Sorry for the bad formatting (getting tangled with the editor).
Additional useful code to supplement the previous note:
function dropdim(A,i)
v = collect(size(A))
splice!(A,i)
return tuple(v...)
end
# now something like this works:
# which gives the sum of the two edge planes of array A3 as a matrix
reshape(map(x->A3[x[1]]+A3[x[2]],mapedges(A3,3)),dropdim(A3,3))
On Thursday, March 24, 2016 at 3:21:02 AM UTC+2, Dan wrote:
>
> # 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
>>>>>
>>>>>
>>>>
>>
>
