Vector of Vectors in C++ STL with Examples

Last Updated : 1 Jul, 2026

A vector of vectors is a two-dimensional dynamic array in C++ where each element of the outer vector is itself a vector. Unlike traditional 2D arrays, a vector of vectors can have a variable number of rows and columns.

  • The outer vector stores the rows.
  • Each inner vector represents a row of elements.
  • The number of rows and columns can grow or shrink dynamically.
C++ 
vector<vector<int>> matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Syntax

vector<vector<data_type>> vec;

where:

  • data_type is the type of elements.
  • vec is the name of the 2D vector.

Initializing a Vector of Vectors

A vector of vectors can be initialized using a fixed size with default values or by using an initializer list.

Using Fixed Size and Default Value

vector<vector<int>> v(3, vector<int>(3, 7));

This creates a 3 × 3 matrix with all values initialized to 7.

Using Initializer List

vector<vector<int>> v = {

{1, 2, 3},

{4, 5, 6},

{7, 8, 9}

};

Example

C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {

    // Fixed size with default value
    vector<vector<int>> v1(3, vector<int>(3, 7));

    // Using initializer list
    vector<vector<int>> v2 = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    return 0;
}

Accessing Elements

Elements of a vector of vectors are accessed using two indices: one for the row and one for the column.

v[row][column]

We can also use the at() function.

C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {

    vector<vector<int>> v = {
        {1, 2, 3},
        {4, 5, 6}
    };

    cout << v[0][0] << endl;

    cout << v.at(1).at(2);

    return 0;
}

Output
1
6

Updating Elements

Elements can be modified by assigning a new value.

C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {

    vector<vector<int>> v = {
        {1, 2, 3},
        {4, 5, 6}
    };

    v[1][0] = 10;

    cout << v[1][0];

    return 0;
}

Output
10

Traversing a Vector of Vectors

A vector of vectors can be traversed using nested loops.

Using Range-Based Loop

C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {

    vector<vector<int>> v = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    for (auto& row : v) {
        for (int val : row) {
            cout << val << " ";
        }
        cout << endl;
    }

    return 0;
}

Output
1 2 3 
4 5 6 
7 8 9

Note: Using auto& creates a reference instead of making a copy, which improves performance and allows modification of elements.

Using Index-Based Traversal

C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<vector<int>> v = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    // Traversing using indices
    for (int i = 0; i < v.size(); i++) {
        for (int j = 0; j < v[i].size(); j++) {
            cout << v[i][j] << " ";
        }
        cout << endl;
    }

    return 0;
}

Output
1 2 3 
4 5 6 
7 8 9

Inserting Elements

In a vector of vectors, insertion can be performed in two ways:

  • Adding a new row.
  • Inserting elements into an existing row.
C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<vector<int>> v = {
        {1, 2, 3},
        {4, 5, 6}
    };

    // Add new row
    v.push_back({7, 8, 9});

    // Insert element in row
    v[1].insert(v[1].begin() + 1, 4);

    for (auto& row : v) {
        for (int val : row) {
            cout << val << " ";
        }
        cout << endl;
    }

    return 0;
}

Output
1 2 3 
4 4 5 6 
7 8 9

Removing Elements

Elements can be removed by:

  • Removing an entire row.
  • Removing specific elements within a row.

Example

C++ 
#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<vector<int>> v = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 6}
    };

    // Delete first row
    v.erase(v.begin());

    // Delete element in row
    v[0].erase(v[0].begin() + 1);

    // Delete last row
    v.pop_back();

    for (auto& row : v) {
        for (int val : row) {
            cout << val << " ";
        }
        cout << endl;
    }

    return 0;
}

Output
4 6

Advantages of Vector of Vectors

A vector of vectors provides several benefits over traditional two-dimensional arrays:

  • Dynamic number of rows and columns.
  • Automatic memory management.
  • Supports STL algorithms and iterators.
  • Easy insertion and deletion of rows.

Disadvantages of Vector of Vectors

Despite its flexibility, a vector of vectors has some limitations:

  • Memory is not stored contiguously like arrays.
  • Slightly slower than fixed-size 2D arrays.
  • Additional memory overhead due to nested vectors.
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