Re: [julia-users] Re: Howto: Julia x[I] = [] ?? vs Matlab x(I) = []

[Steven G. Johnson]

On Wednesday, February 12, 2014 3:41:21 PM UTC-5, Jason Pries wrote:
>
> FWIW, The place I use this the most is when solving PDEs using finite 
> elements with Dirichlet/Periodic boundary conditions. If I want to preserve 
> the symmetry of the matrix before factorization, I end up needing to delete 
> some rows and columns. I could write the code to create the matrix the way 
> I end up using it, but that would require every function that creates a 
> matrix to know about the boundary conditions which complicates things 
> considerably.
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Note that you can do this out-of-place via B = A(I,J), where I and J are 
arrays of the desired indices.  The question is how important is in-place 
operation here; for sparse matrices, the matrix storage is usually small 
compared to that of the factorization (for sparse-direct solvers).