Glas List: [glas] cursor and permuted vector

From: Karl Meerbergen (Karl.Meerbergen_at_[hidden])
Date: 2006-06-17 07:51:29

Hi,

I spoke to our colleague Greg yesterday who is developing finite element 
software and he wanted some permutations of a vector (related to some 
ordering of eigenvalues). It is something we did not yet think of. He was 
refering to where this done in matlab. (Matlab is a great source of 
inspiration for glas for both functionality as syntax).

Concerning permutations, I see three possibilities:
* We can add a permutation algorithm and a related view, without permuting the 
vector explicitly, which has some advantages when the value_type is quite 
large in size.
* Add a function to permute a vector explicitly
* Add both.

For those who are familiar with the current svn tree ...

Toon and I have introduced cursors (can be an index or a pointer or an 
iterator) to walk over an expression and funtions to increment the cursor:
  vector.next(cursor)
  vector.next(cursor,step)
and evaluate
  vector.value(cursor)
  vecor.index(cursor) (for the sparse case only)

Currently cursors can only walk in a forward way over the vector. (cfr 
ForwardIterator)

With such a permutation, this is a case where prev(cursor) and 
prev(cursor,step) should be added to the SparseVectorExpression and 
DenseVectorExpression concepts.

Algorithms carried out on a permutated vector, using next(cursor) and 
next(cursor,step) need the prev() functions to implement the next() of such a 
permuted vector (without explicitly carrying out the permutation), although I 
am unsure it is enough. We might need a random access change of the cursor 
also for efficiency reasons. This should become clear once we implement 
permuations on vector expressions.

Karl