Sum of an empty array

General Usage
1

Hello,
I need to calculate the sum of the values in the column of a DataFrame. But it can also be empty.
How can I do that?

sum([])

gives an long stack trace…

2

Both of these work

julia> sum(Float64[])
0.0

julia> sum([], init=0.0)
0.0

As long as the column is typed, sum should just work:

julia> df = DataFrame(:x => Float64[])
0×1 DataFrame
 Row │ x
     │ Float64
─────┴─────────

julia> sum(df[!,:x])
0.0

julia> combine(df, :x => sum)
1×1 DataFrame
 Row │ x_sum
     │ Float64
─────┼─────────
   1 │     0.0
2 Likes
3

Well, the column type is a union of missing and bool, therefore sum(skipmissing(df.x)) did not work, but sum(skipmissing(df.x), init=0) works… :slight_smile:

2 Likes
4

Are you sure that’s the problem?

julia> sum(skipmissing([true, missing]))
1

julia> typeof([true, missing])
Vector{Union{Missing, Bool}}
5

Yes, that was my problem. Try:

sum(skipmissing([missing]))
6

there is a recent PR for emptymissing in Missings.jl which solves this.

emptymissing(sum)(skipmissing([missing]))

maybe not the most elegant syntax, but it will propagate missing in this scenario.

Also, with Julia 1.6 you can use init as a keyword argument.

julia> emptymissing(sum)(skipmissing([missing]))
missing

julia> sum(skipmissing([missing]); init = 0.0)
0.0