From:

Jesse Perla <jesseperla@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 21 Jan 2009 21:53:49 CST

Message-ID:

<babdf216-e7ef-4ce7-b5c5-392fb445db84@v39g2000pro.googlegroups.com>

wrote:

Jesse Perla wrote:

But you'd have to resum the difference tuple into an integer which

gives the same value as subtracting the pointers in the first place.

Martin

Then I am creating a sequential_iterator which operates on the

underlying data pointer of the container and simultaneously

increments the mixed_radix counter. Last, to implement the

distance() concept of the sequential_multi_index_iterator, I will

subtract the two index arrays.

underlying data pointer of the container and simultaneously

increments the mixed_radix counter. Last, to implement the

distance() concept of the sequential_multi_index_iterator, I will

subtract the two index arrays.

But you'd have to resum the difference tuple into an integer which

gives the same value as subtracting the pointers in the first place.

Martin

{ signature and clc++m banner removed. please remove them yourself. -mod }

I may not understand your concern. With a forward only iterator that

combines a linear data iterator with a multiradix counter, I can just

increment my data pointer with simple pointer arithmetic. Then for

the multiradix increment, for most cases it will only require a couple

of increment operations (one unless we are hitting the "nines" and a

few equality checks on the "nines"). This seems about as efficient as

I can imagine and doesn't require a bunch of relatively expensive

modulo operators for each dimension every time we ask for the current

index in multiradix form.

Of course, one major issue with this is that it only works for dense

storage of the data. However, it will work for fortran vs. C ordering

by choosing left vs. right first order for multiradix counting.

Assuming that I have implemented a multiradix counter with some of the

code here (with a ++ operator to increment, and a .digits() function

to get the current position), here is my code for the hardcoded

boost::multi_array<T, N> container:

template<class T, int N>

class sequential_multidim_iterator : public

boost::forward_iterator_helper<

sequential_multidim_iterator, T, std::ptrdiff_t, T*, T>

{

typedef sequential_multidim_iterator<T, N> self;

private:

T* data_; // The underlying data pointer

T* data_end_;

T* data_iter_; //The current position

mixedradix_counter<N> index_; //Current index for the position.

public:

explicit sequential_multidim_iterator(boost::multi_array<T, N>& c) :

data_(c.data())

{

data_end_ = c.data() + c.num_elements();

etk::lexicographic_order order;

if(c.storage_order().ordering(0) > c.storage_order().ordering(1)) //

i.e. c ordering

order = right_first;

else

order = left_first; //otherwise, fortran. Note we are not using

proper general orders here.

boost::array<int, N> shape;

for(int i = 0; i < N; ++i)

shape[i] = c.shape()[i];

index_.reset(shape, order); // Shape gives the extents

}

T& operator*()

{

return *data_iter_;

}

inline void first()

{

data_iter_ = data_;

index_.begin();

}

inline void end()

{

data_iter_ = data_end_;

index_.end();

}

self& operator++()

{

++data_iter_;

++index_;

return *this;

}

boost::array<int, N>& indices() const

{

return index_.digits();

}

friend bool operator==(const self& x, const self& y) {

return x.data_iter_ == y.data_iter_; } //Don't compare the

indicies.

};

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