Iterating over pair elements in a container of pairs (C++)

Question

If I have a container (vector, list, etc) where each element is a std::pair, is there an easy way to iterate over each element of each pair?

i.e.

std::vector<std::pair<int,int> > a;
a.push_back(std::pair(1,3));
a.push_back(std::pair(2,3));
a.push_back(std::pair(4,2));
a.push_back(std::pair(5,2));
a.push_back(std::pair(1,5));

and then being able to iterate over the value: 1,3,2,3,4,2,5,2,1,5?

Similarly, what type of functor/function would return to me a container (of the same type) with a flat listing of the pair elements as above?

Answer 1

At some point you need to use first and second, even if you create your own iterator class. I don't think there's a way out of it (at least, in a portable manner).

Answer 2

For your first, you have to create your own iterator class, which pairs a flag indicating the within-pair position with a container<pair> iterator

For the second, it's easier, although to be as general as you want (container of same type) you need a template typedef. Here's for just vector:

template <class V>
std::vector<V> flatten_pairs(std::vector<std::pair<V,V> > const& a) {
  typedef std::vector<std::pair<V,V> > A;
  std::vector<V> ret;
  for (typename A::const_iterator i=a.begin(),e=a.end();i!=e;++i) {
    ret.push_back(i->first);
    ret.push_back(i->second);
  }
  return ret;
}

Here's how you fake a template typedef:

template <class C>
struct same_container;

template <class V>
struct same_container<std::vector<V> > {
  template <class W> struct rebind { typedef std::vector<W> type; };
};

template <class V>
struct same_list<std::list<V> > {
  template <class W> struct rebind { typedef std::list<W> type; };
};

template <class C>
typename same_container<C>::rebind<typename C::value_type::first_type>::type
flatten_pairs(C const& a);

Answer 3

There is no simple way of performing the iteration you want, but you may want to take a look at the boost::iterator_adaptor library or implement your own iterator to do it (it should not be too complex). Then, on the second question, you can use std::copy with your new iterator adaptor.

Answer 4

No, there really isn't such a thing for std::pair. You might want to consider using a Boost Tuple instead. A tuple is a bit like an expanded version of std::pair that allows an arbitrary number of elements (up to some limit, but normally at least 10), and gives access to the elements something like a vector/array as well (i.e. you can access the elements by either name or index).

TR1 also includes std::tr1::tuple, which is a subset of Boost's tuple, but if memory serves, it still includes the name/index functionality you're asking for.

Edit: note that in both cases, the index notation requires a compile-time constant for the index, so you can't write a (run-time) loop to iterate over the elements in a tuple -- but you can do the job with a bit of metaprogramming. Boost fusion includes quite a bit to support what you'd need (by some strange coincidence, tuple is part of the fusion library).

Answer 5

The following code will print all values as required:

for ( size_t x = 0; x < a.size(); ++x ) {
    cout << a[x].first << "," << a[x].second << ",";
}

I'd prefer this easy way than creating custom iterator.

Answer 6

To flatten your container of pairs into a second container you could also simply write your own inserter:

template<class C>
struct Inserter {
    std::back_insert_iterator<C> in;
    Inserter(C& c) : in(c) {}
    void operator()(const std::pair<typename C::value_type, typename C::value_type>& p)
    {
     *in++ = p.first;
 *in++ = p.second;
    }
};

template<class C>
Inserter<C> make_inserter(C& c)
{ 
    return Inserter<C>(c); 
}

// usage example:
std::list<int> l;
std::for_each(a.begin(), a.end(), make_inserter(l));