C 中的 Luhn 算法

Question

I'm very new to coding and one of the assignment is to program Luhn's Algorithm. After searching on the internet, everyone's solution looks so different and foreign:( so I don't know where the problem is with my solution. Any help is appreciated!

int main(void)
{
    // get card number from user
    long number;
    do
    {
        number = get_long("Number: ");
    } while (number < 0);

    // isolate digits of card number
    int digit;
    int product;
    int sum;
    int totalSum;
    int counter;
    for (counter = 1; number > 9; counter++) {
        for (int i = 1; number > 9; i = i * -1) {
            digit = number % 10;

            // isolate digits that need to be multiplied by 2
            if (i == 1) {
                product = digit * 2;

                // add products' digits
                if (product > 9) {
                    sum = (product % 10) + 1;
                }
            }
            // add sum of digits that weren't multiplied by 2
            totalSum = product + sum + digit;

            // update "new" number
            number = (number - digit) / 10;
        }
    }

    // checksum
    int check = totalSum % 10;
    if (check != 0) {
        printf("INVALID\n");
    } else {
        printf("VALID\n");
    }
}

Answer 1

There are a number of errors in your code, most having to do with how you use variables.

1. You use totalSum without ever initializing it, which means it can start with any random value!

2. You add both product and sum to totalSum every time, but you only update their values when some condition applies.
   This means at least half the time (maybe more) you add old values you already added previously.

3. Your loops exit when number is 9 or less, meaning you never check the leftmost (highest) digit of number.

As the comments suggested, you should read the pseudo code in Wikipedia , look carefully what they put in each variable, and what they sum and multiply.