Introduction

The adaptor takes two arguments:

• an iterator to the range V on which the permutation will be applied
• the reindexing scheme that defines how the elements of V will be permuted.

Note that the permutation iterator is not limited to strict permutations of the given range V. The distance between begin and end of the reindexing iterators is allowed to be smaller compared to the size of the range V, in which case the permutation iterator only provides a permutation of a subrange of V. The indexes neither need to be unique. In this same context, it must be noted that the past the end permutation iterator is completely defined by means of the past-the-end iterator to the indices.

Reference

template< class ElementIterator
        , class IndexIterator
        , class ValueT        = use_default
        , class CategoryT     = use_default
        , class ReferenceT    = use_default
        , class DifferenceT   = use_default >
class permutation_iterator
{
public:
  permutation_iterator();
  explicit permutation_iterator(ElementIterator x, IndexIterator y);

  template< class OEIter, class OIIter, class V, class C, class R, class D >
  permutation_iterator(
      permutation_iterator<OEIter, OIIter, V, C, R, D> const& r
      , typename enable_if_convertible<OEIter, ElementIterator>::type* = 0
      , typename enable_if_convertible<OIIter, IndexIterator>::type* = 0
      );
  reference operator*() const;
  permutation_iterator& operator++();
  ElementIterator const& base() const;
private:
  ElementIterator m_elt;      // exposition only
  IndexIterator m_order;      // exposition only
};

template <class ElementIterator, class IndexIterator>
permutation_iterator<ElementIterator, IndexIterator>
make_permutation_iterator( ElementIterator e, IndexIterator i);

template< class OEIter, class OIIter, class V, class C, class R, class D >
permutation_iterator(
    permutation_iterator<OEIter, OIIter, V, C, R, D> const& r
    , typename enable_if_convertible<OEIter, ElementIterator>::type* = 0
    , typename enable_if_convertible<OIIter, IndexIterator>::type* = 0
    );

template <class ElementIterator, class IndexIterator>
permutation_iterator<ElementIterator, IndexIterator>
make_permutation_iterator(ElementIterator e, IndexIterator i);

Example

using namespace boost;
int i = 0;

typedef std::vector< int > element_range_type;
typedef std::list< int > index_type;

static const int element_range_size = 10;
static const int index_size = 4;

element_range_type elements( element_range_size );
for(element_range_type::iterator el_it = elements.begin() ; el_it != elements.end() ; ++el_it)
  *el_it = std::distance(elements.begin(), el_it);

index_type indices( index_size );
for(index_type::iterator i_it = indices.begin() ; i_it != indices.end() ; ++i_it )
  *i_it = element_range_size - index_size + std::distance(indices.begin(), i_it);
std::reverse( indices.begin(), indices.end() );

typedef permutation_iterator< element_range_type::iterator, index_type::iterator > permutation_type;
permutation_type begin = make_permutation_iterator( elements.begin(), indices.begin() );
permutation_type it = begin;
permutation_type end = make_permutation_iterator( elements.begin(), indices.end() );

std::cout << "The original range is : ";
std::copy( elements.begin(), elements.end(), std::ostream_iterator< int >( std::cout, " " ) );
std::cout << "\n";

std::cout << "The reindexing scheme is : ";
std::copy( indices.begin(), indices.end(), std::ostream_iterator< int >( std::cout, " " ) );
std::cout << "\n";

std::cout << "The permutated range is : ";
std::copy( begin, end, std::ostream_iterator< int >( std::cout, " " ) );
std::cout << "\n";

std::cout << "Elements at even indices in the permutation : ";
it = begin;
for(i = 0; i < index_size / 2 ; ++i, it+=2 ) std::cout << *it << " ";
std::cout << "\n";

std::cout << "Permutation backwards : ";
it = begin + (index_size);
assert( it != begin );
for( ; it-- != begin ; ) std::cout << *it << " ";
std::cout << "\n";

std::cout << "Iterate backward with stride 2 : ";
it = begin + (index_size - 1);
for(i = 0 ; i < index_size / 2 ; ++i, it-=2 ) std::cout << *it << " ";
std::cout << "\n";

The output is:

The original range is : 0 1 2 3 4 5 6 7 8 9
The reindexing scheme is : 9 8 7 6
The permutated range is : 9 8 7 6
Elements at even indices in the permutation : 9 7
Permutation backwards : 6 7 8 9
Iterate backward with stride 2 : 6 8

The source code for this example can be found here.