Recursion—or, Better Computational Thinking Through Laughter

Abstract

One of the predictable side effects of writing this column is that on a regular basis—at least several times a year—I am compelled to come up with a topic, a computational diversion. I would like to pretend that this process is structured, impeccably organized, planned months in advance; but as anyone who knows me could attest, things just do not work out that way. The result is that I am periodically on the lookout for some subject matter that could make for a column—and more especially, a subject that has not already been done to death in recreational math circles. Last weekend, I was in this sort of quietly panicked condition when I took my 7-yearold son to see the animated movie Kung Fu Panda. (We both enjoyed it.) At one point during the cartoon, the title creature is excitedly taking an introductory tour of his martial arts school, marveling at all the ancient kung fu-related artifacts; and he comes upon a beautifully rendered Chinese painting. The panda exclaims, ‘‘I’ve only seen paintings of that painting!’’ The line brought an appreciative laugh from the audience (including, interestingly, the children as well); and it occurred to me that there might be a relatively unexplored subject for discussion in this. After all, there is currently a push in computer science circles to promote ‘‘computational thinking’’ (a notion whose definition is itself energetically debated). At the same time, one of the purportedly difficult topics in computational thinking is mastery of the idea of recursion. The panda’s line—and the laugh that it received—suggests that people already have an aesthetic sense of the oddness or appeal of recursive thinking. And the panda’s line is, undeniably, recursive: once you have seen a painting of a painting, you could see a painting of a painting of a painting, and so forth. Recursion is funny, in many senses of the word. I am not entirely sure why this is so: after all, conditionals, and while loops, and compound data structures, and variables are not especially funny. But recursion is. Since arriving at this insight, I have been trying to collect a variety of instances in literature, plays, movies, to illustrate this sort of enjoyably emotionally charged computational thinking. I ask readers to help me in adding to this collection, and will print