What is the best and most efficient way to traverse 2 vectors at the same time? [duplicate]

Question

C++11 provides multiple ways to iterate over containers. For example:

Range-based loop

for(auto c : container) fun(c)

std::for_each

for_each(container.begin(),container.end(),fun)

However what is the recommended way to iterate over two (or more) containers of the same size to accomplish something like:

for(unsigned i = 0; i < containerA.size(); ++i) {
  containerA[i] = containerB[i];
}

Answer 1

Rather late to the party. But: I would iterate over indices. But not with the classical for loop but instead with a range-based for loop over the indices:

for(unsigned i : indices(containerA)) {
    containerA[i] = containerB[i];
}

indices is a simple wrapper function which returns a (lazily evaluated) range for the indices. Since the implementation – though simple – is a bit too long to post it here, you can find an implementation on GitHub.

This code is as efficient as using a manual, classical for loop.

If this pattern occurs often in your data, consider using another pattern which zips two sequences and produces a range of tuples, corresponding to the paired elements:

for (auto& [a, b] : zip(containerA, containerB)) {
    a = b;
}

The implementation of zip is left as an exercise for the reader, but it follows easily from the implementation of indices.

(Before C++17 you’d have to write the following instead:)

for (auto&& items : zip(containerA, containerB))
    get<0>(items) = get<1>(items);

Answer 2

For your specific example, just use

std::copy_n(contB.begin(), contA.size(), contA.begin())

For the more general case, you can use Boost.Iterator's zip_iterator, with a small function to make it usable in range-based for loops. For most cases, this will work:

template<class... Conts>
auto zip_range(Conts&... conts)
  -> decltype(boost::make_iterator_range(
  boost::make_zip_iterator(boost::make_tuple(conts.begin()...)),
  boost::make_zip_iterator(boost::make_tuple(conts.end()...))))
{
  return {boost::make_zip_iterator(boost::make_tuple(conts.begin()...)),
          boost::make_zip_iterator(boost::make_tuple(conts.end()...))};
}

// ...
for(auto&& t : zip_range(contA, contB))
  std::cout << t.get<0>() << " : " << t.get<1>() << "\n";

Live example.

However, for full-blown genericity, you probably want something more like this, which will work correctly for arrays and user-defined types that don't have member begin()/end() but do have begin/end functions in their namespace. Also, this will allow the user to specifically get const access through the zip_c... functions.

And if you're an advocate of nice error messages, like me, then you probably want this, which checks if any temporary containers were passed to any of the zip_... functions, and prints a nice error message if so.