How to select unique elements

Question

I would like to extend the Array class with a uniq_elements method which returns those elements with multiplicity of one. I also would like to use closures to my new method as with uniq. For example:

t=[1,2,2,3,4,4,5,6,7,7,8,9,9,9]
t.uniq_elements # => [1,3,5,6,8]

Example with closure:

t=[1.0, 1.1, 2.0, 3.0, 3.4, 4.0, 4.2, 5.1, 5.7, 6.1, 6.2]
t.uniq_elements{|z| z.round} # => [2.0, 5.1]

Neither t-t.uniq nor t.to_set-t.uniq.to_set works. I don't care of speed, I call it only once in my program, so it can be a slow.

Answer 1

Helper method

This method uses the helper:

class Array
  def difference(other)
    h = other.each_with_object(Hash.new(0)) { |e,h| h[e] += 1 }
    reject { |e| h[e] > 0 && h[e] -= 1 }
  end
end

This method is similar to Array#-. The difference is illustrated in the following example:

a = [3,1,2,3,4,3,2,2,4]
b = [2,3,4,4,3,4]

a - b              #=> [1]
c = a.difference b #=> [1, 3, 2, 2] 

As you see, a contains three 3's and b contains two, so the first two 3's in a are removed in constructing c (a is not mutated). When b contains as least as many instances of an element as does a, c contains no instances of that element. To remove elements beginning at the end of a:

a.reverse.difference(b).reverse #=> [3, 1, 2, 2]

Array#difference! could be defined in the obvious way.

I have found many uses for this method: here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here.

I have proposed that this method be added to the Ruby core.

When used with Array#-, this method makes it easy to extract the unique elements from an array a:

a = [1,3,2,4,3,4]
u = a.uniq          #=> [1, 2, 3, 4]
u - a.difference(u) #=> [1, 2]

This works because

a.difference(u)     #=> [3,4]    

contains all the non-unique elements of a (each possibly more than once).

Problem at Hand

Code

class Array
  def uniq_elements(&prc)
    prc ||= ->(e) { e }
    a = map { |e| prc[e] }
    u = a.uniq
    uniques = u - a.difference(u)
    select { |e| uniques.include?(prc[e]) ? (uniques.delete(e); true) : false }
  end
end

Examples

t = [1,2,2,3,4,4,5,6,7,7,8,9,9,9]
t.uniq_elements
  #=> [1,3,5,6,8]

t = [1.0, 1.1, 2.0, 3.0, 3.4, 4.0, 4.2, 5.1, 5.7, 6.1, 6.2]
t.uniq_elements { |z| z.round }
  # => [2.0, 5.1]

Answer 2

class Array
  def uniq_elements
    counts = Hash.new(0)

    arr = map do |orig_val|
      converted_val =  block_given? ? (yield orig_val) : orig_val
      counts[converted_val] += 1
      [converted_val, orig_val]
    end

    uniques = []

    arr.each do |(converted_val, orig_val)|
      uniques << orig_val if counts[converted_val] == 1
    end

    uniques
  end
end

t=[1,2,2,3,4,4,5,6,7,7,8,9,9,9]
p t.uniq_elements

t=[1.0, 1.1, 2.0, 3.0, 3.4, 4.0, 4.2, 5.1, 5.7, 6.1, 6.2]
p  t.uniq_elements { |elmt| elmt.round }

--output:--
[1, 3, 5, 6, 8]
[2.0, 5.1]

Array#uniq does not find non-duplicated elements, rather Array#uniq removes duplicates.