[boost] Re: stride iterators and matricies

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-06-03 13:09:20

christopher diggins wrote:

> Well all of this talk of matricies, and what not got me thinking that
> perhaps we should get back to basics and introduce stride iterators into
> Boost which properly model the Random Access Iterator concept (unless they
> are already existant somewhere and I simplty overlooked them :-p ) I would
> suggest that we introduce a mini-library simply for stride iterators (and
> k-stride iterators, where the stride factor is a compile-time constant.)
> Any interest in seeing the following stride iterators submitted to boost
> as a mini-library?

That would be useful, however your sample code has a problem with the end
iterator.  To iterate over every second element of a container that has an
even length, the one-past-the-end stride-2 iterator is actually TWO past
the end of the original container.  Unless you have a special container
where incrementing beyond the one-past-the-end is allowed, you need another
solution (indexing?).

[...]

>   template<class Iter_T>
>   class stride_iter

[...]

>   template<class Iter_T, int Step_N>
>   class kstride_iter

May I suggest

struct VariableStride {};

template <class Iter_T, class Stride /* = VariableStride ? */ >
class stride_iterator 
// ...

use as

stride_iterator<T, VariableStride> x;
stride_iterator<T, boost::mpl::int_<2> > y;

Cheers,
Ian