My old friend the 3d array called Pop, I want to remove columns (d2), across all of d3 when the value in the end of d1 == 1, so I have this code:
Pop[end, :, 1] .!=1
I thought adding @view in front of this would write the changes back to Pop, outwith producing an additional copy in memory. This code works fine
@view(Pop[ :, Pop[end, :, 1] .!=1, :])
but it does not alter the original 3d array called Pop. I could do
Pop = @view(Pop[ :, Pop[end, :, 1] .!=1, :])
but I believe this will create another copy in memory which I'm trying to avoid. What simple syntax have I missed? Thx. J
@view does not modify the size of the original array, it provides a "view" into it, (e.g., omitting some columns in your case). I don't think there is anything wrong with
Pop = view(Pop, :, Pop[end, :, 1] .≠ 1, :)
since now Pop is a view into your old, full Pop, but it behaves like an array, so you can modify its entries, e.g., you could then do things like
julia> using Random # using a fixed seed for reproducibility
julia> Random.seed!(0) ;
julia> Pop = rand(1:5, (2,4,2)) # original Pop
2×4×2 Array{Int64,3}:
[:, :, 1] =
4 3 5 5
1 1 3 5
[:, :, 2] =
2 2 3 1
2 5 1 1
julia> Pop[end,:,1] .≠ 1 # columns to keep
4-element BitArray{1}:
0
0
1
1
julia> Pop = view(Pop, :, Pop[end, :, 1] .≠ 1, :) # make it a view
2×2×2 view(::Array{Int64,3}, :, [3, 4], :) with eltype Int64:
[:, :, 1] =
5 5
3 5
[:, :, 2] =
3 1
1 1
julia> Pop[end,:,1] .= 1 ; # use your new view to manipulate data
julia> Pop # view of the modified Pop
2×2×2 view(::Array{Int64,3}, :, [3, 4], :) with eltype Int64:
[:, :, 1] =
5 5
1 1
[:, :, 2] =
3 1
1 1
julia> Pop.parent # original full Pop (now Pop.parent) has been modified
2×4×2 Array{Int64,3}:
[:, :, 1] =
4 3 5 5
1 1 1 1
[:, :, 2] =
2 2 3 1
2 5 1 1