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From: Patrick Alken <alken@colorado.edu>
To: Alexis Tantet <alexis.tantet@gmail.com>
Cc: "gsl-discuss@sourceware.org" <gsl-discuss@sourceware.org>
Subject: Re: Sparse matrix extension
Date: Sun, 07 Feb 2016 21:34:00 -0000	[thread overview]
Message-ID: <56B7B85C.10508@colorado.edu> (raw)
In-Reply-To: <CAMWWPT0CX+ti2j6QM8YmHqv+n8yg7sK69dtSdzKOQMFx3pXeBA@mail.gmail.com>

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);
if (ptr)
  *ptr += x; /* sum duplicate values */
else
   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 <alken@colorado.edu> 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
>>>
>>> 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
>
>

  reply	other threads:[~2016-02-07 21:34 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAMWWPT3uJj4Vrn7ut6+F18gY===zd6+1r1UJhz0hcCj--zwtdg@mail.gmail.com>
2016-01-19 16:43 ` Alexis Tantet
2016-01-19 17:02   ` Patrick Alken
2016-01-19 19:55     ` Alexis Tantet
2016-01-19 20:50       ` Patrick Alken
2016-01-20 18:31         ` Alexis Tantet
2016-02-07  0:03           ` Patrick Alken
2016-02-07  1:03             ` Alexis Tantet
2016-02-07 17:25               ` Patrick Alken
2016-02-07 19:32                 ` Alexis Tantet
2016-02-07 20:14                   ` Patrick Alken
2016-02-07 20:31                     ` Alexis Tantet
2016-02-07 21:34                       ` Patrick Alken [this message]
2016-02-08  0:59                         ` Alexis Tantet
2016-02-10 13:16                           ` Alexis Tantet
2016-02-10 13:48                             ` Alexis Tantet
2016-02-10 15:56                             ` Patrick Alken
2016-02-12 10:43                               ` Alexis Tantet
2016-02-13 19:42                                 ` Patrick Alken
2016-02-14 11:06                                   ` Alexis Tantet
2016-02-14 18:11                                     ` Patrick Alken
2016-02-14 18:25                                       ` Brian Gladman
2016-02-15  5:10                                         ` Patrick Alken
2016-02-15 11:09                                           ` Brian Gladman
     [not found]                                             ` <CAMWWPT0J9ENRZjJHLO=cxot4DGdSLer+n2HkBVnhFnO0oiVV8g@mail.gmail.com>
2016-02-15 13:56                                               ` Alexis Tantet
2016-02-15 17:17                                                 ` Patrick Alken

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