尝试在结构字符串中编辑字符串时出错

Question

我的程序包含下一个结构：

typedef struct user {
    char username[LENGTH_USERNAME];
} User

我在 struct user 的主函数中有一个数组

User user_database[NUMBER_USERS];

问题是当我将此数据库传递给一个函数来编辑数组中所有结构用户的用户名字段时，如下所示：

void initialize_empty_user_database(User database[])
{
    int i;
    for(i = 0; i < NUMBER_USERS; ++i)
        database[i].username[0] = '\0';
}

尝试编译时，出现下一个错误：

 error: incompatible types when assigning to type ‘User’ {aka ‘struct user’} from type ‘char *’

完整代码如下：

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

#define LENGTH_USERNAME 21
#define LENGTH_ACTIVITY_NAME    21
#define LENGTH_TASK_DESCR   51
#define NUMBER_USERS    50
#define NUMBER_ACTIVITIES   10
#define NUMBER_TASKS    10000

typedef struct user {
    char username[LENGTH_USERNAME];
    /* username cannot contain blank characters */
} User;


typedef struct activity {
    char name[LENGTH_ACTIVITY_NAME];      
    /* name cannot contain lowercase letters */
} Activity;

typedef struct task {
    int id;             
    char des[LENGTH_TASK_DESCR];                    /* description */
    User username[LENGTH_USERNAME];
    Activity activity[LENGTH_ACTIVITY_NAME];
    int etc;                                        /* Estimated Time of Completion */
    int start_time;     
} Task;

void initialize_empty_user_database(User database[])
{
    int i;
    for(i = 0; i < NUMBER_USERS; ++i)
        database[i].username[0] = '\0';
}

void initialize_empty_activity_database(Activity database[])
{
    int i;
    for(i = 0; i < NUMBER_ACTIVITIES; ++i)
        database[i].name[0] = '\0';
}

void initialize_empty_task_database(Task database[])
{
    int i;
    for(i = 0; i < NUMBER_TASKS; ++i) {
        database[i].id = -1;                        /* id == -1 -> task not initialized */
    }
}

void read_task_descr(char target[])
{
    int i;
    char c;
    c = getchar();
    for(i = 0; i < (LENGTH_TASK_DESCR - 1) || c == '\n'; ++i){
        target[i] = c;
        c = getchar();
    }
    target[i] = '\0';
}

int available_tasks(Task database[])
{
    int i;
    for(i = 0; i < NUMBER_TASKS; ++i) {
        if(database[i].id == -1)
            return NUMBER_TASKS - i;
    }
    return 0;
}

int duplicate_descr(Task database[], char descr[]) 
{
    int i;
    for(i = 0; i < NUMBER_TASKS; ++i) {
        if (strcmp(database[i].des, descr) == 0)
            return 1;
    }
    return 0;
}

int t_exceptions(Task task_database[], char task_descr[])
{
    if (available_tasks(task_database) == 0) {
        printf("too many tasks");
        return 1;
    }
    else if (duplicate_descr(task_database, task_descr) == 1) {
        printf("duplicate description"); 
        return 2;
    }
    return 0;
}

void new_task(Task database[], int etc, char descr[]) 
{
    static int i = 0;
    database[i].id = i + 1;                         /* id */
    strcpy(database[i].des, descr);                 /* descr */
    database[i].username[0] = '\0';                 /* username (not attributed) */
    strcpy(database[i].activity.name, "TO DO");     /* activity */
    database[i].etc = etc;                          /* estimated time for completion */
    database[i].start_time = 0;                     /* start time (not started) */
    ++i;
}


int main()
{
    /* Database declarations */
    User user_database[NUMBER_USERS];
    Activity activity_database[NUMBER_ACTIVITIES] = {{"TO DO"}, {"IN PROGRESS"}, {"DONE"}};
    Task task_database[NUMBER_TASKS];
    
    /* Variable and array definition */
    int time;
    int etc;                                        /* estimated time of completion */
    char task_descr[LENGTH_TASK_DESCR];
    char activity_name[LENGTH_ACTIVITY_NAME];
    char username[LENGTH_USERNAME];
    
    /* Auxiliary variables */
    char command;

    /* Database initilizations */
    initialize_empty_user_database(user_database);
    initialize_empty_activity_database(activity_database);
    initialize_empty_task_database(task_database);

    while(1) {
        command = getchar();
        switch(command) {
            case 'q':
                return 0;
            case 't':
                scanf("%d", &etc);
                getchar();  /* Consume space separating arguments */
                read_task_descr(task_descr);
                if(t_exceptions(task_database, task_descr) == 0)
                    new_task(task_database, etc, task_descr);       
                break;
            default:
                printf("Exception: Unknown command\n");
        }
        getchar(); /* Consume the newline character */
    }
    return -1;
}

有人可以向我解释我做错了什么吗？

谢谢！

原代码中的错误是：

main.c: In function ‘new_task’:
main.c:103:31: error: incompatible types when assigning to type ‘User’ {aka ‘struct user’} from type ‘int’
  103 |     database[i].username[0] = '\0';                 /* username (not attributed) */
      |                               ^~~~
main.c:104:32: error: ‘(Activity *)&(database + (sizetype)((long unsigned int)i * 948))->activity’ is a pointer; did you mean to use ‘->’?
  104 |     strcpy(database[i].activity.name, "TO DO");     /* activity */
      |                                ^
      |                                ->

Answer 1

错误在这一行：

database[i].username[0] = '\0'; 

这是因为username 被声明为User 数组：

typedef struct task {
    int id;             
    char des[LENGTH_TASK_DESCR];                    /* description */
    User username[LENGTH_USERNAME];
    Activity activity[LENGTH_ACTIVITY_NAME];
    int etc;                                        /* Estimated Time of Completion */
    int start_time;     
} Task;

这看起来像是字段声明中的错误，应该是：

char username[LENGTH_USERNAME];

Answer 2

表达式user_database[i].username[0] 的类型为char，因此您需要为其分配char 而不是字符串。

user_database[i].username[0] = '\0';

注意单引号不是双引号'\0'。