Lecture 27

Recursion

MCS 260 Fall 2020
Emily Dumas

Reminders

OOP loose end: protocols

We implemented the sequence protocol last time. There are others.

• Iterator — creates an iterable
• Mapping — creates a dict-like type

Still more can be found in the collections.abc module, which contains classes you can subclass when implementing the protocols.

Recursion

A function in Python can call itself. This can be useful, for example if the result of the function at one argument is easy to obtain from the result at another argument.

This technique is called recursion. A function which uses it is a recursive function.

Factorial

The classic example of recursion (being easiest to understand) is the computation of factorials:

$ n! = n \times (n-1) \times (n-2) \times \cdots 2 \times 1$

e.g. $5! = 5 \times 4 \times 3 \times 2 \times 1 = 120$

Critical observation: $n! = n \times (n-1)!$

Recursive factorial

Let's build a function fact(n) that uses $n! = n \times (n-1)!$ as the basis of its operation.

Call stack

Python keeps track of all the function calls that are underway in a stack. Items on the stack indicate where the call originated.

Calling a function pushes an item on the stack.

Returning pops an item form the stack.

There is a maximum allowed stack size. Exceeding it is a stack overflow.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
        ]
    

Note "top" of stack is the last element.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
            Called fact on line 30 with argument 3
        ]
    

Note "top" of stack is the last element.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
            Called fact on line 30 with argument 3,
            Called fact on line 18 with argument 2
        ]
    

Note "top" of stack is the last element.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
            Called fact on line 30 with argument 3,
            Called fact on line 18 with argument 2,
            Called fact on line 18 with argument 1
        ]
    

Note "top" of stack is the last element.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
            Called fact on line 30 with argument 3,
            Called fact on line 18 with argument 2
        ]
    

Note "top" of stack is the last element.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
            Called fact on line 30 with argument 3
        ]
    

Note "top" of stack is the last element.

Computing fact(3)

If push is list.append and pop is list.pop:

call_stack == [
        ]
    

Note "top" of stack is the last element.

Recursive delete

How can we make a function delete(fn) that will delete fn if it is a file, or which will remove all files and directories inside fn and then remove fn itself if it is a directory?

Recursion pros and cons

Often can solve a problem with recursion or with loops (an iterative solution). Why use recursion?