How to Change an Element by Index in a Vector in C++?

In C++, vectors are dynamic containers that store the data in a contiguous memory location but unlike arrays, they can resize themselves to store more elements. In this article, we will learn how to change a specific element by its index in a vector in C++.

Example:

Input:
myVector ={1,2,3,4,5}

Output:
myVector ={1,2,7,4,5} 
// replaced element at index 2 with 7

Change a Specific Element in a Vector by Index

To replace a specific element in a vector using its index, we can use the [] array subscript operator. It is used with the index and the vector name and we directly assign the new value using the assignment operator.

C++ Program to Change a Specific Element in a Vector by Index

C++

// C++ program to demonstrate changing an element in a
// vector
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    // Initialize a vector
    vector<int> vec = { 1, 2, 3, 4, 5 };

    // Output the original vector
    cout << "Original vector: ";
    for (int i : vec) {
        cout << i << " ";
    }
    cout << endl;

    // Index of the element to be changed
    int index = 2;
    // New value to be set
    int newValue = 10;

    // Check if the index is within range
    if (index >= 0 && index < vec.size()) {
        // Modify the element at the specified index
        vec[index] = newValue;
    }
    else {
        cout << "Index out of range!" << endl;
        return 1;
    }

    // Output the modified vector
    cout << "Modified vector: ";
    for (int i : vec) {
        cout << i << " ";
    }
    cout << endl;

    return 0;
}

Output

Original vector: 1 2 3 4 5 
Modified vector: 1 2 10 4 5 

Time Complexity : O(1)
Auxilary Space: O(1)