I have a concrete Array and want to efficiently construct a similar array of the same dimensions filled with ones. What would be the recommended approach?
Here's a random array to work with:
julia> A = rand(0:1, 10, 5)
10×5 Matrix{Int64}:
or A = rand(0:1., 10, 5) (with a dot on 0. and/or 1.) for a random matrix of floats.
Two approaches are very natural. I could do this:
julia> zero(A) .+ 1
5×10 Matrix{Int64}:
Or I could do it this way:
julia> repeat(ones(size(A)[2])', outer = size(A)[1])
5×10 Matrix{Float64}:
The first approach is more elegant. The second approach feels more clunky and prone to error (accidentally exchanging [1] and [2]), but at the same time it doesn't involve the addition operation and so possibly involves fewer allocations (or maybe not because the compiler is super smart Edit: quick benchmark below suggests the compiler is super smart).
And of course there may be another, better approach.
using BenchmarkTools
A = rand(0:1, 1000, 1000)
@btime zero(A) .+ 1
## 1.609 ms (6 allocations: 15.26 MiB)
@btime repeat(ones(size(A)[2])', outer = size(A)[1])
## 3.032 ms (10 allocations: 7.64 MiB)
Edit 2: Follow-up onBogumił's answer
The following method for a unit-array J, defined for convenience, is efficient:
function J(A::AbstractArray{T,N}) where {T,N}
ones(T, size(A))
end
J(A)
@btime J(A)
## 789.929 μs (2 allocations: 7.63 MiB)