Index Array without Elements

Question

I have a vector

a = Vector(1:4)

[1, 2, 3, 4]

and I want to index it to all elements but the third to get

[1, 2, 4]

in R you could do a[-3]. What do you do in Julia?

(Eventually I want to loop through all the elements and compare each of them to the rest - maybe this is relevant.)

Answer 1

This use case is a common one and is covered by the InvertedIndices.jl package. If you install it then you can run:

julia> using InvertedIndices

julia> a = 1:4
1:4

julia> a[Not(3)]
3-element Array{Int64,1}:
 1
 2
 4

Also some packages (like DataFrames.jl) automatically load this package (so if you e.g. use DataFrames.jl you do not have to install and load InvertedIndices.jl separately).

Answer 2

The Julia syntax will be unfortunately more verbose than those of R:

julia> a[1:end .!== 3]
3-element Array{Int64,1}:
 1
 2
 4

Another option is to mutate a:

julia> deleteat!(a,3)
3-element Array{Int64,1}:
 1
 2
 4

If your data is within a DataFrame than you can get a nicer syntax:

julia> df = DataFrame(a=1:4);

julia> df[Not(3),:]
3×1 DataFrame
│ Row │ a     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 1     │
│ 2   │ 2     │
│ 3   │ 4     │

and when DataFrames is imported Not will also work with a Vector:

julia> a[Not(3)]
3-element Array{Int64,1}:
 1
 2
 4