義為一個結構體，並讓它作為鏈表節點數據域的數據類型；修改linkedlist.h中LinkedlistData的相關定義：

1 typedef struct Word {
2     string eng;
3     string chn;
4 } LinkedListData;

C語言中沒有string類型，可以使用定長的字符數組來定義它；再定義一個函數mygets()用來代替scanf()函數，以確保輸入的內容不會超出緩沖區：

 1 #define MAX_STR_LEN 8
 2 typedef char string[MAX_STR_LEN];
 3 
 4 void mygets(char * s)
 5 {
 6     __fpurge(stdin);
 7     fgets(s, MAX_STR_LEN, stdin);
 8     while (*s++) {
 9         *s = *s == ‘\n‘ ? 0 : *s;
10     }
11 }

在Linux下，使用__fpurge()函數來代替Windows下的fflush()函數清空輸入流。

　　接下來就是電子詞典的增、刪、查、改操作：

 1 void dict_add(const char * eng) {
 2     struct Word word;
 3     strcpy(word.eng, eng);
 4 
 5     printf("The word does not exist, add it?\ny/n>");
 6     if (__fpurge(stdin), getchar() == ‘y‘) {
 7         printf("Ok, what does it mean?\n>");
 8         mygets(word.chn);
 9 
10         linkedlist_insert(list, TRAVELDIR_BACKWARD, 1, word);
11         printf("The word is existed now.\n");
12     }
13 }
14 
15 void dict_delete() {
16     int location;
17     struct Word word;
18 
19     printf("What word do you wanna delete?\n>");
20     mygets(word.eng);
21 
22     if ((location = linkedlist_locate(list, TRAVELDIR_FORWARD, word, dict_cmp))
23             != -1) {    // found
24         struct Word * pWord = linkedlist_get(list, TRAVELDIR_FORWARD, location);
25 
26         printf("Delete: %s %s\nAre you sure?\ny/n>", pWord->eng, pWord->chn);
27         if (__fpurge(stdin), getchar() == ‘y‘) {
28             linkedlist_delete(list, TRAVELDIR_FORWARD, location);
29             printf("The word is deleted now.\n");
30         }
31     } else {            // not found
32         printf("The word does not exist.\n");
33     }
34 }
35 
36 void dict_search(const char * eng) {
37     int location;
38     struct Word word;
39     strcpy(word.eng, eng);
40 
41     if ((location = linkedlist_locate(list, TRAVELDIR_FORWARD, word, dict_cmp))
42             == -1) {    // not found
43         dict_add(eng);
44     } else {            // found
45         printf("%s\n", linkedlist_get(list, TRAVELDIR_FORWARD, location)->chn);
46     }
47 }
48 
49 void dict_modify() {
50     int location;
51     struct Word word;
52 
53     printf("What word do you wanna modify?\n>");
54     mygets(word.eng);
55 
56     if ((location = linkedlist_locate(list, TRAVELDIR_FORWARD, word, dict_cmp))
57             != -1) {    // found
58         struct Word * pWord = linkedlist_get(list, TRAVELDIR_FORWARD, location);
59 
60         printf("Ok, what does it mean?\n>");
61         mygets(pWord->chn);
62         printf("The word is modified now.\n");
63     } else {            // not found
64         printf("The word does not exist.\n");
65     }
66 }

dict_cmp()函數作為參數傳遞給linkedlist_locate()函數，用以比較詞組是否相同，相同則返回0，它的實現如下：

1 int dict_cmp(const void * s1, const void * s2) {
2     return strcmp(((LinkedListData *) s1)->eng, ((LinkedListData *) s2)->eng);
3 }

還需要一個函數來組織這些子函數的調用：

 1 void dict_show() {
 2     while (1) {
 3         string str;
 4         printf("\n>");
 5         mygets(str);
 6 
 7         if (!strcmp(str, "quit;")) {
 8             dict_store();
 9             linkedlist_destory(&list);
10             printf("Bye.\n");
11             return;
12         } else if (!strcmp(str, "delete;")) {
13             dict_delete();
14         } else if (!strcmp(str, "modify;")) {
15             dict_modify();
16         } else {
17             dict_search(str);
18         }
19     }
20 }

　　最後，編寫主函數，電子詞典就大功告成了！

雙向循環鏈表（C語言描述）（四）