Feature #4553

Add Set#pick and Set#pop

Added by adgar (Michael Edgar) over 6 years ago. Updated almost 5 years ago.

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?

pick_pop.diff (1.06 KB) pick_pop.diff implementation and tests for Set#pick and Set#pop adgar (Michael Edgar), 04/05/2011 08:10 AM


#1 [ruby-core:35645] Updated by marcandre (Marc-Andre Lafortune) over 6 years ago

  • Priority changed from Normal to 3


There is no difference between your Set#pick and the existing Set#first. As such, there is no need for pick.

One can get the effect of Set#pop by calling the existing Set#delete with the result of Set#first (with the same O(1) efficiency).

I find problematic that a method called pop would take the first element; it could be renamed to shift, say, or take the last element instead. More importantly, though, I am not convinced that this functionality is that frequently used.

#2 [ruby-core:35714] Updated by adgar (Michael Edgar) over 6 years ago

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

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.


#3 [ruby-core:35724] Updated by zimbatm (zimba tm) over 6 years ago

#pop is often associated to stack operations, which implies an order. Unless a better name is found, isn't set.delete(set.take) enough ?

#take can be an alias of #first but I'm not sure if Enumerable should be included in Set in the first place. In my POV, Enumerable is for elements that have a particular order.


#4 [ruby-core:37543] Updated by adgar (Michael Edgar) about 6 years ago

Any update on this? It seems like a reasonable hole to fill in Ruby 1.9.3.

#5 [ruby-core:43445] Updated by shyouhei (Shyouhei Urabe) over 5 years ago

  • Description updated (diff)
  • Status changed from Open to Assigned
  • Assignee set to knu (Akinori MUSHA)

#6 [ruby-core:43447] Updated by trans (Thomas Sawyer) over 5 years ago

I would expect Array#pick to work too. Which then leads one to think Array might support the equivalent of Set#pop as well. But since Array already has #pop it's not really a good idea to reuse same term, although in the case of Set, #pop might be an alias for such method.

So what might such methods be called then? Well, if we look at hand-dandy facets/random:

Looks like it uses #at_rand/#at_rand! and #pick/pick!, where #pick has the additional option of returning a random subset if a size is given as argument.

#7 [ruby-core:49707] Updated by mame (Yusuke Endoh) almost 5 years ago

  • Target version set to next minor

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