116 lines
3.5 KiB
ReStructuredText
116 lines
3.5 KiB
ReStructuredText
.. Copyright David Abrahams 2006. Distributed under the Boost
|
|
.. Software License, Version 1.0. (See accompanying
|
|
.. file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
|
|
|
|
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
|
|
{
|
|
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>()
|
|
);
|