Question

我是C新生，现在就学习链接列表。当我对链表进行冒泡排序时，发生段错误，GDB指向函数bubble（），i = ptr-＆gt; elem。我不知道是什么原因引起的。请帮忙搞清楚。

`

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

define      i_track(n)  printf ("The %s's is %d.\n", #n, (n))
define      s_track(n)  printf ("%s.\n", #n);


typedef struct tag_node
{
    int elem;
struct tag_node *next;
}NODE, *SLINK; 

void  node_track (SLINK list);
NODE *node_generate (void);
void init_list (SLINK *header, int len);
SLINK locate_cur (SLINK list, int target_elem);
void insert_node (SLINK *list, int new_elem, int tag_elem);
SLINK traverse_list (SLINK list);
void list_info (SLINK list, int node_elem);
void bubble (SLINK list);
void swap (int *a, int *b);

int main (int argc, char *argv[])
{
int len;
SLINK list = node_generate ();
printf ("Input a number for length of the link.\n");
scanf ("%d", &len);
s_track(init_start.);
init_list (&list, len);
s_track(init_end.);
traverse_list (list);
bubble (list);

return EXIT_SUCCESS;
}   /* ----------  end of function main  ---------- */          

NODE *node_generate (void) /* generate a node */
{
NODE *new_node = malloc (sizeof (NODE));
if (NULL == new_node)
{
    printf ("ERROR: malloc failed.\n");
    exit (EXIT_FAILURE);
}
memset (new_node, 0, sizeof(NODE));
return new_node;
}

void insert_node (SLINK *list, int new_elem, int tag_elem)
/* insert a node */
{
NODE *new_node = node_generate ();
NODE *cur = *list;
new_node->elem = new_elem;
if ((int)NULL == tag_elem)
{
    new_node->next = (*list)->next;
    (*list)->next = new_node;
}
else
{
    *list = locate_cur (cur, tag_elem);
    new_node->next = (*list)->next;
    (*list)->next = new_node;
}
}

void init_list (SLINK *header, int len)
/* generate a linked-list and assign it*/
{
srand ((unsigned) time(0));
int elem;
for (int i = 0; i < len; i++)
    /* skip number 4 since I dislike it */
    {
        elem = rand () % 100;

        if (4 == elem / 10)
        {
            elem = elem + 50;
        }
        if (4 == elem % 10)
        {
            elem = elem + 5;
        }
        if (0 == elem % 100)
        {
            elem = elem + 999;
        }
        insert_node (header, elem, (int)NULL);  
    }
}

SLINK traverse_list (SLINK list)
/*traverse list */
{
SLINK ptr = list;
for (int node_num = 0; ptr != NULL; ptr = ptr->next)
{
    ++node_num;
    list_info (ptr, node_num);
}
return list;
}

void list_info (SLINK list, int node_num)
/* used for traversed_list () */
{
if (1 == node_num % 10 && 11 != node_num)
{
    printf ("The %dst element is %d.\n", node_num, list->elem);
}
else if (2 == node_num % 10 && 12 != node_num)
{
    printf ("The %dnd element is %d.\n", node_num, list->elem);
}
else if (3 == node_num % 10 && 13 != node_num)
{
    printf ("The %drd element is %d.\n", node_num, list->elem);
}
else
{
    printf ("The %dth element is %d.\n", node_num, list->elem);
}
}

SLINK locate_cur (SLINK list, int target_elem)
/* locate previous of a node */
{
NODE *prev, *cur;
prev = node_generate ();
cur = node_generate ();
for (prev = list, cur = list->next; NULL != cur && target_elem != cur->elem; prev = cur, cur = cur->next)
    ;
return cur;
}

void node_track (NODE *flag)
{
printf ("The flag element is %d.\n", flag->elem);
}

void bubble (SLINK list) /* bubble sort */
{
s_track(bubble_start);
list = list->next;
SLINK ptr, header;
ptr = list; /*GDB point to here*/
int i =0, j = 0;
for (; NULL != list; list = list->next)
{
    for (ptr = list; NULL != ptr; ptr = ptr->next);
    {
        j = list->elem;
        i = ptr->elem;
    //  if (ptr->elem > list->elem)
        if (i > j)
            swap (&(ptr->elem), &(list->elem));
    }
}
s_track(bubble_end);
}

void swap (int *a, int *b)
{
*a = (*a)^(*b);
*b = (*a)^(*b);
*a = (*a)^(*b);
}

`

Answer 1

tag_elem中init_list()的目的非常不明确。我修改了代码以消除该变量，然后运行它并指定5作为输入值。它给出了：

Input a number for length of the link.
5
init_start..
init_end..
The 1st element is 0.
The 2nd element is 12.
The 3rd element is 8.
The 4th element is 99.
The 5th element is 7.
The 6th element is 63.
bubble_start.
Segmentation fault: 11

为什么要求5个时打印6个项目？重复执行时，第1个元素中的值始终为0（3次重复）。

可以通过修改traverse_list()来解决这个问题：

SLINK traverse_list(SLINK list)
{
    assert(list != 0); 
    SLINK ptr = list->next;
    for (int node_num = 0; ptr != NULL; ptr = ptr->next)
        list_info(ptr, ++node_num);
    return list;
}

有趣的是，bubble()中的代码已经跳过列表中的第一项。

然而，bubble()中的内部循环有一个错误：

for (ptr = list; NULL != ptr; ptr = ptr->next);
{
    j = list->elem;
    i = ptr->elem;
//  if (ptr->elem > list->elem)
    if (i > j)
        swap (&(ptr->elem), &(list->elem));
}

可能更容易看到你是否将它写成：

for (ptr = list; NULL != ptr; ptr = ptr->next)
    ;
{
    j = list->elem;
    i = ptr->elem;  // When this is executed, ptr == NULL!
    if (i > j)
        swap (&(ptr->elem), &(list->elem));
}

你不想要那个分号。修复后，代码运行正常：

Input a number for length of the link.
5
init start.
init end.
The 1st element is 12.
The 2nd element is 19.
The 3rd element is 99.
The 4th element is 92.
The 5th element is 28.
traverse 1 end.
bubble_start.
bubble_end.
sort end.
The 1st element is 99.
The 2nd element is 92.
The 3rd element is 28.
The 4th element is 19.
The 5th element is 12.
traverse 2 end.

显然，稍微修改了一组诊断打印，但数据按降序排序。看起来它也适用于较大的套装;我尝试了55没有崩溃，数据中有一些重复，输出按降序排序。

带链接列表的冒泡排序

1 个答案: