<分区>
这很乱，所以如果您没有时间，请不要打扰。我已尽力解释我的代码中的每个函数如何发挥最佳作用。所以我的问题是函数 Q_drop 我无法正常工作，其他一切(代码的所有其他部分)都运行良好。
所以函数 Q_drop 需要做以下事情:
实现函数 Q_drop，从队列中删除给定的特定成员(由 ID 标识)，并释放为其分配的内存。这个队列成员可以位于队列中的任何位置，并且在移除之后，队列应该保持可操作，即所有指针都应该相应地更新。并且 My_Queue->last 指针需要指向最后一个成员。
如果找到并删除了具有匹配 ID 的物种，该函数返回 1；如果没有匹配且没有删除任何内容，则返回 0。每次调用应该只删除一个队列成员:如果一个物种多次出现在队列中，则只删除第一个条目。
所以我的问题是，如果匹配的节点是最后一个节点，我如何将指针移动到前一个节点？我收到的检查器错误是“My_Queue->last pointer does not point to the last member”。
我的代码:
struct animal {
char id[7]; // animal ID: 6 characters + '\0'
char *species; // species of animal, allocated dynamically
struct animal *next; // next animal in linked list
};
/* For storing the first and last item in linked list
* If list is empty, both <first> and <last> are NULL
* If list has one element, <first> and <last> point to the same place
*/
struct my_queue {
struct animal *first;
struct animal *last;
};
const struct zoo {
const char *id;
const char *species;
} animals[] = {
{"123456", "Dog" },
{"234567", "Bear" },
{"777777", "Pink Fairy Armadillo" },
{"aaaaaaaaa", "Chlamyphorus truncatus" },
{"666666", "Mosquito" }
};
/* Drops the given animal with animal ID <id> from queue <q>.
* Only the first instance with matching ID is dropped.
*
* Returns: 1 if something was removed,
* 0 if nothing was removed, i.e., matching animal ID was not found.
*/
int Q_drop(My_Queue *q, const char *id)
{
struct animal *prev = NULL;
struct animal *curr = q->first;
while (curr != NULL) {
if (!strcmp(curr->id, id)) {
if (prev == NULL) {
q->first = curr->next;
}
if(curr->next == NULL){
//here I cant figure out what to write here tried q->last=prev
}
else {
prev->next = curr->next;
}
free(curr->species);
free(curr);
return 1;
}
else {
prev = curr;
curr = curr->next;
}
}
return 0;
}
/* Allocates and initializes a new queue.
*
* Returns: pointer to an empty queue
*/
My_Queue *Q_init(void)
{
My_Queue *q = calloc(1, sizeof(My_Queue));
return q;
}
/* Adds a new member with given animal ID <id> and species <species> to the
* end of queue <q>.
*
* Returns: 1 if addition was successful, 0 if addition failed. Addition
fails,
* for example if animal ID has more than 6 characters.
*/
int Q_enqueue(My_Queue *q, const char *id, const char *species)
{
int n = strlen(id);
if (n < 7){
struct animal *new = malloc(sizeof(struct animal));
strcpy(new->id, id);
new->species = malloc(strlen(species) + 1);
strcpy(new->species, species);
new->next = NULL;
if(q->first == NULL && q->last == NULL){
q->first = new;
q->last = new;
}
else{
q->last->next = new;
q->last = new;
}
return 1;
}
else{
return 0;
}
}
int main()
{
/* testing exercise. Feel free to modify this function */
My_Queue *q = Q_init();
for (unsigned int i = 0; i < sizeof(animals) / sizeof(struct zoo); i++)
{
if (Q_enqueue(q, animals[i].id, animals[i].species))
printf("i = %d, firstspecies: %s\n", i, Q_firstSpecies(q));
}
Q_drop(q, "777777");