How can I avoid “for” loops with an “if” condition inside them with C++?

Question

With almost all code I write, I am often dealing with set reduction problems on collections that ultimately end up with naive \"if\" conditions inside of them. Here\'s a simple

Answer 1

One can describe your code pattern as applying some function to a subset of a range, or in other words: applying it to the result of applying a filter to the whole range.

This is achievable in the most straightforward manner with Eric Neibler's ranges-v3 library; although it's a bit of an eyesore, because you want to work with indices:

using namespace ranges;
auto mycollection_has_something = 
    [&](std::size_t i) { return myCollection[i] == SOMETHING };
auto filtered_view = 
    views::iota(std::size_t{0}, myCollection.size()) | 
    views::filter(mycollection_has_something);
for (auto i : filtered_view) { DoStuff(); }

But if you're willing to forego indices, you'd get:

auto is_something = [&SOMETHING](const decltype(SOMETHING)& x) { return x == SOMETHING };
auto filtered_collection = myCollection | views::filter(is_something);
for (const auto& x : filtered_collection) { DoStuff(); }

which is nicer IMHO.

PS - The ranges library is mostly going into the C++ standard in C++20.