Ever wonder how squash or tennis rating systems work? Are you interested in designing one? If so, this is the problem for you. We will give you the algorithm for determining the rating changes of two players after they have played a match: you have to design the system to keep the ratings up to date.

Sounds easy? It is, if match results are input in chronological order. However, the real world does not work that way! Players enter results out of order and make mistakes. The system has to maintain the ratings as if the corrected match results were input in the correct order. Not so easy.

Players also want to track the effects of each match on their rating, find challenging opponents (particularly if they are improving quickly) and see their historical performance against a particular opponent. Club pros also get into the game (pun intended) to set initial ratings, to adjust ratings, and to enter or fix match results for members of their clubs.

Veterinary medical systems are very similar to human medical systems. One of the biggest differences is that patients are animals and clients are people who own the animals. This introduces some interesting complications when it comes time to pay for the bills. The system keeps track of what procedures and medicines were used on the patient, but when it comes time to pay, the bill is sent to the client.