From: keystonelemur@...
Date: 2019-01-29T01:05:01+00:00
Subject: [ruby-core:91312] [Ruby trunk Feature#11076] Enumerable method	count_by

Issue #11076 has been updated by baweaver (Brandon Weaver).


mame (Yusuke Endoh) wrote:
> I have learnt the word "tally" in this thread.  Thank you.  It looks good to me, a non-native speaker. I have put this on the agenda of the next developers' meeting.
> 
> By the way, what is the precise semantics of the method?
> 
> Question 1. What identity is the object in the keys?
> 
> ```
> str1 = "a"
> str2 = "a"
> t = [str1, str2].tally
> 
> p t  #=> { "a" => 2 }
> 
> p t.keys.first.object_id #=> str1.object_id or str2.object_id ?
> ```
> 
> IMO: I think it should prefer the first element, so it should be equal to `str1.object_id`.
> 
> Question 2. What is the key of `tally_by`?
> 
> ```
> str1 = "a"
> str2 = "A"
> t = [str1, str2].tally_by(&:upcase)
> 
> p t  #=> { "a" => 2 } or { "A" => 2 } ?
> 
> p t.keys.first.object_id #=> str1.object_id, str2.object_id, or otherwise?
> ```
> 
> IMO: The return values of `sort_by` and `max_by` contains the original elements, not the return value of the block.  According to the analogy to them, I think that `t` should be `{ "a" => 2 }` and its key be `str1.object_id`.

Answer 1: I would say the first, but `tally` could also be effectively represented by `tally_by(&:itself)` as shown in an implementation below:

Answer 2: The transformed value, like `group_by`:

```
[1, 2, 3].group_by(&:even?)
=> {false=>[1, 3], true=>[2]}

[1, 2, 3].tally_by(&:even?)
=> {false => 2, true => 1}
```

The implementation is similar to this:

```
module Enumerable
  # Implementing via group_by
  def tally_by(&fn)
    group_by(&fn).to_h { |k, vs| [k, vs.size] }
  end

  # Implementing via reduction
  def tally_by2(&fn)
    each_with_object(Hash.new(0)) { |v, a| a[fn[v]] += 1 }
  end
end
```

...which would result in the first `object_id` I believe.

----------------------------------------
Feature #11076: Enumerable method count_by
https://bugs.ruby-lang.org/issues/11076#change-76559

* Author: haraldb (Harald B��ttiger)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
I very often use `Hash[array.group_by{|x|x}.map{|x,y|[x,y.size]}]`.

Would be nice with to have a method called `count_by`:

~~~ruby
array = ['aa', 'aA', 'bb', 'cc']
p array.count_by(&:downcase) #=> {'aa'=>2,'bb'=>1,'cc'=>1}
~~~



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