Quick Sort in Python

Published on 17 January 2019 (Updated: 02 May 2020)

Quick Sort in Python

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

Solution

#!/usr/bin/env python
import sys


def quick_sort(xs):
    if len(xs) <= 0:
        return []

    left = quick_sort([l for l in xs[1:] if l <= xs[0]])
    right = quick_sort([r for r in xs[1:] if r > xs[0]])
    return left + xs[:1] + right


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(quick_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(quick_sort(xs))
    except (IndexError,ValueError):
        exit_with_error()

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

Finally we wrap this entire block in a try … except, and we catch two exceptions: IndexError and ValueError. IndexError will be thrown if args isn’t a list, and we try to access args[0]. ValueError will be thrown if we try to convert a non-integer string into an integer. For example if args[0] was “a” -> int(“a”). If any exceptions are raised, then we call the exit_with_error() function.

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) 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.

Quick Sort

def quick_sort(xs):
    if len(xs) <= 0:
        return []

    left = quick_sort([l for l in xs[1:] if l <= xs[0]])
    right = quick_sort([r for r in xs[1:] if r > xs[0]])
    return left + xs[:1] + right

Now onto the main part of the program. This is the function that actually sorts our list. The first part len(xs) <= 0 checks if the list is empty. If it is, then it returns an empty list return [].

In each iteration of the quicksort we have to pick a pivot element. This element is used to break down the current list into two smaller lists. One list contains all numbers larger than the pivot and the other list contains all elements smaller (or equal) to the pivot. There are a few ways to pick a pivot element but in our code we will be using the first element of each list (xs[0]).

We create two sub-lists from our list. left will contain all elements smaller than xs[0], the first element in our list. right will contain all elements larger than xs[0]. These lists aren’t actually sorted yet so we call the quick_sort() function on these sub-lists recursively until they are sorted. Each of these sub-lists is broken down into two lists every time the quick_sort() function is called.

left = quick_sort([l for l in xs[1:] if l <= xs[0]])

Taking a closer look at how we generate the left sub-list, we use a list comprehension which is a way to generate lists in Python. In this example [l for l in xs[1:] if l <= xs[0]] we loop through the unsorted list xs. Examining the first part [l for l in xs[1:]] this would generate a list which would contain every element in xs except the first one. xs[1:] means that the list includes every element except the first one, hence the [1:]. This is called index splicing in Python, and you can learn more about it here.

For example, if xs = [3, 2, 4, 1], then the generated list would be left = [2, 4, 1]. Now including the final part of the list comprehension [l for l in xs[1:] if l <= xs[0]], l is only added to the list if it’s less or equal to the first element of xs, if l <= xs[0]. So again using our example xs = [3, 2, 4, 1] the generated list would beleft = [2, 1], since 4 > 3 and xs[0] = 3. This new list is then passed to our quick sort function.

The right = quick_sort([r for r in xs[1:] if r > xs[0]]) is very similar except it stores elements strictly greater than xs[0]. But other than that it generates the list in much the same way left does. So using the same example xs = [3, 2, 4, 1] then right = [4]. Since 4 is the only element great then xs[0] = 3.

Finally, those two lists are combined into a single list and returned return left + xs[:1] + right, where left contains all elements less than or equal to xs[0] and right contains all the items great than xs[0]. xs[:1] in this example gets the first element, except this also works if xs is empty. If xs = [] and you try to get xs[0] the program will throw an IndexError whereas xs[:1] = [] an empty list.

Let’s take a look at an example where xs = [5, 1, 10]

Main Quick Sort

Left Quick Sort

Since the lists are empty quick_sort will return [] an empty list back so.

Right Quick Sort

Since the lists are empty quick_sort will return [] an empty list back so.

Back to Main Quick Sort

So the final sorted list is [1, 5, 10]

How to Run Solution

If we want to run this program, we should probably download a copy of Quick 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 quick-sort.py "3, 2, 10, 6, 1, 7"

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

Further Reading