def in other
      other = Hash[other.zip(Array.new(other.size, 0))]
      Daru::Core::Query::BoolArray.new(
        @data.each_with_object([]) do |d, memo|
          memo << (other.key?(d) ? true : false)
        end
      )
    end
Comparator for checking if any of the elements in *other* exist in self.

@param [Array, Daru::Vector] other A collection which has elements that
need to be checked for in self.
@example Usage of `in`.
vector = Daru::Vector.new([1,2,3,4,5])
vector.where(vector.in([3,5]))
#=>
##<Daru::Vector:82215960 @name = nil @size = 2 >
#    nil
#  2   3
#  4   5