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