15-112 Lecture 11 (June 5, 2014)

Recursion Practice

We got warmed up with some recursion practice. Please find some similar problems below:

Binary Recursion and a Recursion Tree

We then drove down and took a deeper look at fib(), as implemented below:

```#!/usr/bin/python

def fib(n):
if (n == 0):
return 0
if (n == 1):
return 1

return fib(n-2) + fib(n-1)
```

It was the first example we'd seen of binary recursion, which is the case where a single recursive call can directly make two more recursive calls. Binary recursion is interesting, becuase it can "blow up", doing a lot of work really quickly. As the n increases incrementally (linearly), the tree double sin size, resulting in rapid(exponential) growth in the amount of work needed.

For example, consider the call tree for fib(4), shown below: Memoization: A Better Way?

What is interesting about the tree above is that it not only grows rapidly -- but it is very redundant. It computes fib(0) twice, fib(1) 3 times, and fib(2) twice.

One way to save time is to store the vales the first time we calculate them, so we can used the stored values rather than recomputing for subsequent uses. This technique is called memoization. You'll talk a lot about it in 15-210. But, we worked through a couple of implementations for this example in class.

The first example is somewhat straight-forward. We use a list to store values as we compute them. We pre-popualte it with our base cases of 0 and 1. As we need values, we try to look them up. If they aren't there, we get an exception, add them, and keep chugging along:

`
```#!/usr/bin/python

fibs = [1,1]

def fib(n):
try:
return fibs[n]
except:
fibs.append((fib(n-2) + fib(n-1)))
return fibs[n]

print fib(0)
print fib(1)
print fib(2)
print fib(3)
print fib(4)
print fib(5)
print fib(6)
```
`

One interesting observation is that we learn about the series from lowest-to-highest, from left-to-right in our list. So, instead of using exceptions, we can also just look at the length of the list as in the example below:

`
```#!/usr/bin/python

fibs = [1,1]

def fib(n):
if (n >= len(fibs)):
fibs.append((fib(n-2) + fib(n-1)))
return fibs[n]

print fib(0)
print fib(1)
print fib(2)
print fib(3)
print fib(4)
print fib(5)
print fib(6)
```
`

The cool thing about this approach is that as n grows "linearly", e.g., from 1, to 2, to 3, to 4, to 5, to 6, to 7, etc, so does the amount of work. For example, each time n gets one bigger -- we only need to add one more thing to the list. Before, each increment of n resulted in approximately a doubing of the size of the recursion tree, which illustrates the doubling of the amount of work and time required.

Cool, huh?