An elephant in Cairo is a term used in computer programming to describe a piece of data that matches the search criteria purposefully inserted at the end of a search space, in order to make sure the search algorithm terminates; it is a humorous example of a sentinel value.
The term derives from a humorous essay circulated on the Internet that was published in Byte magazine in September 1989, describing how various professions would go about hunting elephants.
[1] When hunting elephants, the article describes programmers as following this algorithm:[1] This algorithm has a bug, namely a bounds checking error: if no elephants are found, the programmer will continue northwards and end up in the Mediterranean sea, causing abnormal termination by drowning.
Thus experienced programmers modify the above algorithm by placing a known elephant in Cairo to ensure that the algorithm will terminate.
[2] The modified algorithm is therefore as follows: