Multidimensional Arrays

We can have an array of arrays. This would again be fixed-size, so we’ll need to specify both the number of rows and the number of columns at compile time.

Here’s an example:

Listing 99 multid_array.cpp

#include <iostream>
#include <array>
#include <format>

using row_arr_t = std::array<double, 4>;
using fixed_mat_t = std::array<row_arr_t, 3>;

int main() {

    fixed_mat_t M{0.0};

    double val{0.0};
    for (auto &r : M) {
        for (auto &c : r) {
            c = val++;
        }
    }

    for (auto r : M) {
        for (auto c : r) {
            std::cout << std::format("{:4} ", c);
        }
        std::cout << std::endl;
    }
}

Notice that we need to explicitly set the size of both the row_arr_t and the fixed_mat_t. While this solves the issue we had with our std::vector<std::vector<double>> where we could have rows of varying length, it is less flexible in that we need to know the size ahead of time.

There are a few features here that we have not yet seen.

• We use the format library to set the width (number of characters) to use when writing out our numbers. This makes them line up nicely.

• We access the information in the arrays using a reference (with the & operator). This gives us direct access to the memory without needing to make a copy.

Note

We could also do this with std::vector.