How make .+ work with A::Vector{Float64} and B::OffsetArray(::Vector{Float64}, 0:1)

General Usage
2

Hey elvispy! Welcome to the julia discourse.

Summary
For your use-case you should just use:

julia> A .+ (B.parent)
2-element Vector{Float64}:
 2.0
 2.1

Reasoning
Let me speculate.

I would think that the reason this is not working is that one of the arrays is an OffsetArray with indices 0 and 1. So if you interpret the broadcasted addition .+ as an “element-wise addition” you would like to add A[i] + B[i] for all the indices in A in B. But in this case A has indices 0 and 1 and B has indices 1 and 2. So even though they have the same number of elements they don’t “match”. I agree that the error message is not very useful in this case, but making it work by force in general could be a foot gun.

If you are sure that the operation you want to do is the element-wise addition disregarding the fact that both vectors have different indices you could acces the underlying array of B, which you can do with B.parent:

julia> using LegendrePolynomials

julia> A = [1.0, 2.0];

julia> B = collectPl(0.1, lmax = 1) # Just two arrays of the same size
2-element OffsetArray(::Vector{Float64}, 0:1) with eltype Float64 with indices 0:1:
 1.0
 0.1

julia> A
2-element Vector{Float64}:
 1.0
 2.0

julia> A .+ B
ERROR: DimensionMismatch: arrays could not be broadcast to a common size; got a dimension with lengths 2 and 2
Stacktrace:
 [1] _bcs1
   @ ./broadcast.jl:516 [inlined]
 [2] _bcs
   @ ./broadcast.jl:510 [inlined]
 [3] broadcast_shape
   @ ./broadcast.jl:504 [inlined]
 [4] combine_axes
   @ ./broadcast.jl:499 [inlined]
 [5] instantiate
   @ ./broadcast.jl:281 [inlined]
 [6] materialize(bc::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1}, Nothing, typeof(+), Tuple{Vector{Float64}, OffsetArrays.OffsetVector{Float64, Vector{Float64}}}})
   @ Base.Broadcast ./broadcast.jl:860
 [7] top-level scope
   @ REPL[8]:1

julia> A .+ B.parent
2-element Vector{Float64}:
 2.0
 2.1

The problem then is not that broadcasting does not work for OffsetArrays. For example if we re-wrap the contents of B in an OffsetArray with indices 1:2 (the same indices as in A) things work as you would expect.

# Code continues from previous snippet
julia> using OffsetArrays

julia> C = OffsetArray(B.parent, 1:2)
2-element OffsetArray(::Vector{Float64}, 1:2) with eltype Float64 with indices 1:2:
 1.0
 0.1

julia> A .+ C
2-element OffsetArray(::Vector{Float64}, 1:2) with eltype Float64 with indices 1:2:
 2.0
 2.1

This discussion seems to be related: Check that axes start with 1 for AbstractRange operations by timholy · Pull Request #30950 · JuliaLang/julia · GitHub

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