在使用链表的程序中，c 中没有显示输出

Question

我正在用 c 语言开发一个程序，在这个程序中我必须使用链表，在这个程序中，如果用户传递值，我必须在链表的开头插入新节点如果用户在 choice 变量中传递位置 1 的值，则位置 0 并在链表的末尾插入新节点。但是我没有在控制台上得到任何输出，我的程序只写了 Output: 我无法找出我的代码中的问题，这是我的代码。

/* 
  program for making nodes and adding them in memory as per 0
  and 1

  0 means that insert the number at front , in other words insert number after head 

  1 means insert number at the last place

  First you need to input a number and then enter the place you want to insert it by giving input as 0 and 1 
  
  *Recall what does 0 and 1 mean by looking at line 5-7 respectively. 
  
  Just like 

  5 0 6 1 7 0 8 1

*/

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

// declaring struct with typedef for ease of use
typedef struct node
{
  int data;
  struct node *next;
}node;

// declarations of functions use for this program respectively
void free_node(struct node *head);
void insert_at_beg(int num, struct node *head);
void insert_at_end(int num, struct node *head);
void print_node(struct node *head);

int main(void)
{
  struct node *head = NULL;
  int n;
  // taking input
  printf("Input number of nodes: ");
  scanf("%d",&n);

  int num, choice;

  printf("\nInput data for nodes->\n");
  // loop which takes value and choice 
  for (int i = 0; i < n; i++)
  {
    num = 0, choice = 0;
    printf("\nInput data for the %d node: ", i+1);
    scanf("%d",&num);

    do
    {
      printf("Input place for the %d node: ", i+1);
      scanf("%d",&choice);
    }
    while (choice != 1 && choice != 0);
    
    if (choice == 0)
    {
      // function to insert node at front of head
      insert_at_beg(choice, head);

    }
    else 
    {
      // function to insert node at last place
      insert_at_end(choice, head);

    }
  }
  
  // function to print nodes
  print_node(head);
  
  // function to free memory made by malloc()
  free_node(head);
}

// function to free the nodes 
void free_node(struct node *head)
{
  struct node *temp = head;
  while(temp != NULL)
  {
    free(temp);
    temp = temp->next;
  }
}

// function for inserting number at front
void insert_at_beg(int num, struct node *head)
{ 
  struct node *new_node = malloc(sizeof(node));

  if (new_node == NULL)
  {
    printf("Can't allocate memory.");
    exit (1);
  }

  new_node->data = num;  
  new_node->next = head;
  head = new_node;
}

// function for inserting node at end
void insert_at_end(int num, struct node *head)
{
  struct node *new_node, *last_node = NULL;
  new_node = malloc(sizeof(node));

  if (new_node == NULL)
  {
    printf("Can't allocate memory.");
    exit (1);
  }
  if (head == NULL)
  {
    new_node->data = num;
    new_node->next = NULL;
    head = new_node; 
  }
  
  last_node = head;

  new_node->data = num;
  new_node->next = NULL;

  while (last_node->next != NULL)
  {
    last_node = last_node->next;
  } 

  last_node->next = new_node;
}

//function for printing nodes
void print_node(struct node *head)
{
  printf("\nOutput: \n");
  struct node *temp = head;

  while(temp != NULL)
  {
    printf("%d ",temp->data);
    temp = temp->next;
  }
}

Answer 1

C 是一种 pass-by-value 语言 -- 传递给函数的值被复制，并且对函数中参数的更改不会影响调用者。所以 head 在 main 中永远不会变成 non-null； insert_at_end 中对 head 的赋值是局部的，不会更新 main.

中的 head 指针

Answer 2

您需要通过引用您的函数 insert_at_beg 和 insert_at_end 将指针传递到头节点，并且需要函数 free_node.

在 C 中通过引用传递意味着通过指向对象的指针间接传递对象。

因此，例如函数 insert_at_beg 可以如下所示。注意这样的函数不应发出任何消息。是否输出消息由函数的调用者决定。

// function for inserting number at front
int insert_at_beg( struct node **head, int num )
{ 
    struct node *new_node = malloc( sizeof( struct node ) );
    int success = new_node != NULL;

    if ( success )
    {
        new_node->data = num;  
        new_node->next = *head;
        *head = new_node;
    }

    return success;
}

对应的函数insert_at_end可以看成下面的样子

// function for inserting node at end
int insert_at_end( struct node **head, int num )
{
    struct node *new_node = malloc( sizeof( struct node ) );
    int success = new_node != NULL;

    if ( success )
    {
        new_node->data = num;
        new_node->next = NULL;
 
        while ( *head != NULL )
        {
            head = &( *head )->next;
        }

        *head = new_node;
    }

    return success;
}

函数 free_node 具有未定义的行为，因为您正在使用指针 temp 访问已释放的内存。

free(temp);
temp = temp->next;

函数可以这样定义

// function to free the nodes 
void free_node( struct node **head )
{
    while( *head != NULL )
    {
        struct node *temp = *head;
        head = &( *head )->next;
        free( temp );
    }
}

函数可以这样调用

insert_at_end( &head, num );

或

if ( !insert_at_end( &head, num ) )
{
    printf( "There is no enough memory to insert the value %d\n", num );
}

函数print_node的参数应该有修饰符const因为列表在函数

中没有改变

//function for printing nodes
void print_node( const struct node *head )
{
  printf("\nOutput: \n");
  const struct node *temp = head;
  //...

在使用链表的程序中，c 中没有显示输出

In a program using linked-list no output is being shown in c

c

struct

linked-list

pass-by-reference

singly-linked-list