How do I find the maximum value of an object's attribute that occurs exactly twice in an array?

Question

I'm using Ruby 2.4. If I want to find the maximum number of a numeric attribute of my model, I can do

max_num = my_objects_arr.maximum(:numeric_attr)

but how would I find the maximum number of attributes whose values occur exactly twice in my array? That is, let's say my objects array has three objects

obj1 - numeric_attr = 3
obj2 - numeric_attr = 3
obj3 - numeric_attr = 4

The maximum of the attributes above that occur exactly twice would be "3". Although "4" is the maximum of all attributes, it only occurs once in the array.

Answer 1

array = [1, 2, 3, 2, 3, 4]

array.group_by { |e| e } # group_by(&:itself) since 2.3
     .select { |_, v| v.count == 2 }
     .keys
     .max
#⇒ 3

For objects and attributes:

my_objects_arr.group_by { |o| o.numeric_attr }
              .select { |_, v| v.count == 2 }
              .keys
              .max

To get the objects themselves:

my_objects_arr.group_by { |o| o.numeric_attr }
              .select { |_, v| v.count == 2 }
              .max_by(&:first)
              .last

Answer 2

Since you are using rails calculations eg #maximum this should work for you

my_objects_arr
  .group(:numeric_attr)
  .having("count(numeric_attr) = 2")
  .maximum(:numeric_attr)

This will find the maximum value of numeric_attr by grouping them by the numeric_attr and selecting the numeric_attr that have exactly 2

SQL estimation

SELECT 
  MAX(numeric_attr)
FROM 
  [SOME TABLE]
GROUP BY 
  numeric_attr
HAVING 
  COUNT(numeric_attr) = 2

Answer 3

arr = [1, 2, 3, 2, 3, 4]

arr.each_with_object(Hash.new(0)) {|n,h| h[n] += 1}.select {|_,nbr| nbr == 2}.keys.max
  #=> 3

This uses the form of Hash::new that creates a hash h with a default value of zero. That means that if h does not have a key k , h[k] returns zero (without altering the hash). This refers to the method Hash#[] , not to be confused with Hash#[]= .