How do I create an average from a Ruby array?

Without having to repeat the array (e.g. perfect for one-liners):

[1, 2, 3, 4].then { |a| a.sum.to_f / a.size }

You can choose one of the below solutions as you wish.

Bruteforce

[0,4,8,2,5,0,2,6].sum.to_f / [0,4,8,2,5,0,2,6].size.to_f

=> 3.375

Method

def avg(array)
  array.sum.to_f / array.size.to_f
end  

avg([0,4,8,2,5,0,2,6])
=> 3.375

Monkey Patching

class Array
  def avg
    sum.to_f / size.to_f
  end
end

[0,4,8,2,5,0,2,6].avg
=> 3.375

But I don't recommend to monkey patch the Array class, this practice is dangerous and can potentially lead to undesirable effects on your system.

For our good, ruby language provides a nice feature to overcome this problem, the Refinements, which is a safe way for monkey patching on ruby.

To simplify, with the Refinements you can monkey patch the Array class and the changes will only be available inside the scope of the class that is using the refinement! :)

You can use the refinement inside the class you are working on and you are ready to go.

Refinements

module ArrayRefinements
  refine Array do
    def avg
      sum.to_f / size.to_f
    end
  end
end

class MyClass
  using ArrayRefinements

  def test(array)
    array.avg
  end
end

MyClass.new.test([0,4,8,2,5,0,2,6])
=> 3.375

Add Array#average.

I was doing the same thing quite often so I thought it was prudent to just extend the Array class with a simple average method. It doesn't work for anything besides an Array of numbers like Integers or Floats or Decimals but it's handy when you use it right.

I'm using Ruby on Rails so I've placed this in config/initializers/array.rb but you can place it anywhere that's included on boot, etc.

config/initializers/array.rb

class Array

  # Will only work for an Array of numbers like Integers, Floats or Decimals.
  #
  # Throws various errors when trying to call it on an Array of other types, like Strings.
  # Returns nil for an empty Array.
  #
  def average
    return nil if self.empty?

    self.sum.to_d / self.size
  end

end

Another simple solution too

arr = [0,4,8,2,5,0,2,6]
arr.sum(0.0) / arr.size

This method can be helpful.

def avg(arr)
  val = 0.0

  arr.each do |n|
    val += n
  end

  len = arr.length

  val / len 
end

p avg([0,4,8,2,5,0,2,6])

print array.sum / array.count is how i've done it

I really like to define a mean() method so my code is more expressive.

I usually want to ignore nil by default, so here's what I define

def mean(arr)
  arr.compact.inject{ |sum, el| sum + el }.to_f / arr.compact.size
end

mean([1, nil, 5])
=> 3.0

If you want to keep the nils, just remove both the .compacts.

A much faster solution than .inject is :

arr.sum(0.0) / arr.size

See this article for ref: https://andycroll.com/ruby/calculate-a-mean-average-from-a-ruby-array/