C 链表开头额外打印0

Question

该程序尚未完全准备好，因此并非所有功能都已编码。

但是，当涉及到最基本的功能时，我遇到了一个问题：在链表的末尾添加节点。

我不知何故在链表的开头得到了一个神秘的数字“0”，我看不出它是从哪里来的

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

typedef struct USER{
    int number;
    struct USER *next;
}USER;

USER* allocMem(USER* newUser){
    if ((newUser = (USER*)malloc(sizeof(USER))) == NULL ){
        perror("Memory allocation failed.");
        exit(1);
    }
    newUser->number;
    printf("A list has been created.\n");
    return newUser;
}

int newNode(USER* start){
    int num;
    USER *newUser = NULL; 
    USER *value = NULL;
    if ((newUser = (USER*)malloc(sizeof(USER))) == NULL ){
        perror("Memory allocation failed.");
        exit(1);
    }
    printf("\nGive a new number: ");
    scanf("%d", &num);
    newUser->number = num;
    newUser->next = NULL;
    if (start == NULL){
        start = newUser;
    }else{
        value = start;
        while(value->next != NULL){
            value = value->next;
        }
        value->next = newUser;
    }
    return 0;
}

int newNode_position(USER* start){
    USER *newUser = NULL;
    USER *value = NULL;
    int num;
    int position;
    if ((newUser = (USER*)malloc(sizeof(USER))) == NULL ){
        perror("Memory allocation failed.");
        exit(1);
    }
    printf("\nGive a new number: ");

    scanf("%d", &newUser->number);

    printf("Into which node do you want to place the number: ");
    scanf("%d", &position);
    value = start;
    for(int i = 1; i < position; i++){  
        value = value->next;
        
    }
    newUser->next = value->next; 
    value->next = newUser;

    value = NULL;
    value = start;
    printf("The following numbers are in the list: \n");
    while(value->next != NULL){
        printf("%d ", value->number);
        value = value->next;
    }

    return 0;
}


void print(USER *start){
    USER *value = NULL;
    value = start;
    printf("Following numbers are now in the list: \n");
    while(value->next != NULL){
        printf("%d ", value->number);
        value = value->next;
    }
    printf("\n");
}


void freeMem(USER* start){ 
    USER *value = NULL;
    while (start != NULL){
        value = start;   
        start = value->next;
        free(value); 
    }
}



int main(void){
    int choice;
    USER *start = NULL;
    printf("This program let's you create a linked list and manage it.\n");
    do {
        printf("\n1) Create a list\n");
        printf("2) Insert a new node in the end\n");
        printf("3) Insert a new node in the middle, to a specified position\n");
        printf("4) Clear the list\n");
        printf("5) Delete certain node from the list\n");
        printf("6) Print the list\n");
        printf("0) Quit\n");
        printf("Your choice: ");
        scanf("%d", &choice);
        switch(choice){
            case 1:
            start = allocMem(start);  
            break;
            case 2:
            newNode(start);
            break;
            case 3:
            newNode_position(start);
            case 4:
            freeMem(start);
            break;
            case 6:
            print(start);
            break;
            case 0:
            printf("\nThank you for using the program.");
            break;
            default:
            printf("Unknown choice.");
            break;
        }
    } while(choice != 0);
    
}

OUTPUT：

当我创建列表并在列表末尾添加 number: "2" 三次时，output 是：

0, 2, 2

应该在什么时候

2, 2, 2

我找不到为什么它在开头打印神秘的 0。

Answer 1

function allocMem创建一个具有未初始化数据成员number和next的节点。

USER* allocMem(USER* newUser){
    if ((newUser = (USER*)malloc(sizeof(USER))) == NULL ){
        perror("Memory allocation failed.");
        exit(1);
    }
    newUser->number;
    printf("A list has been created.\n");
    return newUser;
}

这会导致程序出现未定义的行为。

function打印错误

void print(USER *start){
    USER *value = NULL;
    value = start;
    printf("Following numbers are now in the list: \n");
    while(value->next != NULL){
        printf("%d ", value->number);
        value = value->next;
    }
    printf("\n");
}

输出存储在第一个节点的数据成员number中的不确定值，而且由于 while 语句的条件，它不是最后一个节点中的值 output

    while(value->next != NULL){

实际上 function allocMem是多余的，应该删除。 本声明

USER *start = NULL;

已经创建了一个空列表。

代码中还有其他缺点。 例如 function freeMem实际上并没有清空链表，因为 main 中声明的指针start在调用 function 后不会改变，因此无效。

要么你应该申报 function 谎言

USER * freeMem(USER* start){ 
    USER *value = NULL;
    while (start != NULL){
        value = start;   
        start = value->next;
        free(value); 
    }

    return start;
}

并称之为

case 4:
start = freeMem(start);
break;

或者你可以声明它

void freeMem( USER **start){ 
    while ( *start != NULL ){
        USER *value = *start;   
        start = &( *start )->next;
        free(value); 
    }
}

并称之为

case 4:
freeMem( &start);
break;

或者例如 function newNode_position再次可以调用未定义的行为至少因为在这个 for 循环中

value = start;
for(int i = 1; i < position; i++){  
    value = value->next;
    
}

没有检查指针value是否等于NULL 。

而且 function 不会改变 main 中声明的指针start 。