What is Ref?

New to Julia
5

You can also write [ x for x in [1,2,3] if x in [3,4,5]], and I am mentioning this because it is, at least in this simple test, faster than intersect:

julia> using BenchmarkTools

julia> @btime [ i for i in [1,2,3] if i in [3,4,5] ]
  173.147 ns (7 allocations: 656 bytes)
1-element Array{Int64,1}:
 3

julia> @btime intersect([1,2,3],[3,4,5])
  494.927 ns (15 allocations: 1.33 KiB)
1-element Array{Int64,1}:
 3

EDIT: the results are not identical if there are repeated values in the vectors. In that case, you need to check what you want:

julia> @btime intersect([1, 4, 4, 5, 6], [4, 6, 6, 7, 8])
  541.625 ns (14 allocations: 1.36 KiB)
2-element Array{Int64,1}:
 4
 6

julia> @btime [ x for x in [1, 4, 4, 5, 6] if x in [4, 6, 6, 7, 8]]
  233.356 ns (9 allocations: 976 bytes)
3-element Array{Int64,1}:
 4
 4
 6

EDIT2: Dropping repeated elements with unique! is still faster than intersect:

julia> v1 = rand(1:10,10); 

julia> v2 = rand(1:10,20);

julia> @btime intersect($v1,$v2)
  773.919 ns (15 allocations: 1.46 KiB)
6-element Array{Int64,1}:
 9
 7
 2
 3
 4
 1

julia> @btime unique!([ x for x in $v1 if x in $v2 ])
  459.558 ns (9 allocations: 912 bytes)
6-element Array{Int64,1}:
 9
 7
 2
 3
 4
 1
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