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Many languages have a way such as null or none to indicate a non-existent value. Because Julia is designed to handle large volumes of (often messy) data, it has multiple forms of nothingness.

The overall aim is to flag missing or suspect values as they are encountered, then continue without raising an exception.

nothing

If a value really does not exist, it is represented by nothing. This is probably closest to what C (NULL) or Python (None) might do.

julia> n = nothing

julia> isnothing(n)
true

julia> typeof(n)
Nothing

So nothing is a singleton value of type Nothing, and we can test for it.

One common use of nothing is as a return (non-)value for functions which are used only for their side effects (printing, network configuration, or whatever).

missing

For situations where a value exists in theory but we don't know what it is, missing is used. For example, when counting vehicles traveling on a road, human observers might need a break or automatic sensors break down, but the traffic continues to flow.

Thus missing is a placeholder, warning humans that they need to make a decision about how to handle this gap in the data.

julia> mv = [1, 2, missing]
3-element Vector{Union{Missing, Int64}}:
 1
 2
  missing

julia> typeof(mv)
Vector{Union{Missing, Int64}} (alias for Array{Union{Missing, Int64}, 1})

julia> ismissing.(mv)  # broadcast function, displays as 1 for true, 0 for false
3-element BitVector:
 0
 0
 1

Few other languages have this feature built in, but close analogues are NA in R or NULL in SQL.

Expressions usually return missing by default if missing values are present. If you want these values to be ignored, use the skipmissing() function to make this explicit:

julia> mv = [1, 2, missing]
3-element Vector{Union{Missing, Int64}}:
 1
 2
  missing

julia> sum(mv)  # missing in, missing out
missing

julia> skipmissing(mv)
skipmissing(Union{Missing, Int64}[1, 2, missing])

julia> collect(skipmissing(mv))
2-element Vector{Int64}:
 1
 2

julia> sum(skipmissing(mv))  # functions like sum() can work with iterators
3

Because skipmissing creates an iterator, wrap it in collect() if you need a vector.

Sometimes it is useful to replace missing values with some default. The @coalesce() macro is useful for this, as it will return the first non-missing value (or missing if there is nothing else).

julia> str = ["I", "exist", missing]
3-element Vector{Union{Missing, String}}:
 "I"
 "exist"
 missing

julia> [@coalesce(s, "-") for  s in str]
3-element Vector{String}:
 "I"
 "exist"
 "-"

NaN

Short for "Not a Number", NaN flags a computation problem in situations where a number was expected.

julia> v = [0, 1, -1]
3-element Vector{Int64}:
  0
  1
 -1

julia> v / 0
3-element Vector{Float64}:
 NaN
  Inf
 -Inf
 
julia> sum(v / 0)
NaN

Any sort of calculation on data including a NaN will give a NaN result.

There is currently no special function to remove NaN values, but the standard filter() function can do this quite simply. Only values for which some given condition is true will be copied to the result array:

julia> filter(!isnan, [1, 2, NaN])
2-element Vector{Float64}:
 1.0
 2.0