How to Insert Elements into 2D Vector in C++?

Last Updated : 22 Nov, 2024

In C++, 2D vectors provide several built-in methods to insert elements. The efficiency of the insertion depends on where the insertion occurs. In this article, we will explore different ways to insert elements into a 2D vector in C++ and their specific use cases.

The simplest way to add elements to a 2D vector is by appending the rows of elements (1D vectors) to the end using the vector push_back() method. Let's take a look at an example:

C++

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<vector<int>> v = {{1, 2, 3}, 
                            {4, 5, 6}};
  
    // Adding a new row at the end
    v.push_back({7, 8, 9});
  
    // Adding an element to the last row
    v[2].push_back(10);

    for (const auto& i : v) {
        for (int j : i)
            cout << j << " ";
        cout << endl;
    }
    return 0;
}

Output

1 2 3 
4 5 6 
7 8 9 10

Inserting elements in the middle is an expensive operation in a vector because all elements after the insertion point need to be shifted.

Using Vector insert()

The vector insert() method is versatile and can insert elements at any position in a 2D vector. It can also insert multiple elements simultaneously.

C++

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<vector<int>> v = {{1, 3, 5}, 
                             {7, 9, 11}};

    // Inserting a new row at position 1
    v.insert(v.begin() + 1, {4, 6, 8});

    // Inserting a value into 2nd row at position 2
    v[1].insert(v[1].begin() + 2, 10);

    for (const auto& i : v) {
        for (int j : i)
            cout << j << " ";
        cout << endl;
    }
    return 0;
}

Output

1 3 5 
4 6 10 8 
7 9 11

Using vector emplace()

The vector emplace() method works similarly to vector insert(), but instead of copying an element, it constructs the element directly in the vector. This avoids unnecessary copies, improving performance.

C++

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<vector<int>> v = {{1, 3, 5}, 
                             {7, 9, 11}};

    // Emplacing a new row at position 1
    v.emplace(v.begin() + 1, vector<int>{4, 6, 8});

    // Emplacing value into 2nd row at position 2
    v[1].emplace(v[1].begin() + 2, 10);

    for (const auto& i : v) {
        for (int j : i)
            cout << j << " ";
        cout << endl;
    }
    return 0;
}

Output

1 3 5 
4 6 10 8 
7 9 11

Using vector emplace_back()

The vector emplace_back() method is the emplacing counterpart of push_back(). It constructs the new element directly in the vector, avoiding extra copies.