Vector of Vectors in C++ STL with Examples

Prerequisite: Vectors in C++ STL

Vectors are known as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container.

Vector of Vectors is a two-dimensional vector with a variable number of rows where each row is vector. Each index of vector stores a vector which can be traversed and accessed using iterators. It is similar to an Array of Vectors but with dynamic properties.

Syntax:

vector<vector<data_type>> vec;

Example:

vector<vector<int>> vec{ { 1, 2, 3 }, 
                         { 4, 5, 6 }, 
                         { 7, 8, 9, 4 } }; 
where vec is the vector of vectors with different
      number of elements in different rows

Insertion in Vector of Vectors

Elements can be inserted into a vector using the push_back() function of C++ STL.

Below example demonstrates the insertion operation in a vector of vectors. The code creates a 2D vector by using the push_back() function and then displays the matrix.

Syntax:

vector_name.push_back(value)

where value refers to the element
      to be added in the back of the vector

Example 1:

v2 = {1, 2, 3}
v1.push_back(v2);

This function pushes vector v2 into vector of vectors v1. Therefore v1 becomes { {1, 2, 3} }.

Example 2:

v2 = {4, 5, 6}
v1.push_back(v2);

This function pushes vector v2 into existing vector of vectors v1 and v1 becomes v1 = { {1, 2, 3}, {4, 5, 6} }

Below is the example to demonstrate insertion into a vector of vectors.

// C++ program to demonstrate insertion
// into a vector of vectors

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

// Defining the rows and columns of
// vector of vectors
#define ROW 4
#define COL 5

int main()
{
    // Initializing the vector of vectors
    vector<vector<int> > vec;

    // Elements to insert in column
    int num = 10;

    // Inserting elements into vector
    for (int i = 0; i < ROW; i++) {
        // Vector to store column elements
        vector<int> v1;

        for (int j = 0; j < COL; j++) {
            v1.push_back(num);
            num += 5;
        }

        // Pushing back above 1D vector
        // to create the 2D vector
        vec.push_back(v1);
    }

    // Displaying the 2D vector
    for (int i = 0; i < vec.size(); i++) {
        for (int j = 0; j < vec[i].size(); j++)
            cout << vec[i][j] << " ";
        cout << endl;
    }
    return 0;
}

Output:

10 15 20 25 30 
35 40 45 50 55 
60 65 70 75 80 
85 90 95 100 105

Removal or Deletion in a Vector of Vectors

Elements can be removed from a vector of vectors using the pop_back() function of C++ STL.

Below example demonstrates the removal operation in a vector of vectors. The code removes elements from a 2D vector by using the pop_back() function and then displays the matrix.
Syntax:

vector_name[row_position].pop_back()

Example 1: Let the vector of vectors be vector v = { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } }

v[2].pop_back()

This function removes element 9 from the last row vector. Therefore v becomes { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8 } }.

Example 2:

v[1].pop_back()

This function removes element 6 from the last second row vector. Therefore v becomes { { 1, 2, 3 }, { 4, 5 }, { 7, 8 } }.

Below is the example to demonstrate removal from a vector of vectors.

// C++ program to demonstrate removal
// from a vector of vectors

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

// Driver Method
int main()
{
    // Initializing 2D vector "vect" with
    // sample values
    vector<vector<int> > vec{ { 1, 2, 3 },
                              { 4, 5, 6 },
                              { 7, 8, 9 } };

    // Removing elements from the
    // last row of the vector
    vec[2].pop_back();
    vec[1].pop_back();

    // Displaying the 2D vector
    for (int i = 0; i < 3; i++) {
        for (
            auto it = vec[i].begin();
            it != vec[i].end(); it++)
            cout << *it << " ";
        cout << endl;
    }
    return 0;
}

Output:

1 2 3 
4 5 
7 8

Traversal of a Vector of Vectors

The vector of vectors can be traversed using the iterators in C++. The following code demonstrates the traversal of a 2D vector.

Syntax:

for i in [0, n)
{
    for (iterator it = v[i].begin();
         it != v[i].end(); it++) 
   {
        // Operations to be done
        // For example to print
        print(*it)
    }
}

Below is the example to demonstrate traversal in a vector of vectors.

// C++ code to demonstrate traversal
// of a 2D vector

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

// Driver Method
int main()
{
    // Initializing 2D vector "vect" with
    // sample values
    vector<vector<int> > vec{ { 1, 2, 3 },
                              { 4, 5, 6 },
                              { 7, 8, 9 } };

    // Displaying the 2D vector
    for (int i = 0; i < 3; i++) {
        for (
            auto it = vec[i].begin();
            it != vec[i].end(); it++)
            cout << *it << " ";
        cout << endl;
    }

    return 0;
}

Output:

1 2 3 
4 5 6 
7 8 9