Trying to implement Luhn's algorithm in C, but not getting expected sum of every other digit

Question

I'm trying to implement Luhn's algorithm. For every other digit you have to take it, multiply it by 2, and add it to a running total. If the ith digit * 2 is greater than 10, you split it up, and just add it to the running total. I create a void function that does this and pass in 123456789 as a test value.

#include <stdio.h>
// #include <cs50.h> // 

void LuhnsAlgorithm(long); // needs to be changed to str return type when submitted
int main()
{


    int num = 123456789;
    LuhnsAlgorithm(num);
}

void LuhnsAlgorithm(long cc) // return type will be str when submitting
{
    int sum = 0;
    // case 1

    // case 2 (every other odd digit, multiplied by 2, and then added)
    long case2 = cc;
    while (case2 > 0)
    {
        if ((case2 / 10 % 10) * 2 >= 10) // checks if the every-other-digit's product when multiplied by 2 is bigger than 10 (aka, has 2 digits)
        {
            // creating this stupid temp variable 
            int digitBreakUp = case2 / 10 % 10 * 2;
            sum += (digitBreakUp / 10) + (digitBreakUp % 10);
        }
        else // if the product is just 1 digit then add it to the sum
        {
            sum += (case2 / 10 % 10) * 2;
        }
        case2 = case2 / 10;
    }

    printf("The sum of every last other digit in 123456789 is %i\n", sum);
}

The expected sum should be 22 (8 --> 1 + 6, 6 --> 1 + 2, 4 --> 8, 2 --> 4). But, I get 36. What's wrong? How do I get to 0 / get it to 'stop iterating' until it reaches the beginning of the number?

Thanks

Luhn's algorithm returns 1 digit so how can the expected result be 22? — Allan Wind, Oct 24 '22 at 00:26
You misunderstand what I'm doing. I'm only working on half the logic. I'm kind of a noob programmer so I'm doing it one piece at a time. — Shah, Oct 24 '22 at 00:42
Do you think it might be confusing when you say "I'm trying to implement Luhn's algorithm.", when in fact you are doing something else? — Allan Wind, Oct 24 '22 at 00:55
yeah, but you could have ignored it if it was so confusing? i followed all the guidelines of posting a question here. — Shah, Oct 24 '22 at 00:57

score 1 · Accepted Answer · answered Oct 24 '22 at 00:22

1

Since your test is for every other digit, the line of code:

case2 = case2 / 10;

Should be:

case2 = case2 / 100;

If you do that, your summation of every other digit does come out to 22.

Give that a try.

answered Oct 24 '22 at 00:22

NoDakker

3,390
1
10
11

Works perfectly, thanks. But I'm a little confused - to get to the second-to-last digit `8` in `123456789`... you just divide it by 10 once. Modding it will give you `8`. So why is it that you need to divide by `10` the first but need to divide by `100` everytime after? I can't figure that out and it hurts my brain a little. – Shah Oct 24 '22 at 00:40

Trying to implement Luhn's algorithm in C, but not getting expected sum of every other digit

1 Answers1