CS 111: Introduction to Computer Science

Recursion notes

Nothing to hand in

Example scenarios from class 11/2

Learning to think recursively takes practice. Roughly, you have to ask the question "does the problem I'm trying to solve have a smaller version of the same problem inside it?" or alternatively "can I solve my problem by first solving a smaller problem of the same type and then doing a little bit of computation?"

Here are some example scenarios that we looked at in class.

• Triangular number T(N) = 1 + 2 + ... + N
• Max value M(L) of a list of integers
• Sum S(L) of a list of integers
• Number E(S) of e's in a string S
• Reverse string R(S) of a string S (e.g. R('bat') = 'tab')
• IsPal(S): True if S is a palindrome, False otherwise. (Think about S[1:-1])

Some code

• The recursive functions we played with in class
• The Sierpinski's Triangle program