带链表的冒泡排序

Question

我是 C 大一新生，现在学习链接列表。当我对链表进行冒泡排序时，出现段错误，GDB指向函数bubble(),i = ptr->elem。我不知道这是什么原因。请帮忙弄清楚。

`

#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

init_list() 中的tag_elem 的用途非常不清楚。我修改了代码以消除该变量，然后运行它并将 5 指定为输入值。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

为什么在请求 5 个项目时打印了 6 个项目？重复执行时，第一个元素中的值始终为 0（重复 3 次）。

这个问题可以通过修改traverse_list()来解决：

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() 中的代码已经跳过了列表中的第一项。

但是bubble()中的内循环有一个错误：

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));
}

如果你写成这样可能会更容易看到：

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，一组稍作修改的诊断打印，但数据是按降序排列的。看起来它也适用于更大的场景；我尝试了 55 没有崩溃，数据中有一些重复，并且输出按降序排序。