Luhn算法（模10算法）检验银行卡号正确性

中文描述：

1、从卡号最后一位数字开始,偶数位乘以2,如果乘以2的结果是两位数，将结果减去9。

2、把所有数字相加,得到总和。

3、如果信用卡号码是合法的，总和可以被10整除。

英文描述

1.Counting from the check digit, which is the rightmost, and moving left, double the value of every second digit.

2.Sum the digits of the products (e.g., 10: 1 + 0 = 1, 14: 1 + 4 = 5) together with the undoubled digits from the original number.

3.If the total modulo 10 is equal to 0 (if the total ends in zero) then the number is valid according to the Luhn formula; else it is not valid.

代码

C++：

char digit;

int oddLengthChecksum=0;

int evenLenthChecksum=0;

int position =1;

cout<<"Enter a number:";

digit=cin.get();

while(digit != 10)

{

if(position%2==0)

{

oddLengthChecksum+=doubleDigitValue(digit-'0');

evenLengthChecksum+=digit-'0';

}

else {

oddLengthChecksum+=digit-'0';

evenLenthChencksum+=doubleDigitValue(digit-'0');

}

digit=cin.get();

position++;

}

int checksum;

//对输入的标识号长度进行奇偶检查

if((position-1)%2==0) checksum=evenLenthChecksum;

//position-1 原因：前段使用cin.get()函数，最后一个字符是表示结束的行末符

else checksum=oddLengthChecksum;

cout<<"Checksum is"<

if(checksum%10==0)

{

cout<<"Checksum is divisible by 10. Valid.\n";

}

else { cout<<"Checksum is not divisible by 10. Invalid. \n"}