Tue, 04 May Marie Rognes wrote: > Anders Logg wrote: > >On Mon, May 03, 2010 at 11:08:13AM +0200, Kent Andre wrote: > >>On Mon, 2010-05-03 at 09:36 +0100, Garth N. Wells wrote: > >>>On 03/05/10 08:48, Kent Andre wrote: > >>>>I am in favour of a(u,v), simply because it is the way I am used to > >>>>read and write it. Also, very few users need to care about this due to > >>>>the high-level syntax in UFL which implicitly takes care of the > >>>>ordering. > >>>> > >>>They do need to worry about it on the DOLFIN side with the creation of a > >>>Form: > >>> > >>> Poissin::Bilinearform a(V0, V1); > >>> > >>>Garth > >>> > >>Yes, of course, sorry, in C++ this is important, > >> > >>Anyway, I think both alternatives have their advantages. > >>But, as Godel put it, you must choose between consistency and > >>conformity :) > > > >I've been thinking some more, and I'm getting more convinced that the > >problem is really the ordering of rows and columns in matrices, which > >should be (column, row) rather than (row, column) because a matrix is > >really a set of columns that span the range of A, so A_i should be > >column i and A_ij should be element j of that column. > > > >So then a(u, v) would be consistent with *that* notation but it would > >be a pain for us to have an unorthodox matrix index notation. > > > >Which leaves us with the following options: > > > >1. Keep a(v, u) as we have now and not change anything. > > > >2. Change to a(u, v) and comment on the need for swapping the indices > >when we define the matrix (tensor) in the text (book and manual). > > > >Alternative (2) would imply the following changes: > > > >(i) Redefine the UFC interface so that the test space is the last > >space, not the first. > > > >(ii) Swapping of indices inside Assembler.cpp. > > > >Further comments? > > > > > Vote in favor of (1).
I also vote in favor of (1) for the source code and technical documentation. However, I still repeat my arguments for a(u, v) in the FEniCS tutorial, at least in the beginning. We may well introduce a(v, u) later in the tutorial and explain the u,v-v,u problem, or postpone that to the introductory chapter on the finite element method. Regardless of what is mostly used in the FEniCS book and technical documentation, there will probably be a lot of application contributions over the next years where people write a(u,v). Therefore I think we should have a small section in the book explaining the issue of a(u,v) vs a(v,u), so we have a reference for resolving potential confusion. Hans Petter _______________________________________________ Mailing list: https://launchpad.net/~fenics Post to : [email protected] Unsubscribe : https://launchpad.net/~fenics More help : https://help.launchpad.net/ListHelp
