Zip Iterator

Example
Reference

The zip iterator provides the ability to parallel-iterate over several controlled sequences simultaneously. A zip iterator is constructed from a tuple of iterators. Moving the zip iterator moves all the iterators in parallel. Dereferencing the zip iterator returns a tuple that contains the results of dereferencing the individual iterators.

The tuple of iterators is now implemented in terms of a Boost fusion sequence. Because of this the 'tuple' may be any Boost fusion sequence and, for backwards compatibility through a Boost fusion sequence adapter, a Boost tuple. Because the 'tuple' may be any boost::fusion sequence the 'tuple' may also be any type for which a Boost fusion adapter exists. This includes, among others, a std::tuple and a std::pair. Just remember to include the appropriate Boost fusion adapter header files for these other Boost fusion adapters. The zip_iterator header file already includes the Boost fusion adapter header file for Boost tuple, so you need not include it yourself to use a Boost tuple as your 'tuple'.

Example

There are two main types of applications of the zip_iterator. The first one concerns runtime efficiency: If one has several controlled sequences of the same length that must be somehow processed, e.g., with the for_each algorithm, then it is more efficient to perform just one parallel-iteration rather than several individual iterations. For an example, assume that vect_of_doubles and vect_of_ints are two vectors of equal length containing doubles and ints, respectively, and consider the following two iterations:

std::vector<double>::const_iterator beg1 = vect_of_doubles.begin();
std::vector<double>::const_iterator end1 = vect_of_doubles.end();
std::vector<int>::const_iterator beg2 = vect_of_ints.begin();
std::vector<int>::const_iterator end2 = vect_of_ints.end();

std::for_each(beg1, end1, func_0());
std::for_each(beg2, end2, func_1());

These two iterations can now be replaced with a single one as follows:

std::for_each(
  boost::make_zip_iterator(
    boost::make_tuple(beg1, beg2)
    ),
  boost::make_zip_iterator(
    boost::make_tuple(end1, end2)
    ),
  zip_func()
  );

A non-generic implementation of zip_func could look as follows:

struct zip_func :
  public std::unary_function<const boost::tuple<const double&, const int&>&, void>
{
  void operator()(const boost::tuple<const double&, const int&>& t) const
  {
    m_f0(t.get<0>());
    m_f1(t.get<1>());
  }

private:
  func_0 m_f0;
  func_1 m_f1;
};

The second important application of the zip_iterator is as a building block to make combining iterators. A combining iterator is an iterator that parallel-iterates over several controlled sequences and, upon dereferencing, returns the result of applying a functor to the values of the sequences at the respective positions. This can now be achieved by using the zip_iterator in conjunction with the transform_iterator.

Suppose, for example, that you have two vectors of doubles, say vect_1 and vect_2, and you need to expose to a client a controlled sequence containing the products of the elements of vect_1 and vect_2. Rather than placing these products in a third vector, you can use a combining iterator that calculates the products on the fly. Let us assume that tuple_multiplies is a functor that works like std::multiplies, except that it takes its two arguments packaged in a tuple. Then the two iterators it_begin and it_end defined below delimit a controlled sequence containing the products of the elements of vect_1 and vect_2:

typedef boost::tuple<
  std::vector<double>::const_iterator,
  std::vector<double>::const_iterator
  > the_iterator_tuple;

typedef boost::zip_iterator<
  the_iterator_tuple
  > the_zip_iterator;

typedef boost::transform_iterator<
  tuple_multiplies<double>,
  the_zip_iterator
  > the_transform_iterator;

the_transform_iterator it_begin(
  the_zip_iterator(
    the_iterator_tuple(
      vect_1.begin(),
      vect_2.begin()
      )
    ),
  tuple_multiplies<double>()
  );

the_transform_iterator it_end(
  the_zip_iterator(
    the_iterator_tuple(
      vect_1.end(),
      vect_2.end()
      )
    ),
  tuple_multiplies<double>()
  );

Boost C++ Libraries

Zip Iterator

Example