On Thursday, September 17, 2015 at 11:11:57 PM UTC-4, Patrick Kofod
Mogensen wrote:
>
> Second, I can create a vector, and store [F1; F2] as F, and simply do F[1]
> to access F given a = 1, and F[2] to access F given a = 2. I think it makes
> the code so much easier to read when looping over A = {1, 2, ..., J} - but
> I am not sure if it is a good idea or not performance wise. Since I've done
> this so many times, I really just wanted to put my ignorance out there, and
> ask if there is a particular reason why I shouldn't be doing this, and if
> there is some other great way to do it.
>
This is perfectly reasonable, and there is no reason it can't be fast. (I
think you mean [F1, F2] rather than [F1; F2], as the latter concatenates
into a single matrix.)
The only case I can think of where it would be better to use a single
concatenated matrix would be if your individual F1 and F2 matrices were
tiny (say 2x2), in which case it might be better to avoid the slight cost
of the nested indexing operations. But you wouldn't want to use slicing
either in that case (which does indeed create a temporary array): you would
want to index directly into the big matrix via F[i, j+offset] (taking care
to concatenate along the right dimensions for locality!) and inline
everything. But it doesn't sound like that level of micro-optimization is
going to be worth it for you.
