If you want this to be very efficient, your best bet is to construct
CartesianRange objects Rstart, Rend and then
for R in (Rstart, Rend)
for I in R
A[I] = 0
end
end
or whatever.
--Tim
On Wednesday, March 23, 2016 04:59:59 PM Tomas Lycken wrote:
> Is there an effective pattern to iterate over the “endpoints” of an array
> along a given dimension?
>
> What I eventually want to accomplish is to apply a function (in this case
> an equality test) to the two end points along a particular dimension of an
> array. I think the pattern is easiest explained by considering 1D, 2D and
> 3D:
>
> # assume the existence of some scalar-valued function f(x,y)
>
> A1 = rand(10)
> f(A1[1], A1[end]) # d == 1 (the only possible value) -> one evaluation
>
> A2 = rand(10, 15)
> map(f, A2[1,:], A2[end,:]) # d == 1 -> 15 evaluations
> map(f, A2[:,1], A2[:,end]) # d == 2 -> 10 evaluations
>
> A3 = rand(10, 15, 8)
> map(f, A3[1,:,:], A3[end,:,:]) # d == 1 -> 15x8 evaluations
> map(f, A3[:,1,:], A3[:,end,:]) # d == 2 -> 10x8 evaluations
> map(f, A3[:,:,1], A3[:,:,end]) # d == 3 -> 10x15 evaluations
>
> I just want to consider one dimension at a time, so given A and d, and in
> this specific use case I don’t need to collect the results, so a for-loop
> without an allocated place for the answer instead of a map is just fine
> (probably preferrable, but it’s easier to go in that direction than in the
> other). What I’m struggling with, is how to generally formulate the
> indexing expressions (like [<d-1 instances of :>, 1, <size(A,d)-d instances
> of :>], but not in pseudo-code…). I assume this can be done somehow using
> CartesianIndexes and/or CartesianRanges, but I can’t get my mind around to
> how. Any help is much appreciated.
>
> // T
>
