Inspecting the Size of a JuMPArray

[ccoffrin]

Initially I tried to inspect the size of a JuMPArray as follows,

using JuMP

m = Model()
v = @variable(m, [i in [1,3,5,7,9], j=1:8], basename="$(i)_$(j)", lowerbound = 0, upperbound = 1)

length(v) # ERROR: JuMPArray does not define size().

v_slice = v[3,:]

length(v_slice) # ERROR: JuMPArray does not define size().

After inspecting the source, I determined this would do what I need,

length(indices(v)) # dimensions
length.(indices(v)) # dimension length check
length(indices(v_slice)) # dimensions
length.(indices(v_slice))[1] # dimension length check

It might be helpful to have some default behavior for the standard Julia array functions like length and size.

[mlubin]

I was following https://docs.julialang.org/en/latest/devdocs/offset-arrays/, but the guidance may have changed for 0.6/0.7.

[ccoffrin]

I see, the logic outlined in offset arrays makes sense to me, but I had not been exposed to them yet so I was lost for the function naming pattern. A more helpful error message would surely mitigate the issue.

Possibly the root of my confusion is Array in the name, it made me think it would behave like typical Julia array, but it is more flexible than that. Maybe something like JuMPCollection{N}, would be more semantics agnostic, and the dimension number N would give a hint that it has some features similar to standard multidimensional arrays.

[mlubin]

The use of Array in JuMPArray is consistent with OffsetArrays and AxisArrays. I agree that the error messages should be more helpful.

[ccoffrin]

An improved error message sounds good to me.

[mlubin]

Both OffsetArrays and AxisArrays now define size, so I think it's safe for us to do so also.