performancejuliavectorizationcontains

Julia: Easiest and efficient way to check if each element of a vector is in another list? .∈ doesn't seem to work

In Julia, assuming I have a vector and a list of candidates:

x = [1, 2, 3, 4, 5]
targetlist = [1, 2]

I would like to iteratively check for each element of the vector if it is contained in the target list.

Using \in ∈ works for one element:

x[1] ∈ targetlist
# true

But does not seem to vectorize correctly?

x .∈ targetlist

ERROR: DimensionMismatch: arrays could not be broadcast to a common size; got a dimension with lengths 5 and 2
Solution

  • You need to wrap targetlist in a Ref to make it behave like a scalar value:

    julia> x .∈ Ref(targetlist)
5-element BitVector:
 1
 1
 0
 0
 0

    Additionally, if your lists are large, you might want to convert targetlist into a Set to make this operation O(n) instead of O(n^2):

    julia> x .∈ Ref(Set(targetlist))
5-element BitVector:
 1
 1
 0
 0
 0

    Time comparison for 10k Int32 elements in x and y each:

    julia> x = rand(Int32, 10000); y = rand(Int32, 10000);

julia> @time x .∈ Ref(y);
  0.118258 seconds (5 allocations: 5.594 KiB)

julia> @time x .∈ Ref(Set(y));
  0.000830 seconds (13 allocations: 86.133 KiB)