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IdentityRanges

IdentityRanges are Julia-language a helper type for creating "views" of arrays. They are a custom type of AbstractUnitRange that makes it easy to preserve the indices of array views. The key property of an IdentityRange r is that r[i] == i (hence the name of the type/package), and that they support arbitrary start/stop indices (i.e., not just starting at 1).

julia> A = reshape(1:24, 4, 6)
4×6 reshape(::UnitRange{Int64}, 4, 6) with eltype Int64:
 1  5   9  13  17  21
 2  6  10  14  18  22
 3  7  11  15  19  23
 4  8  12  16  20  24

julia> V = view(A, 2:3, 3:5)
2×3 view(reshape(::UnitRange{Int64}, 4, 6), 2:3, 3:5) with eltype Int64:
 10  14  18
 11  15  19

julia> axes(V)
(Base.OneTo(2),Base.OneTo(3))

julia> V[1,1]
10

julia> using IdentityRanges

julia> Vp = view(A, IdentityRange(2:3), IdentityRange(3:5))
view(reshape(::UnitRange{Int64}, 4, 6), IdentityRange(2:3), IdentityRange(3:5)) with eltype Int64 with indices 2:3×3:5:
 10  14  18
 11  15  19

julia> axes(Vp)
(2:3,3:5)

julia> Vp[1,1]
ERROR: BoundsError: attempt to access view(reshape(::UnitRange{Int64}, 4, 6), IdentityRange(2:3), IdentityRange(3:5)) with eltype Int64 with indices 2:3×3:5 at index [1, 1]
Stacktrace:
 [1] throw_boundserror(::SubArray{Int64,2,Base.ReshapedArray{Int64,2,UnitRange{Int64},Tuple{}},Tuple{IdentityRange{Int64},IdentityRange{Int64}},false}, ::Tuple{Int64,Int64}) at ./abstractarray.jl:484
 [2] checkbounds at ./abstractarray.jl:449 [inlined]
 [3] getindex(::SubArray{Int64,2,Base.ReshapedArray{Int64,2,UnitRange{Int64},Tuple{}},Tuple{IdentityRange{Int64},IdentityRange{Int64}},false}, ::Int64, ::Int64) at ./subarray.jl:206
 [4] top-level scope at none:0

julia> Vp[2,3]
10

julia> A[2,3]
10