Luhn算法整理

Question

I am sorting this code:

bool checkLuhn(const string& cardNo) 
{ 
    int nDigits = cardNo.length(); 

    int nSum = 0, isSecond = false; 
    for (int i = nDigits - 1; i >= 0; i--) { 

        int d = cardNo[i] - '0'; 

        if (isSecond == true) 
            d = d * 2; 

        nSum += d / 10; 
        nSum += d % 10; 

        isSecond = !isSecond; 
    } 
    return (nSum % 10 == 0); 
} 

There is one mystery what i do not know. Have googled, but still mystery.

That code:

if (isSecond == true) 
d = d * 2;

Where on the code program detects is it splitable with 2? I understand if it's not splitable with 2, program multiply it with 2.

I understand operational principle of the program, but there must be some method or something which tells program what is that isSecond. Help me guys.

Answer 1

Now i understand it.

Before the for loop isSecond is false because first ordernumber is (0). Then this isSecond = !isSecond; change false to true. Then it's turn of the second ordernumber (1) and isSecond = true; so method multiplies it with 2. And then it goes false again and not multiplies number 2. Then it goes true and multiply number 3.

What a brain-teaser :D