Lecture 14

Recursion vs Iteration II

MCS 275 Spring 2021
Emily Dumas

Lecture 14: Recursion vs Iteration II

Course bulletins:

• Project 2 description available.
• Project 2 due 6pm CST Friday, February 26.
• Check out the recursion sample code.

Plan

• A bit about project 2
• More on recursion, iteration, counting function calls
• Start on backtracking

Project 2 topic

Focuses on recursion. Based on special classes of strings that have the "pattern" ABB:

• Egg: A and B are single characters, e.g. egg, off, aaa
• Superegg: A is a superegg or single character and B is a single character, e.g. add, addee
• Hyperegg: A and B are each hypereggs or single characters, e.g. anoonoo, offeggegg, gooss

Project 2 task

You'll write functions to test whether a string belongs to these classes.

Details in the project description.

Later I will provide test data, but you'll need to write your own test code.

Fibonacci timing

Measured on a 4.00Ghz Intel i7-6700K CPU (2015 release date) with Python 3.8.5

fib call graph

Most Fibonacci numbers are computed many times!

fib call graph

Most Fibonacci numbers are computed many times!

Memoization

fib computes the same terms over and over again.

Instead, let's store all previously computed results, and use the stored ones whenever possible.

This is called memoization. It only works for pure functions, i.e. those which always produce the same return value for any given argument values.

math.sin(...) is pure; time.time() is not.

Memoizing fib

Let's add a simple memoization feature to our recursive fib function.

memoized fib call graph

memoized fib call graph

Fibonacci timing summary

Measured on a 4.00Ghz Intel i7-6700K CPU (2015 release date) with Python 3.8.5

Memoization summary

Recursive functions with multiple self-calls often benefit from memoization.

Memoized version is conceptually similar to an iterative solution.

Memoization does not alleviate recursion depth limits.

Call counts

One way to measure the expense of a recursive function is to count how many times the function is called.

Let's do this for recursive fib.

Corollary: $T(n) = 2F_{n+1}-1$.

Proof of corollary: Let $S(n) = 2F_{n+1}-1$. Then $S(0)=S(1)=1$, and $$\begin{split}S(n) &= 2F_{n+1}-1 = 2(F_{n} + F_{n-1}) - 1\\ &= (2 F_n - 1) + (2 F_{n-1}-1) + 1\\ & = S(n-1) + S(n-2) + 1\end{split}$$ Therefore $S$ and $T$ have the same first two terms, and follow the same recursive definition based on the two previous terms.

Corollary: Every time we increase $n$ by 1, the naive recursive fib does $\approx61.8\%$ more work.

(The ratio $F_{n+1}/F_n$ approaches $\frac{1 + \sqrt{5}}{2} \approx 1.61803$.)

Recursion with backtracking

How do you solve a maze?

Recursion with backtracking

How do you solve a maze?