Understanding the Insertion Sort Algorithm in C++

Introduction

Sorting is a fundamental operation in computer science and plays a crucial role in various applications. One of the simplest yet effective sorting algorithms is the Insertion Sort algorithm. In this blog post, we will explore the provided C++ code, step by step, to understand how the Insertion Sort algorithm works and how it can be implemented.

Code Overview

Let’s begin by examining the code snippet you provided:

#include 
using namespace std;

void insertionSort(int *p, int size);
void insertionSort(int *p, int size)
{
    int i, j, temp;
    for (i = 1; i < size; i++) { temp = p[i]; j = i - 1; while (j >= 0 && p[j] > temp)
        {
            p[j + 1] = p[j];
            j--;
        }
        p[j + 1] = temp;
    }
    for (i = 0; i < size; i++)
    {
        cout << p[i] << " ";
    }
}

int main()
{
    int n;
    int *p;
    cout << "Enter the number of elements you want to store in an array:" << endl; cin >> n;
    p = new int[n];
    cout << "Now enter the values" << endl;
    for (int i = 0; i < n; i++) { cin >> p[i];
    }
    insertionSort(p, n);
}

Now, let’s break down this code step by step to understand its functionality:

1. Including Necessary Libraries

#include 
using namespace std;

These lines include the necessary header files for input and output operations and declare the standard namespace. This allows you to use the cin and cout functions for user input and output.

2. Defining the InsertionSort Function

void insertionSort(int *p, int size);

Here, a function insertion sort is declared, which takes two parameters: a pointer to an integer array p and the size of the array size. This function will be used to sort the array using the Insertion Sort algorithm.

3. Implementation of Insertion Sort

void insertionSort(int *p, int size)
{
    int i, j, temp;
    for (i = 1; i < size; i++) { temp = p[i]; j = i - 1; while (j >= 0 && p[j] > temp)
        {
            p[j + 1] = p[j];
            j--;
        }
        p[j + 1] = temp;
    }
    for (i = 0; i < size; i++)
    {
        cout << p[i] << " ";
    }
}

This is the core of the Insertion Sort algorithm. Here’s how it works:

It iterates through the array from the second element (index 1) to the last element (index size – 1).
For each element at index i, it stores the element’s value in the temp variable.
It then starts a loop with index j set to i – 1 and checks whether the element at index j is greater than temp. If so, it shifts the elements to the right until it finds the correct position for temp.
Finally, it inserts the temp value into its correct position in the sorted part of the array.

4. Main Function

int main()
{
    int n;
    int *p;
    cout << "Enter the number of elements you want to store in an array:" << endl; cin >> n;
    p = new int[n];
    cout << "Now enter the values" << endl;
    for (int i = 0; i < n; i++) { cin >> p[i];
    }
    insertionSort(p, n);
}

In the main function:

The user is prompted to enter the number of elements they want to store in the array.
Memory is dynamically allocated for an integer array p with a size of n.
The user is then prompted to enter the values for the elements of the array.
Finally, the insertion sort function is called with the array and its size as arguments, effectively sorting the array and printing the sorted result.

Conclusion

In this blog post, we’ve walked through the provided C++ code that implements the Insertion Sort algorithm. We’ve discussed each part of the code, from including necessary libraries to the core sorting algorithm and the main function that orchestrates user input and sorting. Understanding this code is a valuable step toward mastering sorting algorithms and their practical implementations in programming. Insertion Sort, though simple, is a useful sorting technique for small to moderately-sized arrays and can serve as a foundation for more complex sorting algorithms.

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