C Program For Insertion Sort

Insertion sort is a simple sorting algorithm used to sort a collection of elements in a given order. It is less efficient on large lists than more advanced algorithms such as quicksort, heapsort, or merge sort but it is simple to implement and is suitable to sort small data lists.

In this article, we will learn about the insertion sort, how it works, and how to implement insertion sort in a C program.

What is Insertion Sort?

Insertion sort is one of the simple and comparison-based sorting algorithms. The basic idea behind the algorithm is to virtually divide the given list into two parts: a sorted part and an unsorted part, then pick an element from the unsorted part and insert it in its place in the sorted part. It does this till all the elements are placed in the sorted part.

How Insertion Sort Works in C?

Insertion sort divides the list into sorted and unsorted part. Initially, the first element is already considered sorted, while the rest of the list is considered unsorted. The algorithm then iterates through each element in the unsorted part, picking one element at a time, and inserts it into its correct position in the sorted part.

Let's take an example of array arr = {12, 11, 13, 5, 6} that we need to sort in ascending order.

Insertion Sort Algorithm in C

• Start with the second element (index 1) as the key.
• Compare the key with the elements before it.
• If the key is smaller than the compared element, shift the compared element one position to the right.
• Insert the key where the shifting of elements stops.
• Repeat steps 2-4 for all elements in the unsorted part of the list.

C Program to Implement Insertion Sort

// C program to implement insertion sort
#include <math.h>
#include <stdio.h>

void insertionSort(int arr[], int N) {

    // Starting from the second element
    for (int i = 1; i < N; i++) {
        int key = arr[i];
        int j = i - 1;

        // Move elements of arr[0..i-1], that are
      	// greater than key, to one position to
      	// the right of their current position
        while (j >= 0 && arr[j] > key) {
            arr[j + 1] = arr[j];
            j = j - 1;
        }

        // Move the key to its correct position
        arr[j + 1] = key;
    }
}

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

    printf("Unsorted array: ");
    for (int i = 0; i < N; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    // Calling insertion sort on array arr
    insertionSort(arr, N);

    printf("Sorted array: ");
    for (int i = 0; i < N; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    return 0;
}

Unsorted array: 12 11 13 5 6 
Sorted array: 5 6 11 12 13 

Time Complexity: O(N²)
Auxiliary Space: O(1)

Advantages of Insertion Sort

• Insertion sort is that it is an in-place sorting algorithm.
• It is simple to implement which is great for small datasets.
• Insertion sort is adaptive in nature, i.e. it is appropriate for data sets which are already partially sorted.
• Best-case scenario only requires O(n) time.

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Article Tags :

• Sorting
• C Programs
• C Language
• DSA
• InsertionSort
• C-DSA

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Practice Tags :

• Sorting