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Examples
........
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>()
);
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