DavidOConnor:
Using an external interrupt with the reed switch is a better way to go. There are so many complications with polling.
My opinion is exactly the opposite. Using interrupts introduces a lot of design issues and complexities and should be avoided as far as possible, and if necessary only used reluctantly and to the minimum extent necessary. I don't see any need for them here. Using a polling based approach will be perfectly adequate in performance terms, and will make it easy to debounce the signal (which you're quite likely to need, with a reed switch).