How to initialize an array of vector<int> in C++ with predefined counts?

arrays, c++, initialization, stl, vector

Solution

This creates a vector containing 10 `vector<int>`, each one of those with 5 elements:

std::vector<std::vector<int>> v(10, std::vector<int>(5));

Note that if the size of the outer container is fixed, you might want to use an `std::array` instead. Note the initialization is more verbose:

std::array<std::vector<int>, 10> v{{std::vector<int>(5), 
                                    std::vector<int>(5), 
                                    std::vector<int>(5), 
                                    std::vector<int>(5), 
                                    std::vector<int>(5),
                                    std::vector<int>(5), 
                                    std::vector<int>(5), 
                                    std::vector<int>(5), 
                                    std::vector<int>(5), 
                                    std::vector<int>(5)
                                    }};

Also note that the contents of array are part of the array. It's size, as given by `sizeof`, is larger than the `vector` version, and there is no O(1) move or swap operation available. An `std::array` is akin to a fixed size, automatic storage plain array.

Note also that, as @chris suggests in the comments, you can chose to set the elements of the array after a default initialization, e.g. with `std::fill` if they are all to have the same value:

std::array<std::vector<int>, 10> v; // default construction
std::fill(v.begin(), v.end(), std::vector<int>(5));

otherwise, you can set/modify the individual elements:

v[0] = std::vector<int>(5); // replace default constructed vector with size 5 one
v[1].resize(42); // resize default constructed vector to 42

and so on.

Problem

Original source