Assembling a matrix

[Douglas Arnold]

For me, two copies of the matrix is not a problem.  That is what I was 
looking for but didn't find, something that efficiently copied a sparse 
matrix into a bigger sparse matrix.  I would appreciate
receiving the code for the sequential case, if it is not much
bother for you.

Thanks.  -- Doug

On 12/18/2009 06:12 PM, Barry Smith wrote:
>
> The issue is that the data structures that PETSc uses to store the
> sparse matrices are not designed to allow changing the matrix size, thus
> if you have the matrix A as a PETSc sparse matrix creating B means
> copying the entire matrix, thus two copies of the matrix, more memory
> usage. Now if memory is not an issue for you, the copying can be done
> efficiently order nz work. I can send you code for the sequential case.
>
> If you don't want two copies of the matrix for memory reasons, could you
> "lie" to the part of the code that generates A and have it create the A
> with an extra row and column that it simply does not use? So in the
> MatSetSizes() you just set it 1 larger then it is. Then call the
> MatSetValues() to fill up A then call the MatSetValues() to put in the v
> and c part without calling MatAssemblyBegin/End() in between and you
> will have an efficient code in both time and memory usage.
>
> Barry
>
> On Dec 18, 2009, at 5:59 PM, Douglas Arnold wrote:
>
>> Anirban Chatterjee asked how to assemble an (n+1)x(n+1) sparse matrix
>>
>> B = [A v;
>> v^T c]
>>
>> from an nxn sparse matrix A, a vector v, and a scalar c. Matt
>> Knepley and Barry Smith advised against it.
>>
>> I have pretty much the same problem and I want to make sure I
>> understand the situation. In my case, the matrix A comes to me
>> from another application that I would rather not fiddle with.
>> The rank of A is n-1 and v is a vector in its nullspace, c=0. I would
>> like to solve a system with the matrix B by a direct solver, and
>> so I think I need to assemble it. Do I understand correctly that
>> there is no efficient way to do this in PETSc? Even in a sequential
>> computation?
>>
>> -- Doug
>>
>>
>>> Where will the "new" row live? On the last process, why that one?
>>>
>>> Does the matrix A have to be completely assembled before the v is
>>> computed? Or can the entries of A be computed at the same time as the v?
>>>
>>> PETSc doesn't have code to efficiently dynamically change the size
>>> of a matrix. Thus generating a new matrix from a given sparse matrix
>>> plus a new row requires a complete copy of the starting matrix into
>>> the new matrix data structure. If you assemble the entire matrix
>>> together without first generating the smaller matrix A then it is
>>> easy; just generate the preallocation structure of A with the one
>>> additional row/column information and call MatSetValues() to put in
>>> the parts of A and the parts of v.
>>>
>>> Barry
>>>
>>> On Dec 8, 2009, at 4:05 PM, Anirban Chatterjee wrote:
>>>
>>>> Hi,
>>>>
>>>> I have an nxn matrix A, a column vector v and a scalar c. I want to
>>>> assemble a matrix B as follows,
>>>> B = [A v;
>>>> v^T c]
>>>> Can someone tell me the easiest and most efficient way to do this.
>>>>
>>>> --Anirban
>