From: "mame (Yusuke Endoh) via ruby-core" <ruby-core@...>
Date: 2025-07-25T20:08:57+00:00
Subject: [ruby-core:122866] [Ruby Feature#21518] Statistical helpers to `Enumerable`

Issue #21518 has been updated by mame (Yusuke Endoh).


Naturally, these methods have been desired by some people for a very long time, but Ruby has historically been very cautious about introducing them. Even the obviously useful `#sum` method was only added in 2016, which is relatively recent in Ruby's history.

One reason behind this caution is the reluctance to add methods to Array that assume all elements are Integer or Float. Since Array can contain Strings or other non-numeric objects, there's a question of whether it is appropriate to add methods that make no sense in such cases.

The reason why `#sum` was eventually added was the growing attention to an algorithm called the Kahan-Babuska Summation Algorithm. This is a clever algorithm that reduces floating-point error when summing, and it is actually implemented in `Array#sum`. Before this algorithm gained attention, I remember the prevailing opinion was that it should be written explicitly, like `ary.inject(0, &:+)`.

For now, you may want to try using https://github.com/red-data-tools/enumerable-statistics to get a better idea of what you actually need.

----------------------------------------
Feature #21518: Statistical helpers to `Enumerable`
https://bugs.ruby-lang.org/issues/21518#change-114167

* Author: Amitleshed (Amit Leshed)
* Status: Open
----------------------------------------
**Summary**

I'd like to add two statistical helpers to `Enumerable`:

- `Enumerable#average` (arithmetic mean)
- `Enumerable#median`

Both are small, well-defined operations that many Rubyists re-implement in apps and gems. Providing them in core avoids repeated, ad-hoc code and aligns with `Enumerable#sum`, which Ruby already ships.


**Motivation**

- These are among the most common ���roll-your-own��� helpers for arrays/ranges of numbers.
- They are conceptually simple, universally useful beyond web/Rails.
- Similar to `sum`, they���re primitives for quick data analysis, ETL scripts, CLI tooling, etc.
- Including them encourages consistent semantics (what to do with empty sets, mixed numerics, etc.).


## Proposed API & Semantics

```ruby
Enumerable#average -> Float or nil
Enumerable#median  -> Numeric or nil
```

```ruby
[1, 2, 3, 4].average      # => 2.5
(1..4).average            # => 2.5
[].average                # => nil

[1, 3, 2].median          # => 2
[1, 2, 3, 10].median      # => 2.5
(1..6).median             # => 3.5
[].median                 # => nil
```

Ruby implementation 

```ruby
module Enumerable
  def average
    count = 0
    total = 0.0
    each do |x|
      raise TypeError, "non-numeric value for average" unless x.is_a?(Numeric)
      total += x
      count += 1
    end
    count.zero? ? nil : total / count
  end

  def median
    arr = to_a
    return nil if arr.empty?
    arr.each { |x| raise TypeError, "non-numeric value for median" unless x.is_a?(Numeric) }
    arr.sort!
    mid = arr.length / 2
    arr.length.odd? ? arr[mid] : (arr[mid - 1] + arr[mid]) / 2.0
  end
end
```

**Upon approval I'm more than willing to implement spec and code in C.**




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