From: redmine@...
Date: 2011-04-12T09:05:37+09:00
Subject: [ruby-core:35714] [Ruby 1.9 - Feature #4553] Add Set#pick and Set#pop


Issue #4553 has been updated by Michael Edgar.


The ruby-core e-mail I sent appeared to not attach to the issue on Redmine. I apologize for the noise.

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The existence of `Set#first` is an artifact of Set including Enumerable and the very idea of "first" on a Set is an odd notion. A set by itself does not have ordering. I have implemented it to pick the first element because it is simplest, but someone calling "pick" on a set does not care which element it is. If one would prefer implementing it as an alias to first, I'd be fine with that.

On a personal note to emphasize why I find this issue compelling: one reason I enjoy using Ruby because often my code reads just as I would describe it to a colleague - reading "element = set.first" is not something many would agree is a sensible way to describe the act of picking an arbitrary element from a mathematical set.

Yes, one could implement #pop with #delete and #first, which again relies on a notion of ordering in a set. One can implement #proper_subset with #subset and a comparison, as can one implement #proper_superset with #superset and a comparison. Yet they are in the Set class because they are fundamental operations. #add? and #delete? can also be implemented with #add and #include?/#delete and #include? respectively, yet they are included in the Set class because they are useful and common operations on a set.

If `pop` is too close the Array method of the same name, I understand the confusion, but I don't get why picking first/last makes a difference. Anyone asking to remove an arbitrary element from a Set surely doesn't care where it lies in an underlying hash table. I am surprised one would argue that the Set class rarely needs the notion of selecting an element from it. Rarely am I presented with an algorithm using sets that does not require simply getting an element or picking one out of a set.

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Feature #4553: Add Set#pick and Set#pop
http://redmine.ruby-lang.org/issues/4553

Author: Michael Edgar
Status: Open
Priority: Low
Assignee: 
Category: 
Target version: 


A very common operation on sets is to take an arbitrary element from them at O(1) cost. I know typically
it's suggested that library additions go out as a gem to see community review. However, to me it seems
to be a glaring omission to lack such an operation on a built-in, fundamental data structure, so I've
gone straight to the bug tracker.

I wasn't too sure which method names to use as I've often heard "take" in lieu of "pop," so I just used the
names Wikipedia uses. Consider them flexible. "Pick" selects an arbitrary element, and "pop" selects and
deletes it.

I've provided a very simple patch that implements the necessary behavior. Thoughts?


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