我写了一个Luhn算法的实现。当我用数字无效的数字测试程序时,我不会得到程序错误。相反,程序在主函数中执行printf()
调用。但是,每当我按照算法使用有效的卡号测试程序时,我会得到一个0除错误。用DBG调试会给我一个算术错误,而MSVC给我一个浮点异常,它缩小到0的除法范围。这是代码:
#include <stdio.h>
#include <math.h>
#include "cs50.h" // For MSVC, I used scanf_s() from stdio.h instead since cs50.h is not supported
long long Prompt(void); // Returns credit card number
int Validate(long long); // Returns checksum
void Report(long long); // Prints success
int main()
{
long long CardNumber = Prompt();
int CheckSum = Validate(CardNumber);
if (!(CheckSum % 10)) {
Report(CardNumber);
} else {
printf("INVALID\n");
}
return 0;
}
long long Prompt(void)
{
printf("CARD NUMBER: ");
long long CardNumber;
CardNumber = GetLongLong();
return CardNumber;
}
int Validate(long long CardNumber)
{
int CheckSum = 0;
int Digit = 0;
int DigitCount = (int)(floor(log10l(CardNumber) + 1));
for (int i = 2; i < DigitCount; i += 2) {
Digit = 2 * ((CardNumber % (10 ^ i)) / (10 ^ (i - 1))); // BUG
if (Digit > 9) {
int BiDigit = Digit;
Digit = 0;
int SubDigit = 0;
for (int j = 0; j < 2; i++) {
SubDigit = (BiDigit % (10 ^ j)) / (10 ^ (j - 1)); // BUG
Digit += SubDigit;
}
}
CheckSum += Digit;
}
for (int i = 1; i < DigitCount; i += 2) {
Digit = (CardNumber % (10 ^ i)) / (10 ^ (i - 1));
CheckSum += Digit;
}
return CheckSum;
}
void Report(long long CardNumber)
{
int DigitCount = (int)(floor(log10l(CardNumber) + 1));
int Digit1 = (CardNumber % (10 ^ (DigitCount - 1))) / (10 ^ (DigitCount - 2)); // BUG
int Digit2 = (CardNumber % (10 ^ (DigitCount - 2))) / (10 ^ (DigitCount - 3)); // BUG
if (Digit1 == 4 && (DigitCount == 13 || DigitCount == 16))
printf("VISA\n");
else if (Digit1 == 3 && (Digit2 == 4 || Digit2 == 7) && DigitCount == 15)
printf("AMERICAN EXPRESS\n");
else if (Digit1 == 4 && (Digit2 >= 1 || Digit2 <= 5) && DigitCount == 16)
printf("MASTERCARD\n");
else
printf("INVALID\n");
return;
}
最佳答案
Digit = 2 * ((CardNumber % (10 ^ i)) / (10 ^ (i - 1)));
^
不是幂运算符,而是C
和C++
中的按位异或。一旦i
达到10
,表达式就变为零。顺便说一句,为了从整数中获取数字,最好避免浮点运算(
10 ^ i
,floor
)。可能的算法如下:int DigitIndex = 0;
while (CardNumber > 0)
{
int Digit = CardNumber % 10;
CardNumber /= 10;
// calculate check sum depending on parity of DigitIndex
DigitIndex++;
}