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Re: Sparse matrix extension
- From: Patrick Alken <alken at colorado dot edu>
- To: Alexis Tantet <alexis dot tantet at gmail dot com>
- Cc: "gsl-discuss at sourceware dot org" <gsl-discuss at sourceware dot org>
- Date: Sun, 7 Feb 2016 14:34:20 -0700
- Subject: Re: Sparse matrix extension
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By design, gsl_spmatrix_set won't allow you to do this.
If you add element (i, j, x) and then later to try add element (i, j,
y), gsl_spmatrix_set will detect that there exists an element in the (i,
j) spot and it will simply change x to y - the value of x will be
overwritten by y. This is the same behavior as gsl_matrix_set.
So no duplicates are allowed by design. If you have such an application
where you want to keep track of duplicates, you could do the following:
double *ptr = gsl_spmatrix_ptr(m, i, j);
*ptr += x; /* sum duplicate values */
gsl_spmatrix_set(m, i, j, x); /* initalize to x */
On 02/07/2016 01:31 PM, Alexis Tantet wrote:
> I'm not sure I got your last point. I have the following situation in mind:
> Start to construct a transition matrix in triplet format, adding one
> element after another.
> In this particular example, each element is one count of a transition
> from (state, box, etc.) i to j,
> so I add elements (i, j, 1) to the triplet object, with possibly duplicates.
> What happen to these duplicates in the binary tree?
> Eventually, when I compress to CRS or CCS, I would like the duplicates
> to be summed up, so that element (i, j) counts transitions from i to j
> (and no duplicates exist after compression).
> Is this more clear?
> On Sun, Feb 7, 2016 at 9:14 PM, Patrick Alken <email@example.com> wrote:
>> 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
>> 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?