Bubble Sort in Python

Published on 19 December 2018 (Updated: 29 January 2019)

In this article, we’ll be tackling Bubble Sort in Python.

How to Implement the Solution

At this point, let’s dig into the code a bit. The following sections break down the Bubble Sort in Python functionality.

Solution

#!/usr/bin/env python
import sys
from functools import reduce


def bubble_sort(xs):
    def pass_list(xs):
        if len(xs) <= 1:
            return xs
        x0 = xs[0]
        x1 = xs[1]
        if x1 < x0:
            del xs[1]
            return [x1] + pass_list(xs)
        return [x0] + pass_list(xs[1:])
    return reduce(lambda acc, _ : pass_list(acc), xs, xs[:])


def input_list(list_str):
    return [int(x.strip(" "), 10) for x in list_str.split(',')]


def exit_with_error():
    print('Usage: please provide a list of at least two integers to sort in the format “1, 2, 3, 4, 5”')
    sys.exit(1)


def main(args):
    try:
        xs = input_list(args[0])
        if len(xs) <= 1:
            exit_with_error()
        print(bubble_sort(xs))
    except (IndexError,ValueError):
        exit_with_error()


if __name__ == "__main__":
    main(sys.argv[1:])

The Main Function

Breaking down this solution bottom up,

if __name__ == "__main__":
    main(sys.argv[1:])

This bit of code checks to see if this is the main module run. If it is it then calls the main function and passes user input to it. In this case the user input would be a string of numbers like so "2, 1, 10, 5, 3" (to sort).

def main(args):
    try:
        xs = input_list(args[0])
        if len(xs) <= 1:
            exit_with_error()
        print(bubble_sort(xs))
    except (IndexError,ValueError):
        exit_with_error()

This is the main function of this file. It parses the input, then calls our bubble sort function (and prints the results). It also deals with any errors raised.

Transform Input Parameters

def input_list(list_str):
    return [int(x.strip(" "), 10) for x in list_str.split(',')]

This function takes a string like "2, 1, 10, 5, 3", and turns into a list of numbers. It does this using a list comprehension, first we need to convert our string into a list list_str.split(',') which is a list of strings split by comma (,). So our original input string becomes ["2", " 1", " 10", " 5", " 3"]. Then for each element in the list for x in ... , we do something to it.

In this example we convert it into a decimal integer, int(x.strip(" "), 10). Then x.strip(" "), removes any whitespace so " 1" becomes "1". Then int("1", 10) converts the string "1" into a decimal number in this case 1. This is done for every item in the list so our original input of "2, 1, 10, 5, 3" becomes [2, 1, 10, 5, 3].

Throw Errors

def exit_with_error():
    print('Usage: please provide a list of at least two integers to sort in the format “1, 2, 3, 4, 5”')
    sys.exit(1)

This function prints a message and then exits the script with an error, sys.exit(1). If any non-zero value is returned then the program didn’t complete properly. This function is called if the user input isn’t correct.

Bubble Sort

def bubble_sort(xs):
    def pass_list(xs):
        if len(xs) <= 1:
            return xs
        x0 = xs[0]
        x1 = xs[1]
        if x1 < x0:
            del xs[1]
            return [x1] + pass_list(xs)
        return [x0] + pass_list(xs[1:])
    return reduce(lambda acc, _ : pass_list(acc), xs, xs[:])

Finally we’re at the real meat and potatoes of the script. This function takes an unsorted integer list and returns a sorted list, using the bubble sort algorithm. This function bubble_sort has another function inside of it pass_list, also called a closure. The nested function can access the variables of the parent function but as read-only, they cannot change the variable values.

The pass_list function does one pass of the list and makes sure it’s sorted, it does by deleting elements from the list if they’re sorted and comparing the next two.

For example, if [3, 2, 5, 1] is the input:

• First we compare 2, 3
• 2 < 3
• Delete 2 from the list del xs[1]
• Call pass_list, [2] + pass_list(3,5,1)

So we combine the output of pass_list with 2.

Taking a look at the time pass_list is called.

• Compare 3, 5
• 3 < 5
• This time we don’t delete 3, we then pass every element of the list except the first one (3) xs[1:]
• Call pass_list [3] + pass_list(5,1).

The final bit of the code is what calls pass_list, where acc parameter is the xs list. The reduce function is used to call pass_list, multiple times and xs[:] is a copy of the xs so when xs is changed xs[:] is unaffected. The reduce function is used to loop through every element, it (reduce) it uses the output of the last iteration as input to the next one.

For example:

• For input [10, 3, 2, 5, 7] output is [3, 2, 5, 7, 10]
• Then input [3, 2, 5, 7, 10] output is [2, 3, 5, 7, 10]
• Then input is [2, 3, 5, 7, 10] output is [2, 3, 5, 7, 10]
• …

How to Run Solution

If we want to run this program, we should probably download a copy of Bubble Sort in Python. After that, we should make sure we have the latest Python interpreter. From there, we can run the following command in the terminal:

python bubble-sort.py "3, 2, 10, 6, 1, 7"

Alternatively, we can copy the solution into an online Python interpreter and hit run.