How to count multiple values in an array

Question

I can count a value using Array#count.

numbers = [1, 2, 5, 5, 1, 3, 1, 2, 4, 3]
numbers.count(1) #=> 3

How can I count multiple values in an array?

What I wrote were:

numbers.count(1) + numbers.count(2) #=> 5
[1,2].map{|i| numbers.count(i)}.sum #=> 5

I think these are a bit redundant.

Answer 1

count can also take a block, so you can write this in a way that only traverses the array once:

numbers.count {|i| [1,2].include? i } # => 5

Or for fun, in a slightly more functional/point-free style:

numbers.count &[1,2].method(:include?) # => 5

Answer 2

You mean something pretty like this?

[1, 2, 2, 3, 3, 3, 4, 4, 4, 4].count_all(3, 4) # => 7

While there's nothing in Ruby's core library that provides that functionality directly, it's pretty trivial to add it.

You could just write helper method:

def count_all(array, values_to_count)
  array.count { |el| values_to_count.include?(el) }
end

count_all([1, 2, 2, 3, 3, 3, 4, 4, 4, 4], [3, 4]) # => 7

You could instead use Ruby's new refinements to add a method to Array when you need it:

module ArrayExtensions
  refine Array do
    def count_all(*values_to_count)
      self.count { |el| values_to_count.include?(el) }
    end
  end
end

# Then inside some module or class

using ArrayExtensions

[1, 2, 2, 3, 3, 3, 4, 4, 4, 4].count_all(3, 4) # => 7

Or you could opt to take the more hacky path and modify Array directly:

class Array
  def count_all(*values_to_count)
    self.count { |el| values_to_count.include?(el) }
  end
end

[1, 2, 2, 3, 3, 3, 4, 4, 4, 4].count_all(3, 4) # => 7