Credit card numbers typically have a structure to them: American Express uses 15-digit numbers all starting with 34 or 37, MasterCard uses 16-digit numbers starting with 51, 52, 53, 54, or 55 , and Visa uses 13 and 16-digit numbers all beginning with 4. These values are decimal numbers (0 through 9), not binary, which means, for instance, that American Express could print as many as 10^(15) = 1,000,000,000,000,000 unique cards! Credit cards rely on a built in "checksum", this

Harvard's CS50 spring term has just started, and the first assignment has come and gone. Here's what I worked on..