The Python Journey - Recursion

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Recursion (Wikipedia)Recursion Introduction (@ RealPython)

Page Contents

Introduction
When Do You Use It?
The Rules
Examples
AoC Examples

Introduction

I have to admit, I used to struggle with recursion. A lot of people do. It can be a bit mind-bending. But it’s a pretty simple concept and can be very useful.

In short: a recursive function is a function that calls itself. Thus, the code that defines the function will include a call to the same function.

As an anology, take a look at examples of recursive acronyms. See how the acronym definition includes the acronym itself!

Acronym	Definition
GNU	= GNU’s not Linux
LAME	= LAME Ain’t an MP3 Encoder
YAML	YAML Ain’t Markup Lanugage

When Do You Use It?

Typical use cases include:

Any time you need to calculate the next value of something, based on the current value of something.
To traverse some sort of tree or nested structure.

We’ll look at example of these in a bit.

The Rules

When creating a recursive function, there are only two rules you need to know:

The function must have an exit condition, called the base case. You need this, otherwise your recursive function will never end!
Each recursive call should move us closer to the base case.

Examples

Countdown

We want to create recursive function that counts down from an arbitary number n to 0. We can do it like this:

def countdown(n):
    print(n)
    if n == 0:
        return             # Terminate recursion
    else:
        countdown(n - 1)   # Recursive call, one closer to the base case

As per the rules:

We’ve defined a base condition. I.e. when we get to 0, we exit.
We’ve defined a condition that always moves us closer to the base condition. In this case, by decrementing the value of n each time by 1.

We can simplify this code:

def countdown(n):
    print(n)
    if n > 0:
        countdown(n - 1)   # Recursive call, one closer to the base case

Let’s try it. I’ve added the above code to a file called scratch.py, in my snippets folder. I’ll now execute it from the Python REPL:

>>> from snippets.scratch import *
>>> countdown(5)
5
4
3
2
1
0

Factorial

Recall the defition of factorial:

\(k! = k * (k-1)\)

This is slightly tricker than the previous example, since we’re not just printing a value with each iteration. Instead, we’re always multiplying the current iteration by the result of the previous iteration.

So we can code it like this:

def factorial(n):
     return 1 if n <= 1 else n * factorial(n - 1)

The base condition is when n == 1. In this situation, factorial should always return 1.
If not the base condition, we always multiply by a recursive call where n is decremented by 1.

Note that it’s common for any recusive function that calculates a product to have an exit condition that returns 1.

We can see how function works by adding some debugging statements:

def factorial(n):
    print(f"factorial() called with n = {n}")
    return_value = 1 if n <= 1 else n * factorial(n -1)
    print(f"-> factorial({n}) returns {return_value}")
    return return_value

Let’s run it from the REPL:

>>> from snippets.scratch import *
>>> factorial(4)
factorial() called with n = 4
factorial() called with n = 3
factorial() called with n = 2
factorial() called with n = 1
-> factorial(1) returns 1
-> factorial(2) returns 2
-> factorial(3) returns 6
-> factorial(4) returns 24
24

Note how each return is the product of n and the previous return value.

Counting Leaf Items in a Nested List

Here we create a recursive function that counts all the individual elements in a list. If the list is nested, the function recurses into each sub list, adding the elements of that list to the overall count.

def count_leaf_items(item_list):
    """Recursively counts and returns the number of leaf items
       in a (potentially nested) list. """
    count = 0
    for item in item_list:
        if isinstance(item, list): # if the element is itself a list, recurse...
            count += count_leaf_items(item)
        else: # count the item
            # this is the exit condition, i.e. when we've reached a leaf (element) rather than a nested list
            count += 1  

    return count

Let’s try this…

nested_list = [2, [3,5], [[10,20],30]]
print(nested_list)

res = count_leaf_items(nested_list)
print(res)

Output:

[2, [3, 5], [[10, 20], 30]]
6

Fibonacci

The Fibonacci sequence is an infinite sequence that generates the next number by adding the two preceding numbers.

1, 1, 2, 3, 5, 8, 13, 21...

I.e. to determine the nth value in the sequence:

\(f(n) = f(n-2) + f(n-1)\)

The base case is where n is 1, which returns a value of 1.

def fib(num: int) -> int:
    """ Recursive function to determine nth value of Fibonacci sequence.
    I.e. 1, 1, 2, 3, 5, 8, 13, 21...
    fib(n) = fib(n-2) + fib(n-1)

    Args:
        num (int): value of n, i.e. to determine nth value

    Returns:
        int: The nth value of the Fibonacci sequence
    """
    if num > 2:
        return fib(num-2) + fib(num-1)
    else:
        return 1

while True:
    try:
        input_val = input("Enter the value of n, or q to quit: ")
        if input_val.upper() == "Q":
            break
        
        print(fib(int(input_val)))
    except ValueError as err:
        print("Invalid input")

Note: this isn’t a particularly efficient function. It doesn’t scale well!

AoC Examples

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