Four Apt Elementary Examples of Recursion

We give four elementary examples of recursion that are reallife, non-trivial, more natural than the corresponding iterative approach, and do not involve any sophisticated algorithms, data structures, or mathematical problems. The examples are two forms of writing numbers in words, coalescing page references for an index, and finding unclosed begin blocks. My New Zoo, McGrew Zoo, will make people talk. My New Zoo, McGrew Zoo, will make people gawk At the strangest odd creatures that ever did walk. I’ll get, for my zoo, a new sort-of-a-hen Who roosts in another hen’s topknot, and then Another one roosts in the topknot of his, And another in his, and another in HIS, And so forth and upward and onward, gee whiz! —Dr. Seuss: If I Ran the Zoo