How can I do standard deviation in Ruby?

Question

I have several records with a given attribute, and I want to find the standard deviation.

How do I do that?

Answer 1

I'm not a big fan of adding methods to Enumerable since there could be unwanted side effects. It also gives methods really specific to an array of numbers to any class inheriting from Enumerable, which doesn't make sense in most cases.

While this is fine for tests, scripts or small apps, it's risky for larger applications, so here's an alternative based on @tolitius' answer which was already perfect. This is more for reference than anything else:

module MyApp::Maths
  def self.sum(a)
    a.inject(0){ |accum, i| accum + i }
  end

  def self.mean(a)
    sum(a) / a.length.to_f
  end

  def self.sample_variance(a)
    m = mean(a)
    sum = a.inject(0){ |accum, i| accum + (i - m) ** 2 }
    sum / (a.length - 1).to_f
  end

  def self.standard_deviation(a)
    Math.sqrt(sample_variance(a))
  end
end

And then you use it as such:

2.0.0p353 > MyApp::Maths.standard_deviation([1,2,3,4,5])
=> 1.5811388300841898

2.0.0p353 :007 > a = [ 20, 23, 23, 24, 25, 22, 12, 21, 29 ]
 => [20, 23, 23, 24, 25, 22, 12, 21, 29]

2.0.0p353 :008 > MyApp::Maths.standard_deviation(a)
 => 4.594682917363407

2.0.0p353 :043 > MyApp::Maths.standard_deviation([1,2,2.2,2.3,4,5])
 => 1.466628787389638

The behavior is the same, but it avoids the overheads and risks of adding methods to Enumerable.