Re: Sparse matrix extension

[Patrick Alken <alken@colorado.edu>]

Hi Alexis,

>> I'm not sure what you mean. I've added a new function gsl_spmatrix_ptr
>> to the git, which as far as I can tell does exactly what your
>> sum_duplicate flag does. It searches the matrix for an (i,j) element,
>> and if found returns a pointer. If not found a null pointer is returned.
>> This makes it easy for the user to modify A(i,j) after it has been added
>> to the matrix. Are you thinking of something else? Can you point me to
>> the Eigen routine?
>>
> What I meant is to have the equivalent of gsl_spmatrix_compress,
> with the difference that gsl_spmatrix_ptr is used instead of gsl_spmatrix_set,
> so has to build the compressed matrix from triplets, summing the
> duplicates, instead of replacing them.
> This is what is done here :
> The http://eigen.tuxfamily.org/dox/classEigen_1_1SparseMatrix.html#a5bcf3187e372ff7cea1e8f61152ae49b
>
> Best,
> Alexis

I'm not sure why a user would ever need to do this. The whole point of
the binary tree structure in the triplet storage is to efficiently find
duplicate entries, so that if a user tries to call gsl_spmatrix_set on
an element which is already been previously set, it can find that
element with a binary search (rather than linearly searching the arrays)
and change the value of that element.

Therefore, the way the triplet storage is designed, there is will never
be a duplicate element in the triplet arrays. All of the (i[n],j[n])
will be unique for each n <= nz.

Am I missing something?

Patrick