# something along these lines:
topleft(A,d) = tuple(ones(Int,ndims(A))...)
bottomleft(A,d) = tuple([i==d ? 1 : e for (i,e) in enumerate(size(A))]...)
topright(A,d) = tuple([i==d ? e : 1 for (i,e) in enumerate(size(A))]...)
bottomright(A,d) = size(A)
mapedges(A,d) = zip(
CartesianRange{CartesianIndex{ndims(A)}}(
CartesianIndex{ndims(A)}(topleft(A,d)...),
CartesianIndex{ndims(A)}(bottomleft(A,d)...)),
CartesianRange{CartesianIndex{ndims(A)}}(
CartesianIndex{ndims(A)}(topright(A,d)...),
CartesianIndex{ndims(A)}(bottomright(A,d)...)
))
A3 = rand(10,15,8)
julia> mapedges(A3,1) |> length
120
julia> mapedges(A3,2) |> length
80
julia> mapedges(A3,3) |> length
150
On Thursday, March 24, 2016 at 2:53:48 AM UTC+2, Tomas Lycken wrote:
>
> …but not really. Reading the docstring more carefully:
>
> Transform the given dimensions of array A using function f. f is called on
> each slice of A of the form A[…,:,…,:,…]. dims is an integer vector
> specifying where the colons go in this expression. The results are
> concatenated along the remaining dimensions. For example, if dims is [1,2]
> and A is 4-dimensional, f is called on A[:,:,i,j] for all i and j.
>
> What I want to do, is rather call f on A[:,:,1,:] and A[:,:,end,:], but
> nothing in between 1 and end for that dimension. mapslices still
> eventually visit the entire array (either by slicing, or by iteration), but
> I only want to visit the “edges”. I might be missing something, though.
>
> // T
>
> On Thursday, March 24, 2016 at 1:48:36 AM UTC+1, Tomas Lycken wrote:
>
> Yes, probably - thanks for the tip! I'll see if I can cook something up...
>>
>> On Thursday, March 24, 2016 at 1:45:32 AM UTC+1, Benjamin Deonovic wrote:
>>>
>>> Can mapslices help here?
>>>
>>>
>>> On Wednesday, March 23, 2016 at 6:59:59 PM UTC-5, 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
>>>>
>>>>
>>>
>
