Understanding Insertion Sort: A Step-by-Step Guide

Understanding Insertion Sort: A Step-by-Step Guide

Data Structure Algorithm


Sorting is a fundamental operation in computer science, and various algorithms have been developed to efficiently arrange elements in a specific order. One such algorithm is Insertion Sort, a simple yet effective sorting technique. In this blog post, we will delve into the details of Insertion Sort, including its pseudocode, time and space complexity analysis, and a practical implementation in C++.

Insertion Sort: Definition and Explanation

Insertion Sort is a sorting algorithm that builds the final sorted array one element at a time. It is much less efficient on large lists than more advanced algorithms such as quicksort, heapsort, or merge sort. However, insertion sort provides several advantages, including simplicity and efficiency on small datasets.

The algorithm works by iteratively selecting elements from the unsorted part of the array and placing them in the correct position within the sorted part. At each iteration, the selected element is compared with the elements in the sorted part, and it is moved to its proper place, shifting other elements if necessary.

Pseudocode for Insertion Sort

    n = length(arr)
    for i = 1 to n - 1:
        key = arr[i]
        j = i - 1
        while j >= 0 and arr[j] > key:
            arr[j + 1] = arr[j]
            j = j - 1
        arr[j + 1] = key

The pseudocode outlines the key steps of the Insertion Sort algorithm. It uses a loop to traverse the unsorted part of the array, and a nested loop to find the correct position for each element in the sorted part.

Time Complexity

The time complexity of Insertion Sort in the worst case is O(n^2), where 'n' is the number of elements in the array. This occurs when the array is in reverse order, and each element must be compared and shifted.

In the best case (already sorted array), the time complexity is O(n), making Insertion Sort efficient for small datasets or nearly sorted lists.

Space Complexity

Insertion Sort has a space complexity of O(1) since it only requires a constant amount of additional memory for the key and index variables. It sorts the array in-place without the need for additional data structures.

C++ Implementation

#include <iostream>
using namespace std;

void insertionSort(int arr[], int n) {
    for (int i = 1; i < n; ++i) {
        int key = arr[i];
        int j = i - 1;

        while (j >= 0 && arr[j] > key) {
            arr[j + 1] = arr[j];

        arr[j + 1] = key;

int main() {
    int arr[] = {12, 11, 13, 5, 6};
    int n = sizeof(arr) / sizeof(arr[0]);

    insertionSort(arr, n);

    cout << "Sorted array: ";
    for (int i = 0; i < n; ++i) {
        cout << arr[i] << " ";

    return 0;

This C++ program demonstrates the application of the Insertion Sort algorithm on an array of integers.


Insertion Sort, though not the most efficient algorithm for large datasets, is a valuable tool in certain situations. Its simplicity and low space complexity make it suitable for small lists or nearly sorted arrays. Understanding its pseudocode and analyzing time and space complexity provides insights into when and how to best apply Insertion Sort in real-world scenarios.

Remember, choosing the right sorting algorithm depends on the specific requirements and characteristics of the data you are working with.

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