Julia: Finding values larger than 0 in vector with missing

Question

I'm fairly new to Julia and as a Matlab/R User I find it, for the most part, really nice to work with.

However, I'm a little confused by the missing values and how to work with them.

Let's say I have a vector:

a=[missing -1 2 3 -12] #Julia
a=[NaN -1 2 3 -12] #Matlab

In Matlab I would just do the following to find the values below 0

a(a<0)

which gives me

-1 -12

The same unfortunately doesn't work in Julia and when I try

a[a.<0]

in Julia I just get the following error

ERROR: ArgumentError: unable to check bounds for indices of type Missing

I also tried the following

a[findall(skipmissing(a).<0)]

which gives me

missing
3

since, of course, I skipped the missing value in the findall-function. I'm pretty sure there is an easy and logical way to do this, but I don't seem to be able to find it.

Can someone please show me the way?

Best, Richard

Note that the Matlab example is not equivalent to the Julia one. If you use NaN instead of missing in Julia, then a[a.<0] works exactly like in Matlab. — DNF

Bogumił Kamiński Bogumił Kamiński · Accepted Answer · 2019-04-12T10:26:07

Here is the simplest way to do it:

julia> a=[missing -1 2 3 -12]
1×5 Array{Union{Missing, Int64},2}:
 missing  -1  2  3  -12

julia> a[isless.(a, 0)]
2-element Array{Union{Missing, Int64},1}:
  -1
 -12

This uses the fact that missing is considered larger than any number by isless.

Another way to write it:

julia> filter(x -> isless(x, 0), a)
2-element Array{Union{Missing, Int64},1}:
  -1
 -12

Now in order to avoid this special trick with isless you can do the following (using coalesce is a general approach that can be used for safe handling of missing values):

julia> a[coalesce.(a .< 0, false)]
2-element Array{Union{Missing, Int64},1}:
  -1
 -12

or

julia> filter(x -> coalesce(x < 0, false), a)
2-element Array{Union{Missing, Int64},1}:
  -1
 -12

finally you can be more explicit like:

julia> filter(x -> !ismissing(x) && x < 0, a)
2-element Array{Union{Missing, Int64},1}:
  -1
 -12

or

julia> [v for v in a if !ismissing(v) && v < 0]
2-element Array{Int64,1}:
  -1
 -12

(you could use comprehension syntax also in the examples above)

Julia: Finding values larger than 0 in vector with missing

1 Answers