[ruby-core:91252] [Ruby trunk Feature#11076] Enumerable method count_by

[mame@ruby-lang.org]

Issue #11076 has been updated by mame (Yusuke Endoh).


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`.

----------------------------------------
Feature #11076: Enumerable method count_by
https://bugs.ruby-lang.org/issues/11076#change-76498

* Author: haraldb (Harald Böttiger)
* 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}
~~~



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