When indexing more than one level for an array, it works fine. But when I used it to assign values, it did not. Does anyone know why A does not change below?
In [4]: A = rand(6) Out [4]: 6-element Array{Float64,1}: 0.111552 0.155126 0.78485 0.147477 0.362078 0.959022 In [5]: A[3:5][[true,false,true]] Out [5]: 2-element Array{Float64,1}: 0.78485 0.362078 In [6]: A[3:5][[true,false,true]] = [99, 999] Out [6]: 2-element Array{Int64,1}: 99 999 In [7]: A Out [7]: 6-element Array{Float64,1}: 0.111552 0.155126 0.78485 0.147477 0.362078 0.959022
This is because indexing arrays by ranges and vectors returns a new array with the output (instead of a view into the original array). Your statement is equivalent to the following:
julia> A = rand(6) 6-element Array{Float64,1}: 0.806919 0.445286 0.882625 0.556251 0.719156 0.276755 julia> B = A[3:5] 3-element Array{Float64,1}: 0.882625 0.556251 0.719156 julia> B[[true,false,true]] = [99, 999] 2-element Array{Int64,1}: 99 999 julia> A' 1x6 Array{Float64,2}: 0.806919 0.445286 0.882625 0.556251 0.719156 0.276755 julia> B' 1x3 Array{Float64,2}: 99.0 0.556251 999.0
You can actually see that this is what Julia is doing through some of its expression utilities. Note the explicit parentheses ― it's calling setindex! on the result of indexing, which has made a copy. (GenSym() is an internal way of specifying a temporary variable):
julia> :(A[3:5][[true,false,true]] = [99, 999]) :((A[3:5])[[true,false,true]] = [99,999]) julia> expand(:(A[3:5][[true,false,true]] = [99, 999])) :(begin GenSym(0) = (top(vect))(99,999) setindex!(getindex(A,colon(3,5)),GenSym(0),(top(vect))(true,false,true)) return GenSym(0) end)
The goal is to eventually have all array indexing return views instead of copies, but that's still a work-in-progress.
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