How to Find the Unique Elements in an Array in Ruby

How do I get unique elements in this array?

You can just use the method uniq. Assuming your array is ary, call:

ary.uniq{|x| x.user_id}

and this will return a set with unique user_ids.

How to select unique elements

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]

Get unique elements for an array of objects matching two elements only

You want to use the form of Array#uniq that takes a block.

Code

arr.uniq { |instance| [instance.name, instance.value] }

Example

class Something
  attr_accessor :id, :name, :value, :comment
  def initialize(id, name, value, comment)
    @id = id
    @name = name
    @value = value
    @comment = comment
  end
end

arr = [Something.new(34175, "abc", 123.3, "something here"),
       Something.new(34176, "xyz", 123.3, "something here"),
       Something.new(34177, "xyz", 227.3, "something here sdfg"),
       Something.new(34178, "xyz", 123.3, "something here sdfg")]
  #=> [#<Something:0x000001012cc2f0 @id=34175, @name="abc", @value=123.3,
  #      @comment="something here">,
  #    #<Something:0x000001012cc278 @id=34176, @name="xyz", @value=123.3,
  #      @comment="something here">,
  #    #<Something:0x000001012cc200 @id=34177, @name="xyz", @value=227.3,
  #      @comment="something here sdfg">,
  #    #<Something:0x000001012cc0e8 @id=34178, @name="xyz", @value=123.3,
  #      @comment="something here sdfg">]

arr.uniq { |instance| [instance.name, instance.value] }

  #=> [#<Something:0x000001012cc2f0 @id=34175, @name="abc", @value=123.3,
  #      @comment="something here">,
  #    #<Something:0x000001012cc278 @id=34176, @name="xyz", @value=123.3,
  #      @comment="something here">,
  #    #<Something:0x000001012cc200 @id=34177, @name="xyz", @value=227.3,
  #      @comment="something here sdfg">]

How to get the unique elements of an array with a maximum value of attribute

Something like this, maybe?

a = [
    {id:1, price:10},
    {id:2, price:9},
    {id:3, price:8},
    {id:1, price:7}
]

b = a.group_by{|h| h[:id]}. 
      map{|_, v| v.max_by {|el| el[:price]}}

b # => [{:id=>1, :price=>10}, {:id=>2, :price=>9}, {:id=>3, :price=>8}]

How do I find the unique number in an array and return only that number in ruby?

The following doesn't use tallies and will short circuit the search when a unique item is found. First, it returns nil if the array has fewer than 3 elements, since there's no way to answer the question in that case. If that check is passed, it works by comparing adjacent values. It performs an up-front check that the first two elements are equal—if not, it checks against the third element to see which one is different. Otherwise, it iterates through the array and returns the first value it finds which is unequal. It returns nil if there is not a distinguished element in the array.

def find_uniq(arr)
  return nil if arr.size < 3
  if arr[0] == arr[1]
    arr.each.with_index do |x, i|
      i += 1
      return arr[i] if arr[i] != x
    end
  elsif arr[1] == arr[2]
    arr[0]
  else
    arr[1]
  end
end

This also works with non-numeric arrays such as find_uniq(%w(c c c d c c c c)).

Thanks to Cary Swoveland for reminding me about each_cons. That can tighten up the solution considerably:

def find_uniq(arr)
  return nil if arr.size < 3
  if arr[0] != arr[1]
    return arr[1] == arr[2] ? arr[0] : arr[1]
  end
  arr.each_cons(2) { |x, y| return y if y != x }
end

What is the Ruby way to determine if an array has only unique entries?

array.uniq.size == array.size
array.to_set.size == array.size
array.all? { |element| array.count(element) == 1 }

ruby: how to find non-unique elements in array and print each with number of occurrences?

puts a.uniq.
       map { | e | [a.count(e), e] }.
       select { | c, _ | c > 1 }.
       sort.reverse.
       map { | c, e | "#{e}:#{c}" }