[petsc-users] How can I do matrix addition with different nonzeros patterns correctly?

[Barry Smith]

> On Mar 3, 2017, at 3:31 PM, Fangbo Wang <fangbowa at buffalo.edu> wrote:
> 
> Hi, 
> 
> I am a little bit confused on how to appropriately do matrix addition with different nonzeros patterns. 
> 
> Suppose I want to do D=2*A+3*B+4*C, A, B and C all have different nonzero patterns.
> I know I can use MatDuplicate, MatCopy, MatConvert to create a matrix D, which way is the right way?

   There is no particular "right way". You could use a MatDuplicate() then a MatScale and then two MatAXPY()

   D=2*A+3*B+4*C looks like a MATLAB thing, not something you would need to do when solving PDEs, where do you get this need? Perhaps there is an alternative way to get what you want.

> 
> What's the difference between MatDuplicate and MatCopy?

   MatDuplicate() CREATES a new matrix while MatCopy() copies values from an already existing matrix to another already existing matrix.

   
> 
> Thank you very much!
> 
> Best regards,
> 
> Fangbo
> 
> 
> -- 
> Fangbo Wang, PhD student
> Stochastic Geomechanics Research Group
> Department of Civil, Structural and Environmental Engineering
> University at Buffalo
> Email: fangbowa at buffalo.edu