Error trying to edit character string inside struct string - arrays

Question

My program contains the next structure:
typedef struct user {
char username[LENGTH_USERNAME];
} User
and I have an array inside my main function of struct user
User user_database[NUMBER_USERS];
The problem is that when I pass this database to a function to edit the username field of all the struct user in the array like this:
void initialize_empty_user_database(User database[])
{
int i;
for(i = 0; i < NUMBER_USERS; ++i)
database[i].username[0] = '\0';
}
when trying to compile, I get the next error:
error: incompatible types when assigning to type ‘User’ {aka ‘struct user’} from type ‘char *’
Here is the complete code:
#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;
}
can somebody explain to me what am I doing wrong?
Thanks!
Errors in the original code are:
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

The error is on this line:
database[i].username[0] = '\0';
This is because username is declared to be a User array:
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;
That looks like an error in the field declaration and should instead be:
char username[LENGTH_USERNAME];

Answer 2

The expression user_database[i].username[0] is of type char, so you need to assign a char and not a string to it.
user_database[i].username[0] = '\0';
Note the single quotes not double quotes '\0'.