Question

我使用以下代码将节点添加到双链表并按照word按字母顺序排列它们。节点由char * word，struct NODES * prev和struct NODES * next组成。我在阅读file testing a后，列表看起来像NULL <- a <-> file <-> testing -> NULL，在word = c之前添加包含c a的节点而不是a，我遇到了问题在file和"Add: Placing c before list head c"之间。该函数打印c < a，似乎正在评估a。但即使评估不正确，我也不知道在进行任何节点操作之前它是如何替换NODES * arg的。如果有人知道什么可能导致这个问题，我会很感激这个建议。附：传入arg -> next == NULL; arg -> prev == NULL; arg -> word != NULL;始终采用list -> prev的形式，但如果尚未添加任何节点，则列表可以将所有字段设为NULL，int addToList(struct NODES * list, struct NODES * arg){ fprintf(stderr,"Add: Adding %s\n", arg->word); if(list->word == NULL){ list->word = (char *)malloc(strlen(arg->word)); strcpy(list->word, arg->word); list->next = NULL; list->prev = NULL; fprintf(stderr,"Add: Head %s\n", list->word); return 2; } struct NODES * abc = list; while(abc->word != NULL){ if(strcmp(abc->word, arg->word)<0){ fprintf(stderr, "abc %s < arg %s", abc->word, arg->word); if (abc->next != NULL) abc = abc->next; else{ abc->next = malloc(sizeof(NODE)); abc->next->prev = abc; abc = abc->next; abc->next = NULL; abc->word = NULL; } } else if(abc == list){ fprintf(stderr, "Add: Placing %s before list head %s\n", arg->word, list->word); arg->next = list; list->prev = arg; arg->prev = NULL; list = arg; fprintf(stderr, "Add: Placed %s before %s\n", list->word, list->next->word); return 3; } else{ fprintf(stderr, "Add: Placing %s between %s and %s\n", arg->word, abc->word, abc->next->word); arg->next = abc; arg->prev = abc->prev; if(abc->prev != NULL) abc->prev->next = arg; abc->prev = arg; fprintf(stderr, "Added %s after %s and before %s\n", arg->word, arg->prev, arg->next->word); return 1; } } abc->word = (char *)malloc(strlen(arg->word)); strcpy(abc->word, arg->word); fprintf(stderr, "Added %s after %s and before %s\n", abc->word, abc->prev->word, abc->next); return 1; }在调用函数时应始终为NULL该函数终止。

int addToList(struct NODES ** list, struct NODES * arg){
fprintf(stderr,"Add: Adding %s current head %s\n", arg -> word, (*list)->word);
if((*list) -> word == NULL){
    (*list) -> word = malloc(strlen(arg->word)+1);
    strcpy((*list) -> word, arg -> word);
    (*list) -> next = NULL;
    (*list) -> prev = NULL;
    fprintf(stderr,"Add: Head %s\n", (*list) -> word);
    return 2;
}
struct NODES * abc = (*list);
//while arg > abc
fprintf(stderr,"Comparing %s and %s\n", abc->word,arg->word);
while(strcmp(abc->word, arg->word)<0){
    fprintf(stderr,"Comparing %s and %s\n", abc->word,arg->word);
    if (abc -> next == NULL)
        break;
    abc = abc -> next;
}
if (abc == (*list)){
    if(!(strcmp(abc->word, arg->word)<0)){
        arg -> next = abc;
        arg -> prev = NULL;
        abc -> prev = arg;
        *list = arg;
    }
    else{
        abc -> next = arg;
        arg -> prev = abc;
        abc -> next = NULL;
    }
    return 5;
}
if(abc -> next != NULL){
    fprintf(stderr, "Inserting %s between %s and %s\n", arg -> word, abc->prev->word,abc->word); 
    arg -> next = abc;
    arg -> prev = abc -> prev;
    arg -> prev -> next = arg;
    abc -> prev = arg;
    fprintf(stderr, "Added %s before %s and after %s\n", arg->word, arg->prev->word,arg->next->word);
    return 3;
}
return 0
}

更新以反映建议：

{{1}}

Answer 1

函数收到的list参数是调用者具有的列表指针的副本。要返回修改后的列表指针，函数可能如下所示：

int addToList(struct NODES ** list, struct NODES * arg)

它将被称为这样的东西：

result = addToList(&list, arg);

该函数将提供一个像这样的新列表指针

*list = arg;

您目前拥有的所有列表访问权限都是间接的一步

if(list->word == NULL)

会变成

if((*list)->word == NULL)

<强>更新

尝试这个简化的代码，我发现这比让你的头脑更容易。

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

struct NODES {
    struct NODES *prev;
    struct NODES *next;
    char *word;
};

void showList(struct NODES * list) {
    while (list) {
        if (list->prev)
            printf("%-10s", list->prev->word);
        else
            printf("%-10s", "NULL");
        printf(" <- %-10s -> ", list->word);
        if (list->next)
            printf("%-10s", list->next->word);
        else
            printf("%-10s", "NULL");
        printf("\n");
        list = list->next;
    }
}

int addToList(struct NODES ** list, char *word){
    struct NODES *wptr, *lptr = *list, *pptr = NULL;
    if ((wptr = malloc(sizeof(struct NODES))) == NULL)
        return -1;
    wptr->prev = NULL; 
    wptr->next = NULL;
    if ((wptr->word = strdup(word)) == NULL)
        return -2;

    if (lptr == NULL) {
        *list = wptr;                 // first list item
        return 0;
    }

    while (lptr) {
        if (strcmp(word, lptr->word) <= 0) {
            wptr->next = lptr;        // insert before current node
            wptr->prev = pptr;
            if (pptr)
                pptr->next = wptr;
            else
                *list = wptr;
            lptr->prev = wptr;
            return 1;
        }
        pptr = lptr;
        lptr = lptr->next;
    }

    wptr->prev = pptr;                // insert at tail
    pptr->next = wptr;
    return 2;
}

int main()
{
    struct NODES *list = NULL;
    addToList(&list, "one");
    addToList(&list, "two");
    addToList(&list, "three");
    addToList(&list, "four");
    addToList(&list, "five");
    showList(list);
    return 0;
}

节目输出：

NULL       <- five       -> four
five       <- four       -> one
four       <- one        -> three
one        <- three      -> two
three      <- two        -> NULL

Answer 2

我怀疑你的问题在这里：

list->word = (char *)malloc(strlen(arg->word));
strcpy(list->word, arg->word);

由于您没有为终止空字符分配空间，因此稍后对strcmp的调用将超出分配的缓冲区。这可能有各种各样的行为，很可能也是你看到的行为。

另外，drop the (char *) cast，它可能会隐藏您的代码的其他问题。

C：比较字符串

2 个答案: