c++ resize 1d vector into 2d vector code example

Example 1: initialize 2d vector of ints c++

auto M = 4;	// num of rows
auto N = 3; // num of cols in each row
auto default_value = 1; // default value of all int elements
std::vector<std::vector<int>> matrix(M, std::vector<int>(N, default_value));

Example 2: resize two dimensional vector c++

//vector<vector<int>> M;
//int m = number of rows, n = number of columns;
M.resize(m, vector<int>(n));