Extract data and store in struct

Question

I have three std::vectors

std::vector<std::string> vec1{ "one","two","three" };
std::vector<std::string> vec2{ "1","2","3" };
std::vector<std::string> vec3{ "i","ii","iii" };

and a struct

struct MyData {
  std::string str1;
  std::string str2;
  std::string str3;
};

I need to obtain a std::vector<MyData>, which is filled from the data of the three vectors:

std::vector<MyData> myData;
myData.reserve(vec1.size());
for (int i = 0; i < vec1.size(); ++i) {
  myData.emplace_back(vec1[i],vec2[i],vec3[i]);
}

Is there a more elegant way instead of using this for-loop (like using views or similar) for C+20? It is guaranteed that the three vectors are of same length.

Answer 1

You can use views::zip_transform (C++23) like:

auto to_MyData = [](auto... elems)  { return MyData(elems...); };
auto myData = std::views::zip_transform(to_MyData, vec1, vec2, vec3)
            | std::ranges::to<std::vector>();

Answer 2

Since you mentioned views, you might use C++23 zip_view:

    for (auto [str1, str2, str3]: std::views::zip(vec1, vec2, vec3)) {
        myData.emplace_back(str1, str2, str3);
    }

If explicitly writing 0,1,2 is still too ugly, you can construct your structure directly from tuple:

    for (const auto& elem : std::views::zip(vec1, vec2, vec3)) {
        myData.emplace_back(std::make_from_tuple<MyData>(elem));
    }

Answer 3

If you have to use c++20, you could define a view that iterates MyData objects from your vectors, e.g.

auto vw = std::views::iota(size_t{0},vec1.size())
  | std::views::transform ([&] (auto i) {
    return MyData {vec1[i],vec2[i],vec3[i]};
 });

where you use std::views::iota in order to iterate all the required indexes, and then you can create on the fly a MyStruct object through the std::views::transform function. Note that the provided functor needs to access the vectors, so you have to use a reference &.

You might notice that this is not a great deal compared to your initial approach. However, you have some improvement since you can iterate over the objects rather than having to create a container (a std::vector for instance) and then iterate it. When possible, it is always a Good Thing to iterate rather than having an explicit container that may duplicate information. You could write for instance

for (auto [x, y, z] : vw) {
  std::cout << x << " " << y << " " << z << '\n';
}

without the need to create a full-fledged std::vector<MyData>. Of course, you can still fill a vector from such a view.

Answer 4

You can use ranges for this:

std::vector<MyData> myData =
        std::views::zip(vec1, vec2, vec3) |
        std::ranges::views::transform([](const auto& t) {
            return MyData{get<0>(t), get<1>(t), get<2>(t)};
        }) |
        std::ranges::to<std::vector>();