我有麻烦写一个函数，检查信用卡是否有效，使用卢恩的算法（乘2每隔一位数，从倒数第二开始;把没有乘以2的数字相加;如果总数的最后一位是0，则该数字有效）。
当我开始写代码时，我检查了这些行：

long number2 = ((card % x) / y)\*2;
long number1 = ((card % (x/10)) / (y/10));

而且它们看起来工作正常。当我现在再次尝试检查时，它有点忽略以下几行：
x一个一个一个一个x一个一个二个x
函数总是返回真，另外，我声明变量digits两次可以吗？

#include <stdio.h>
#include <cs50.h>

bool luhns_algorithm (long, int);
int number_of_digits(long);

int main (void)
{
  int digits = 0;//if i remove this then on line 15 appears mistakes
  long card = get_long("credit card: ");

  number_of_digits(card);

  luhns_algorithm(card, digits);//use of undeclared identifier 'digits'; variable 'digits' is uninitilazed when used here

}

int number_of_digits(long card)
{
  int digits = 0;//cant be used in do while loop if undeclared and uninitialized so I need to write it twice
  do
  {
    digits++;
    card = card/10;
  } while(card != 0);

  printf("%i\n", digits);
  return digits;
}

bool luhns_algorithm (long card, int digits)//always valid for some reason
{
   int sum = 0;
   long x = 100;
   long y = 10;
   for (int i = 0; i < digits/2; i++)
   {
     long number2 = (card % x)/y;
     long number1 = (card % (x/10))/(y/10);

     printf("%li\n", number1);//doesn't work
     printf("%li\n", number2);//doesn't work

     sum = sum + number2 + number1;

     printf("%i\n", sum);//doesn't work

     x = x*100;
     y= y*100;
  }

  if((sum/10) == 0)
  {
    printf("VAlID\n");
    return true;
  }
  printf("INVALID\n");
  return false;
}