SENTINELS

A sentinel is a special value that marks the end of a sequence of values.
• Simply ask the user, at the end of each loop iteration, if there is another value to process. If the sequence of values is long, however, asking this question at the end of each loop iteration might make the program cumbersome for the user.
• Ask the user at the beginning of the program how many items are in the sequence. This might also inconvenience the user, however. If the sequence is very long, and the user does not know the number of items it contains, it will require the user to count them.