Remove items from two vectors depending on the values inside one vector

Question

I have two integer vectors of equal length. Let's say I want to remove all items in the first vector which are NAN. Obviously, I use the remove_if algorithm. Let's say this removes elements that were at indexes 1,2,5. I then want to remove items from the second vector at these indexes.

What's the most canonical C++ way of doing this?

Topdress

If they are related values, the most canonical way would be to put both pieces of data in the same object and have one vector containing those objects... – Vivacious

like @Vivacious said, if you know they have the same length and the values are linked you should probably have a std::vector<std::pair<int,int>> in the first place – Ratan

Answer 1

This can be done using Boost by creating a zip_iterator and then iterating over the tuple of iterators from both containers in parallel.

First pass a pair of zip_iterators to std::remove_if, and have the predicate inspect the elements of the first vector for NaN

auto result = std::remove_if(boost::make_zip_iterator(boost::make_tuple(v1.begin(), v2.begin())),
                             boost::make_zip_iterator(boost::make_tuple(v1.end(),   v2.end())),
                             [](boost::tuple<double, int> const& elem) {
                                 return std::isnan(boost::get<0>(elem));
                             });

Then use vector::erase to remove the unneeded elements.

v1.erase(boost::get<0>(result.get_iterator_tuple()), v1.end());
v2.erase(boost::get<1>(result.get_iterator_tuple()), v2.end());

Live demo

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The boilerplate required to create the zipped iterator ranges can be further reduced by using boost::combine and Boost.Range's version of remove_if.

auto result = boost::remove_if(boost::combine(v1, v2),
                               [](boost::tuple<double, int> const& elem) {
                                    return std::isnan(boost::get<0>(elem));
                               });

Live demo

Feat

This is awesome. Thanks! I chose to use boost::combine to produce a range which I could call begin() and end() on in remove_if, I presume there's no real difference? – Guttery

@Guttery That's an excellent idea, cleans the code up quite a bit. I added that to the answer. Thanks! – Feat

Answer 2

Use a vector<pair<int, int>> to tie the two vector together. Then, perform your remove based on the first element to and get rid of both at the same time.

Kernite