C 变量指定为长整型，但被识别为 int

Question

我正在开发一个程序，用于检查我正在参加的 CS50 类的信用卡号的有效性（我发誓哈哈是合法的），我目前正在努力正确获取每个 CC# 的前两个数字以检查它来自哪家公司。为了清楚起见，我已经评论了每个部分的作用，并评论了我的问题出现的地方。

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <math.h>
#include <string.h>
int main(void)
{
  long long ccn = get_long_long("Enter CCN: n");
  int count = 0;
  long long ccn1 = ccn;
  // finds the amount of digits entered and stores that in int count.
  while (ccn1 != 0)
  {
     ccn1 /= 10;
     +count;
  }
  printf("%i n", count);
  // ln 17- 19 should take int count, subtract two, put that # as the power of 10, 
  // then divide the CC# by that number to get the first two numbers of the CC#.
  long long power = count - 2;
  // here is where i get the error. its a long long so it 
  // should hold up to 19 digits and im only storing 14 max 
  // but it says that 10^14th is too large for type 'int'
  long long divide = pow(10,power);
  long long ft = ccn / divide;
  printf("power: %i n", power); //ln 20-22 prints the above ints for debug
  printf("Divide: %lli n", divide);
  printf("First two: %lli n", ft);
  string CCC;
  // ln 24-35 cross references the amount of digits in the CC#
  // and the first two digits to find the comapany of the credit card
  if ((count == 15) && (ft =  34|37))
  {
    CCC = "American Express";
  }
  else if ((count == 16) && (ft = 51|52|53|54|55))
  {
    CCC = "MasterCard";
  }
  else if ((count = 13|16) && (ft <=49 && ft >= 40))
  {
      CCC = "Visa";
  }
  printf("Company: %sn", CCC);
}

Answer 1

第一个问题是循环中的+count。这应该是++count.因此，count保持在 0 和 power = -2 .你可以避免所有这些权力的东西。你已经有了循环，你可以用它来获取前两位数字：

int ft = 0;
while (ccn1 != 0)
{
    // When you are down to 10 <= ccn1 < 100, store it
    if (ccn1 < 100 && ccn1 > 9) ft = ccn1;
    ccn1 /= 10;
    ++count;
}

你的第二个问题是你如何进行比较。

if ((count == 15) && (ft =  34|37))

首先，=是分配，==测试相等性。其次，|是按位 OR，||是逻辑 OR。第三，你不能像这样测试多个值。正确方法：

if ((count == 15) && (ft == 34 || ft == 37))