A package for arbitrary array indices?

General Usage
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1

Hello,

I am a looking for a package that allows me to dynamically change the indexing of an array. I’m aware of OffsetArrays but this is not what I need.

Say I have an 8 by 8 symmetric matrix A. I have an algorithm that defines the “merging” of two indices. Say that I merge indices 3 and 5 by doing A[3,:] .= (A[3,:] .+ A[5,:])./2 and the same for the columns. Then I can get rid of 5, in practice I do it with a view and an InvertedIndex to exclude 5. So this view is a 7x7 matrix now.

The new elements at index 3 are in fact that of another node from a graph (that was defined by A), node 9. What I would like is a way to change the indexing of A such that
A[9,:] gives me what’s in A[3,:], and A[3,:] should be out of bounds. In other words, indexing A maps 3 to 9.

I suppose I could implement that myself but I’d like to avoid the risk of making mistakes if this already exists.

Thanks.

2

I’m not aware of any packages that allow you to redefine array indices. Maybe a different data structure like a dictionary would be the answer?

D = Dict(i => A[:,a] for (i,a) in enumerate(axes(A,2)))

Which would allow for things like:

D[9] = (D[3] + D[5])/2
delete!(D,3)
delete!(D,5)

There are probably more efficient methods though. If A is based on a graph perhaps MetaGraphs.jl might be useful?

3

Your application seems highly specialized. Maybe you should just build your own array type. Then getindex can do whatever you would like.

https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-array

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