C语言 链表在开头打印额外的 0

Question

我有一个非常基本的单链表实现。然而，我的实现的问题在于它在列表的开头打印了一个额外的零，而我没有为这个额外的节点显式分配任何存储空间。相同的代码如下 -

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#define LEN 7
/* List node data structure */
typedef struct _ll_node_ {
    int data;
    struct _ll_node_ *next;
} node;
/*
 * @brief   Utility to print the state of the list
 */
void print_list(node *head)
{
    int i = 0;
    node *tmp = head;
    while (tmp)
    {
        printf("Node:t%d,tValue:t%dn", ++i, tmp->data);
        tmp = tmp->next;
    }
    printf("n");
}
/*
 * @brief   Utility to add nodes to the list
 */
node *add_node(node *head, int data)
{
    node *tmp;
    if (head == NULL)
    {
        head = malloc(sizeof(node));
        assert(head != NULL);
        head->data = data;
        head->next = NULL;
    }
    else
    {
        tmp = head;
        while (tmp->next)
            tmp = tmp->next;
        tmp->next = malloc(sizeof(node));
        assert(tmp->next != NULL);
        tmp = tmp->next;
        tmp->data = data;
        tmp->next = NULL;
    }
    return head;
}
/*
 * @brief   Driver function
 */
int main(int argc, char *argv[])
{
    node *head = NULL;
    int i = 0;
    /* Allocate memory */
    head = malloc(LEN * sizeof(node));
    assert(head != NULL);
    /* Populate the list */
    for (; i < LEN; i++)
        head = add_node(head, rand() % 1000);
    /* Print its state */
    print_list(head);
    return 0;
}

有人可以帮我找出我做错的地方吗？

System information:
    Distributor ID: Ubuntu
    Description:    Ubuntu 14.04.3 LTS
    Release:        14.04
    Codename:       trusty

Answer 1

您已经在 main 中为 head 分配内存，因此从未为第一个节点分配数据，因此默认情况下它将其视为 0。试试这个：

int main(int argc, char *argv[])
{
    node *head = NULL;
    int i = 0;
    /* Populate the list */
    for (; i < LEN; i++)
        head = add_node(head, rand() % 1000);
    /* Print its state */
    print_list(head);
    return 0;
}

Answer 2

此语句

head = malloc(LEN * sizeof(node));

没有意义。删除它。

您分配了未初始化的数组。因此，使用函数add_node会导致未定义的行为。

考虑到如果通过引用传递头部，函数add_node可以写得更简单。例如

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <time.h>
#define LEN 7
/* List node data structure */
typedef struct _ll_node_ {
    int data;
    struct _ll_node_ *next;
} node;
/*
 * @brief   Utility to print the state of the list
 */
void print_list(node *head)
{
    int i = 0;
    node *tmp = head;
    while (tmp)
    {
        printf("Node:t%d,tValue:t%dn", ++i, tmp->data);
        tmp = tmp->next;
    }
    printf("n");
}
/*
 * @brief   Utility to add nodes to the list
 */
int add_node( node **head, int data )
{
    int success;
    while ( *head != NULL ) head = &( *head )->next;
    *head = malloc( sizeof( node ) );
    success = *head != NULL;
    if (success )
    {
        ( *head )->data = data;
        ( *head )->next = NULL;
    }
    return success;
}
/*
 * @brief   Driver function
 */
int main( void )
{
    node *head = NULL;
    int i = 0;
    srand( ( unsigned int )time( NULL ) );
    /* Populate the list */
    for ( ; i < LEN; i++ )
        add_node( &head, rand() % 1000);
    /* Print its state */
    print_list( head );
    return 0;
}