c语言 - 为什么我的代码不能像我想要的那样工作？我如何检查日期的有效性？

我想让用户插入她的名字，并检查他是否插入了char，否则就让他插入一个有效的名字，并为卡号插入一个6整数，否则就允许他插入有效的数字，然后插入截止日期。现在，我如何检查插入的有效日期？

int  main()
{
char name[20];
int cardnumber, pin;
float balance;    
printf("Enter your first and last name :  ");
scanf("%s",name);
for (int i = 0; name[i] != ''; ++i)
{
if (!(name[i] >= 'a' && name[i] <= 'z')
|| !(name[i] >= 'A' && name[i] <= 'Z'))
{  
printf("The entered name is not all  Alphabet char plz insert a valid namen");
break;
}
}
printf("insert your card number ~6 digit~n ");
scanf("%d", &cardnumber);
if (!(sizeof(cardnumber) != 6)
|| !(cardnumber >= '0' && cardnumber <= '9'))
{
printf("plz insert a valid 6digit card numbern "); 
scanf("%d", cardnumber);
}
struct date datei;
printf("insert Expiration date of your card :n");
printf("day :");  
scanf("%d", &datei.day);
printf("month :");
scanf("%d", &datei.month);
printf("year :"); 
scanf("%d", &datei.year);
printf("Enter your acount Balance " );
scanf("%d", &balance);
printf("Enter your pin ~3digit~ :");
// scanf("%d", &pin);
// if (sizeof(pin)>3)
// {printf("Enter a valid pin n");
// scanf("%d", &pin);}
scanf("%d", pin);
while (sizeof(pin) == 3)
{
printf("ENTER YOUR SECRET PIN NUMBER:");
scanf("%d", &pin);
if (sizeof(pin) != 3)
printf("PLEASE ENTER VALID PASSWORDn");
}
}