Question

The lambda notation has made stl algorithms more accessible. I am still learning to decide when it’s useful and when to fall back to good old fashioned for-loops.
Often, it becomes necessary to iterate over two (or more) containers of the same size, such that corresponding elements are related, but for some reason are not packed into the same class.

A function using a for-loop to achieve that would look like this:

template<typename Data, typename Property>
void foo(vector<Data>& data, vector<Property>& prop) {
    auto i_data = begin(data);
    auto i_prop = begin(prop);
    for (; i_data != data.end(); ++i_data, ++i_prop) {
        if (i_prop->SomePropertySatistfied()) {
            i_data->DoSomething();
        }
    }
}

In order to use for_each, I need a version of it that handles multiple ranges; something like:

template<typename InputIter1, typename InputIter2, typename Function>
Function for_each_on_two_ranges(InputIter1 first1, InputIter1 last1, InputIter2 first2, Function f) {
    for (; first1 != last1; ++first1, ++first2) {
        f(*first1, *first2);
    }
    return f;
}

With this version, the above code would look like this:

template<typename Data, typename Property>
void foo_two_ranges(vector<Data>& data, vector<Property>& prop) {
    for_each_on_two_ranges(begin(data), end(data), begin(prop), [](Data& d, Property& p) {
        if (p.SomePropertySatistfied()) {
            d.DoSomething();
        }
    });
}

Is there an equivalent way of achieving the same result using stl algorithms?

EDIT

I found the exact answer to my question in the form of boost::for_each running on boost::range. I added the answer, with example code for the sake of completeness.

Answer 1

1) The algorithms in the STL are not meant to cover every possible case, if you need for_each_on_two_ranges then write it (as you have) and use it. The beauty of the STL is it’s so extensible, and you’ve extended it with a useful new algo.

2) If that doesn’t work, you don’t have to use good old fashioned for-loops, you can use fancy new for-loops instead!

As another answer said, boost::zip_iterator is your friend here, but it doesn’t have to be hard to use. Here’s a solution using a range adaptor that is implemented with zip_iterator

template<typename Data, typename Property>
void foo(vector<Data>& data, vector<Property>& prop) {
    for (auto i : redi::zip(data, prop))
        if (i.get<1>().SomePropertySatistfied())
            i.get<0>.DoSomething();
}

That zip function creates an adaptor with begin() and end() members that return a boost::zip_iterator, so the loop variable is a tuple of the elements of each underlying container (and as it’s a variadic template you can do it for any number of containers, so you don’t need to write for_each_for_three_ranges and for_each_for_four_ranges etc.)

You could also use it with for_each

auto z = redi::zip(data, prop);
typedef decltype(z)::iterator::reference reference;

for_each(begin(z), end(z), [](reference i) {
    if (i.get<1>().SomePropertySatistfied()) {
        i.get<0>().DoSomething();
    }
});