Re: matrix and vector iterators

From: jcampbell3_at_[hidden]
Date: 2001-06-10 21:33:34

--- In boost_at_y..., Toon Knapen <toon_at_s...> wrote:
> "Peter Schmitteckert (boost)" wrote:
> > > One thing that we need to decide on is the interface for 
traversing
> > > vectors and matrices. I see a couple options:
> > >
> > > 1. MTL-style iterators
> > >
> > >   *i          // return a matrix/vector element
> > >   i.index()   // return the index of the element pointed to 
(for vectors)
> > >   i.row()     // return the row index (for matrices)
> > >   i.colummn() // return the column indexn (for matrices)
> > >
> > > 2. iterator over "matrix elements" or "vector elements"
> > >   *i         // returns a matrix or vector element object.
> > >   value(*i)  // return the element value
> > >   row(*i)    // returns the row index
> > >   column(*i) // return the column index
> > >   index(*i)  // return the index (for vectors)
> > >
> 
> AFAIK this does not seem compatible with the multi-dimensional 
approach
> taken so far. There has been a lot of discussion and effort (on your
> part in development of the prototype) to take a real n-dimensional
> generic approach.
> Using `row()` and `column()` renders this effort meaningless since 
how
> will you get the index for the 3th, 4th, ... dimension. 
> 
> I certainly prefer the 'richer' iterator and thus the MTL approach 
but
> how about : i.index(dim) and maybe also i.distance() if you want 
more
> information about linear-memory-mapping.
> 
> I also would like add another issue to the boost.numeric 
iterators : For
> matrices e.g. will you have iterators of iterators or true 2D-
iterators.
> The latter would e.g. allow you to go the next row or column (both
> operations supported by the same iterator object). I'm not sure 
about
I need some clarification on this.  For example, given a rank n
array analagous to the following c array:

  T a[sn][sn_1][sn_2]...[s1]; 

then the following statements:

 iterator<n> in = a.begin();
 iterator<n-1> in_1 = *in;
...
 iterator<1> i1 = *i2;
 T& i0 = *i1;

Then this would be an example of the iterator of iterators idea?
To be more specific, the difference between

  successive elements of i1 would be 1;
  successive elements of i2 would be s1;
  successive elements of i3 would be s2*s1;
...
  successive elements of in would be sn_1*sn_2*...*s2*s1.
   
If this is what you mean, then I think this is the best
way to go since I think it most closely mirrors the way
c arrays and pointers to parts of c arrays work.