Data Structures and Algorithms, IT Technology

How does C++ STL use vector’s vector? Code samples

Posted on by Thao Nguyen

Vectors are known as dynamic arrays, capable of automatically resizing themselves when elements are inserted or deleted, and containers automatically handle their storage.

A vector of a vector is a two-dimensional vector with a variable number of rows, where each row is a vector. Each index of a vector stores a vector, which can be traversed and accessed using the following methods. It is similar to a vector array, but with dynamic properties.

The syntax is as follows:

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

Inserts in a vector of vectors

Element can use the function of pushing back ()C++ STL.

The following example illustrates an insertion operation in a vector carrier. The code uses the push_back() function to create a 2D vector and then displays the matrix.

The syntax is as follows:

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 the vector of vector v1. So v1 becomes {{1, 2, 3}.

Example 2:

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

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

The following is an example that demonstrates the insertion vector vector.

// 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;
}

The output is as follows:

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

Delete or delete in a vector vector

You can use vectors to remove elements from vectors pop_back the functionality of ()C++ STL.

The following example illustrates a delete operation in a vector vector. The code uses the pop_back() function to remove elements from the 2D vector and then display the matrix.

The syntax is as follows:

vector_name[row_position].pop_back()

Example 1: Let the vector of a vector 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 of vectors. So 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. So v becomes {{1, 2, 3}, {4, 5}, {7, 8}}.

The following is an example that demonstrates the removal from a vector carrier.

// 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;
}

The output is as follows:

1 2 3 
4 5 
7 8

Traversal of vectors

Vectors of vectors can be used with iterators in C++. The following code demonstrates the traversal of a 2D vector.

The syntax is as follows:

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)
    }
}

The following is an example that demonstrates traversal in a vector vector.

// 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;
}

The output is as follows:

1 2 3 
4 5 6 
7 8 9

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Thao Nguyen