2.5 hours tonight. I finished the last problem from pset1 in cs50 on edx. This was more complicated than I had expected it to be! It was partially implementing the Lund algorithm. That is a quick checksum for seeing if credit card numbers are theoretically valid. It does some math on every other digit and then sums everything and looks for a zero in the end. The concept was something that I had run into before, but never in c. It had been humbling to see how far down you need to go in order to get functionality that I consider pretty basic. My initial thought was to see if I can access digits of a number like indexed chars in a string. That is not possible. It is also not very easy to convert a number into a string. It is also a bit harder to coerce that same number as a string to become a number again. I ended up not doing the num to string to num route, but I found a lot of bad ways to try it before I found even one that I could get to work.

In the end I looped through the number once for every character and subtracted the number from (the number / 10 and then * 10.) Since they are ints and just lose the digit when dividing this was able to get me the last digit. I then set the number to one tenth of itself and looped again.

After the (relatively simple) math was all done you just have to do a modulo operator on the sum in order to see if it ends in a zero. Then you need to do testing on the length and starting characters in order to see if it is a valid mastercard, visa, or amex. I used a lot of conditional logic in this section and I am unsure how it could be cleaned up more. I also stuck a bunch of return statements in there. I know that is controversial, but if I know it is a valid amex, then I should say that and stop thinking about it.

I’m surprised how difficult the second week homework is. I did pick the more difficult problem, but still. This is going to get interesting.