In this article will discuss how to create 2D Matrix using vector of vectors in c++.

Requirement

Represent a 2D Matrix using vector in C++ i.e.

1 , 1 , 1 , 1
1 , 1 , 1 , 1
1 , 1 , 1 , 1
1 , 1 , 1 , 1
1 , 1 , 1 , 1

Declaration of 2D vector or vector of vector in C++

std::vector <std::vector<int> > vec2D

Initializing Vector of Vector – 2D vector

A vector can be initialized using parametrized constructor i.e.

std::vector <NUMBER OF ELEMENTS, VALUE OF EACH ELEMENT>

So,

std::vector<int> (4, 1)

Will create a vector of 4 integers whose values will be 1.

 

Now to create a vector of 5 vectors in which each vector is initialized as above, we will use following syntax,

std::vector <std::vector > vec2D(5, std::vector(4, 1));

Let’s see the code to initialize and print 2D vector as follows,
std::vector <std::vector<int> > vec2D(5, std::vector<int>(4, 1));

for(auto vec : vec2D)
{
	for(auto x : vec)
		std::cout<<x<<" , ";
	std::cout << std::endl;
}

Output

1 , 1 , 1 , 1 ,
1 , 1 , 1 , 1 ,
1 , 1 , 1 , 1 ,
1 , 1 , 1 , 1 ,
1 , 1 , 1 , 1 ,

Iterator over 2D vector in C++

We can iterate over a vector of vector using [][] . Checkout the code below,

for(int i = 0; i < 5; i++)
	for(int j = 0; j < 5; j++)
		vec2D[i][j] = i*j;

Adding a new row in 2D vector

To add a new row, just push_back a new vector in the vector of vector i.e.

vec2D.push_back(std::vector<int>(4, 11));

Complete working Code is as follows,
#include <iostream>
#include <vector>

int main()
{
	std::vector <std::vector<int> > vec2D(5, std::vector<int>(4, 1));

	for(auto vec : vec2D)
	{
		for(auto x : vec)
			std::cout<<x<<" , ";

		std::cout << std::endl;
	}

	std::cout << std::endl;

	for(int i = 0; i < 5; i++)
		for(int j = 0; j < 5; j++)
			vec2D[i][j] = i*j;

	for(auto vec : vec2D)
	{
		for(auto x : vec)
			std::cout<<x<<" , ";

		std::cout << std::endl;
	}


	vec2D.push_back(std::vector<int>(4, 11));

	std::cout << std::endl;

	for(auto vec : vec2D)
	{
		for(auto x : vec)
			std::cout<<x<<" , ";

		std::cout << std::endl;
	}
	return 0;
}

Compile the above code using following command,

g++ –std=c++11 2dvector.cpp

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