Binary search is one of my favorite examples for introducing people to loop invariants. There are tons of opportunities to mess up with an off-by-one error but

Binary search is one of my favorite examples for introducing people to loop invariants. There are tons of opportunities to mess up with an off-by-one error but if you state the invariant beforehand getting it right is super easy. There are also many variations of the binary search problem so you can't just memorize the implementation.