What is a quick sort in C++ and how is it implemented?

Table of Contents

• Introduction
• Key Concepts of Quick Sort
  • 1. Pivot Selection:
  • 2. Partitioning:
  • 3. Recursive Sorting:
  • 4. In-place Sorting:
• Implementation of Quick Sort in C++
  • Step 1: Partitioning Function
  • Step 2: Recursive Quick Sort
  • Quick Sort Code in C++
  • Code Explanation:
  • Output:
• Practical Example: Sorting an Array of Student Scores
• Conclusion

Introduction

Quick sort is one of the most efficient sorting algorithms, following the divide-and-conquer approach. It is favored for its average-case time complexity of O(n log n), making it suitable for large datasets. Unlike merge sort, quick sort is in-place, meaning it does not require additional storage. It works by selecting a pivot element from the array and partitioning the other elements into two sub-arrays: one with elements less than the pivot and one with elements greater than the pivot.

In this article, we will walk through the steps of the quick sort algorithm and provide an implementation in C++.

Key Concepts of Quick Sort

1. Pivot Selection:

A pivot element is chosen from the array, which is used to partition the array into two parts. The choice of pivot can greatly affect the efficiency of the algorithm.

2. Partitioning:

The array is rearranged so that elements smaller than the pivot are placed before it, and elements greater than the pivot are placed after it.

3. Recursive Sorting:

After partitioning, quick sort is applied recursively to the sub-arrays.

4. In-place Sorting:

Quick sort rearranges elements in the original array, making it memory efficient.

Implementation of Quick Sort in C++

Step 1: Partitioning Function

The core of the quick sort algorithm is the partition function, which rearranges the array based on the pivot.

Step 2: Recursive Quick Sort

The main quick sort function recursively applies quick sort to the sub-arrays created by partitioning.

Quick Sort Code in C++

Code Explanation:

1. Partition Function:
   • Selects a pivot (in this case, the last element).
   • Rearranges elements, placing those smaller than the pivot to the left and those greater to the right.
   • Swaps elements as necessary to achieve correct partitioning.
2. quickSort Function:
   • Recursively applies quick sort to the left and right sub-arrays after partitioning.
   • The base case for recursion is when the sub-array has only one or zero elements.
3. printArray Function:
   • Prints the array to the console for verification.

Output:

Practical Example: Sorting an Array of Student Scores

Let's modify the quick sort implementation to sort an array of student scores in ascending order:

This modification will output the student scores in sorted order after applying quick sort.

Conclusion

Quick sort is a highly efficient sorting algorithm that follows a divide-and-conquer strategy to sort elements. Its in-place sorting mechanism ensures minimal memory usage, while its recursive nature and partitioning make it an ideal choice for large datasets. Though its worst-case complexity is O(n²), with proper pivot selection, quick sort is often one of the fastest sorting algorithms in practice. Understanding the implementation and working of quick sort in C++ equips you with the tools to handle efficient sorting in various real-world applications.