在c语言用什么方法才能知道剩下还有几个数

用什么办法才能统计A1到D3剩下还有多少

void InitSeatTable()
{
    char* seatNum[] =
    {

        "A1","A2","A3","B1","B2","B3",
        "C1","C2","C3","D1","D2","D3"

    };
    for (int i = 0; i < SeatCount; i++)
    {
        strcpy(seats[i].seatID, seatNum[i]);
        seats[i].IsAssigned = 0;
        strcpy(seats[i].FirsName, "0");
        strcpy(seats[i].LastName, "0");
        
    
    }
}

//count empty seat
 int EmptySeat()
{
     char option = getchar();
     if (option == 1)
     {
         printf("the number of empty seats is %d", SeatCount - 1);//这样好像不行
    }

}

int getlen(char * seatNum)
{
    int len = 0;
    int posA1 = 0;
    int posD3 = 0;
    char * p =seatNum;
    while(*p != '\0')
    {
        if(*p == 'A1')
        {
            posA1 = len;
        }
        if(*p == 'D3')
        {
            posD3 = len;
            break;
        }
        len++;
        p++;
    }
    if(posD3 == 0 || posA1 == 0)
    {
        return 0;
    }
    return (posD3 - posA1 - 1);
}