Julia: How to count efficiently the number of missings in a `Vector{Union{T, Missing}}`

Question

Consider

x = rand([missing, rand(Int, 100)...], 1_000_000)

which yields typeof(x) = Array{Union{Missing, Int64},1}.

What's the most efficient way to count the number of missings in x?

Answer 1

The cleanest way is probably just

count(ismissing, x)

Simple, easy to remember, and fast

Since you're asking for the "most efficient" way, let me give some benchmark results. It is slightly faster than @xiaodai's answer, and as fast as a simple loop implementation:

julia> @btime count($ismissing,$x);
  278.499 μs (0 allocations: 0 bytes)

julia> @btime mapreduce($ismissing, $+, $x);
  293.901 μs (0 allocations: 0 bytes)

julia> @btime count_missing($x)
  278.499 μs (0 allocations: 0 bytes)

where

julia> function count_missing(x)
           c = 0
           @inbounds for i in eachindex(x)
               if ismissing(x[i])
                   c += 1
               end
           end
           return c
       end

Abstraction for no cost, just the way you'd want it to be.