Removing elements marked for removal with Ranges-V3

Question

I've got two vectors:

struct MyData{
     double value; 
};
std::vector<int> remove_flags = {0, 1, 0, 0, 0, 0, 1, 0};
std::vector<MyData> data =      {{},{},{},{},{},{},{},{}};

The remove_flags vector contains an array of flags of the exact same size as data, each flag is either 0, or 1, where 1 means the data should be removed.

I would like to use remove_flags to remove elements from data in place, ie performing the erase remove idiom, but erasing based on values in remove_flags. The end result should be data with elements erased, and hopefully remove_flags with those same elements erased.

Doing this manually is annoying, and I wanted to use Range-v3 for this. I'm currently using C++17.

After looking through the documentation, I don't think I've found a solution, the closest thing I could come up with is:

auto result = ranges::views::zip(remove_flags, data) | ranges::actions::remove_if([](std::pair<const int&, const MyData&> pair){
    return pair.first != 0;
});

remove_flags.erase(result.first, remove_flags.end());
data.erase(result.second, data.end());

But actions cannot operate on the view zip, so this does not compile. If I switch ranges::actions::remove_if to ranges::views::remove_if a strange views object is returned, presumably one that has not actually performed the std::remove equivalent operation on the two vectors.

I could use contaner_to but that would involve a copy, and I don't want to pay that kind of unnecessary penalty for convenience. I've seen what I want accomplished in boost where actual zip pair iterators can be used to return two separate removal results.

Is this kind of pattern possible in Range-v3?

Is there a reason why you actually want a `std::vector` of the results? Isn't a view enough? — Enlico, Jun 03 '21 at 22:47
@Enlico Please clarify your question, no where in my post do I mention or demonstrate a "standard vector of result". In fact, it's impossible for my example to show this, since I say `result.first` and `result.second` — Krupip, Jun 03 '21 at 23:57
The last two lines of your snippet show that you alter both `data` and `remove_flags`, so I think it's fair to presume you want to use _those_ afterward, i.e. those two _`std::vector`s_ contain the "output/result/effect" you want to achieve. That's where you "mention" in your post that you want the result of the code to be in a `std::vector` (_result_, not _`result`_, just as in my previous comment). However, let's put it in another way: your example is not a [repro]; can you either make it one or describe in words what the desired outcome/result/effect of the code is? — Enlico, Jun 04 '21 at 04:39
@Enlico the above example now represents an MVCE, and I've clarified what I want. Note, the title was not separate from my post, and that the boost example (first part) gives effectively what I want exactly, just with out range syntax. — Krupip, Jun 04 '21 at 11:44
Thanks for clarifying. I've deleted my answer for now, because it's clearly not a good answer at the moment. I don't have any idea at the moment. I'd be curious to know why you need to remove the elments in place, and having a view on them is not enough. — Enlico, Jun 04 '21 at 12:50
@Enlico In real code, `MyData` represents Dear ImGUI table row items. Because ImGUI is immediate mode, in order to remove elements from the UI, I must first mark elements some how (ie selection) *then* remove them afterwards, as displaying rows is literally a for loop that runs each time. Again, it's UI is immediate mode, selection doesn't persist with out **me** creating an array that maintains the selection (effectively `remove_flags`). Removing these elements removes the UI rows. These rows also represent markers, removing the `MyData` then also removes the markers (which is desired). — Krupip, Jun 04 '21 at 13:38
Ok, thanks for clarifying this. I have just one misunderstanding left: how is the loop done? If it was done via `for (auto x : result)` or `for (auto it = result.begin(); it != result.end(); ++it)`, then a view would be fine. — Enlico, Jun 04 '21 at 13:48
@Enlico So the loop effectively looks something like (in half psuedo code) `if(ImGui::BeginTable()){ ...; clipped.Begin(data.size()); for(int row_n = clipped.Start; row_n < clipped.End; ++i){ auto& my_data = data[row_n]; if(ImGui::Selectable("",remove_flags[row_n],...){...};if(ImGui::DisplayColumn(0)){ ImGui::InputText("input", &(my_data.name))}; if(ImGui::DisplayColumn(1)){...}}ImGui::EndTable();}` The loop is not trivial, I need references to mydata inside the loop itself, I use the values of my_data inside the loop itself, need to modify it from within the generated UI. — Krupip, Jun 04 '21 at 14:07
@Enlico if this code looks weird, that's because this his how ImGUI is architected, see here: https://github.com/ocornut/imgui/blob/c6c82b9f1de6ef3e298050656a1cf0a499d4e808/imgui_demo.cpp#L5074 — Krupip, Jun 04 '21 at 14:13

T.C. · Answer 1 · 2021-06-05T02:31:51.447

Eager operation on existing ranges is the domain of algorithms. The only slightly tricky bit here is recovering iterators into the vectors since zip doesn't give you a direct way to get those. So instead recover them using the distance between the new end and the start of the range:

auto z = ranges::views::zip(remove_flags, data);
auto e = ranges::remove_if(z, [](auto&& r){ return r.first; });
data.erase(data.begin() + (e - z.begin()), data.end());
remove_flags.erase(remove_flags.begin() + (e - z.begin()), remove_flags.end());

Demo.

Removing elements marked for removal with Ranges-V3

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