Fantastic, thank you! Except for what I assume is a typo in `dropdim` (we
want `splice!(v,i)`), this does exactly what I want.
// T
On Thursday, March 24, 2016 at 2:37:12 AM UTC+1, Dan wrote:
>
> 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
>>>>>>
>>>>>>
>>>>>
>>>
>>
