Convert type Array{Union{Missing, Float64},1} to Array{Float64,1} in Julia

Question

I have an array of floats having some missing values, hence its type is Array{Union{Missing, Float64},1}. Is there a command to convert the non-missing part into Array{Float64,1}?

Answer 1

Here are three solutions, in order of preference (thanks to @BogumilKaminski for the first one):

f1(x) = collect(skipmissing(x))
f2(x) = Float64[ a for a in x if !ismissing(a) ]
f3(x) = x[.!ismissing.(x)]

f1 lazy-loads the array with skipmissing (useful for e.g. iteration) and then builds the array via collect.

f2 uses a for loop but is likely to be slower than f1 since the final array length is not computed ahead of time.

f3 uses broadcasting, and allocates temporaries in the process, and so is likely to be the slowest of the three.

We can verify the above with a simple benchmark:

using BenchmarkTools
x = Array{Union{Missing,Float64}}(undef, 100);
inds = unique(rand(1:100, 50));
x[inds] = randn(length(inds));
@btime f1($x);
@btime f2($x);
@btime f3($x);

Resulting in:

julia> @btime f1($x);
  377.186 ns (7 allocations: 1.22 KiB)

julia> @btime f2($x);
  471.204 ns (8 allocations: 1.23 KiB)

julia> @btime f3($x);
  732.726 ns (6 allocations: 4.80 KiB)