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Recursion

We introduce recursion from the beginning, repeating the materials given in the lecture just before classes got temporarily cancelled.

Slides for all 3 videos of this lecture: pdf

Part 1: Introduction and a matryoshka (Russian dolls) example

A recursive function is a function that calls itself. We discuss the nesting of Russian dolls and write a recursive function that simulates opening a set of Russian dolls.

• video
• audio only
• Be sure to modify and run playWithDolls.py as suggested in the video. You will also need this module, russian.py, which defines the class doll.

Part 2: More examples

We discuss two more examples, doing a count down and computing the factorial of a number, to reinforce the pattern discussed previously: in a recursive function there is a part for the simple case (called the "base case") and a part for the complex case (recursive case).

• video
• audio only
• Code: blastoff.py, factorial.py

Part 3: Divide and Conquer

Recursion is great for "divide and conquer"! There are 3 steps: (1) Decide how to deal with "small data" (the base case), (2) Decide how to break up the problem, (3) Decide how to combine the answers from the subproblems. The examples discussed are counting the number of 'e's in a string and removing blanks from a string.

• video
• audio only
• Be sure to visualize a call of the deblank function using Python Tutor! Code: num_es.py, deblank.py