[ruby-core:122857] [Ruby Feature#21518] Statistical helpers to `Enumerable`

24 Jul 2025

      Issue #21518 has been updated by Amitleshed (Amit Leshed).

Dan0042 (Daniel DeLorme) wrote in #note-1:
...
In favor, just careful about the bug in #median
```ruby
x = [1, 3, 2]
x.median #=> 2
x #=> [1, 2, 3] modified by #median
```
You'll want to use `arr = entries` rather than `arr = to_a`
Right. Great catch.

Dan0042 (Daniel DeLorme) wrote in #note-1:
...
In favor, just careful about the bug in #median
```ruby
x = [1, 3, 2]
x.median #=> 2
x #=> [1, 2, 3] modified by #median
```
You'll want to use `arr = entries` rather than `arr = to_a`
Thanks, great catch!

----------------------------------------
Feature #21518: Statistical helpers to `Enumerable`
https://bugs.ruby-lang.org/issues/21518#change-114148

* 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.**

-- 
https://bugs.ruby-lang.org/