Python: Quick Sort on Lists

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1. Introduction

In this blog post, we will learn how to write a Python program to implement Quick Sort on a list.

Quick Sort is a popular and efficient divide-and-conquer sorting algorithm that works by selecting a 'pivot' element from the array and partitioning the other elements into two sub-arrays, according to whether they are less than or greater than the pivot. The sub-arrays are then sorted recursively.

2. Program Overview

In this program, we will:

1. Define a function to partition the array.

2. Define the main quickSort function that selects a pivot, partitions the array, and then recursively sorts the two sub-arrays.

3. Test the quick sort implementation on a sample list.

3. Code Program

def quickSort(arr):
    # Base case: If the array has one or zero elements, it's already sorted.
    if len(arr) <= 1:
        return arr

    pivot = arr[len(arr) // 2]
    left = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right = [x for x in arr if x > pivot]

    # Recursively sort the left and right half and combine them with the middle (pivot)
    return quickSort(left) + middle + quickSort(right)

# Test the function
arr = [38, 27, 43, 3, 9, 82, 10]
sorted_arr = quickSort(arr)
print("Original Array:", arr)
print("Sorted Array:", sorted_arr)

Output:

Original Array: [38, 27, 43, 3, 9, 82, 10]
Sorted Array: [3, 9, 10, 27, 38, 43, 82]

4. Step By Step Explanation

1. A pivot element is chosen from the array. For simplicity, we've chosen the middle element here.

2. Elements less than the pivot are moved to its left, and elements greater than the pivot are moved to its right.

3. quickSort is then called recursively on the left and right sub-arrays.

4. The process continues until the base case is reached, i.e., the array to be sorted has 1 or 0 elements.

5. The sorted array is constructed by combining the sorted left sub-array, the pivot, and the sorted right sub-array.