Declaring a 2D vector

Question

In some cases only the below line works.Why so?

vector< vector<int>> a(M,N); 

This works in every case.

vector< vector<int>> a(M, vector<int> (N)); 

What's the difference?

Answer 1

std::vector has a fill constructor which creates a vector of n elements and fills with the value specified. a has the type std::vector<std::vector<int>> which means that it is a vector of a vector. Hence your default value to fill the vector is a vector itself, not an int. Therefore the second options is the correct one.

std::vector<std::vector<int>> array_2d(rows, std::vector<int>(cols, 0));

This creates a rows * cols 2D array where each element is 0. The default value is std::vector<int>(cols, 0) which means each row has a vector which has cols number of element, each being 0.

Answer 2

For declaring a 2D vector we have to first define a 1D array of size equal to number of rows of the desired 2D vector. Let we want to create a vector of k rows and m columns

 "vector<vector<int>> track(k);" 

This will create a vector of size k. Then use resize method.

for (int i = 0; i < k; i++) {     track[i].resize(m); 

In this way you can declare a 2D vector