Array range complement

Question

Is there a way to overwrite [] to have complement of range in array?

julia> a=[1:8...]
8-element Array{Int64,1}:
 1
 2
 3
 4
 5
 6
 7
 8

jul         


        

Answer 1

Directly overwriting [](i.e. getindex) is prone to break many indexing-related things in Base, but we can write an array wrapper to work around it. We only need to define the following three methods to get your specific test cases passed:

immutable ComplementVector{T} <: AbstractArray{T,1}
    data::Vector{T}
end
Base.size(A:: ComplementVector) = size(A.data)
Base.getindex(A:: ComplementVector, i::Integer) = i > 0 ? A.data[i] : A.data[setdiff(1:end, (-i))]
Base.getindex(A:: ComplementVector, I::StepRange) = all(x->x>0, I) ? A.data[I] : A.data[setdiff(1:end, -I)]

julia> a = ComplementVector([1:8...])

julia> a[-1] == a[2:8]
true

julia> a[-(1:3)] == a[4:8]
true

julia> a[-end] == a[1:7]
true

If you would like to extend ComplementVector further more, please read the doc about Interfaces.

Update:

For safety sake, we'd better not extend AbstractArray as @Fengyang Wang suggested in the comment blow:

immutable ComplementVector{T}
    data::Vector{T}
end
Base.endof(A::ComplementVector) = length(A.data)
Base.getindex(A::ComplementVector, i::Integer) = i > 0 ? A.data[i] : A.data[setdiff(1:end, (-i))]
Base.getindex(A::ComplementVector, I::OrdinalRange) = all(x->x>0, I) ? A.data[I] : A.data[setdiff(1:end, -I)]