Feature #4553


Add Set#pick and Set#pop

Added by adgar (Michael Edgar) almost 12 years ago. Updated over 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

Updated by marcandre (Marc-Andre Lafortune) almost 12 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.

Updated by adgar (Michael Edgar) almost 12 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.


Updated by zimbatm (zimba tm) almost 12 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.


Updated by adgar (Michael Edgar) over 11 years ago

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

Updated by shyouhei (Shyouhei Urabe) almost 11 years ago

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

Updated by trans (Thomas Sawyer) almost 11 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.

Updated by mame (Yusuke Endoh) about 10 years ago

  • Target version set to 2.6

Updated by knu (Akinori MUSHA) over 5 years ago

  • Status changed from Assigned to Rejected

As marcandre says, this pick method does exactly what first does, and I'm not sure if pop would be an important primitive method.

Array now has sample that picks a random element(s) from an array without deleting it which may or may not be the function you want, but I can't think of a way to randomly pick a key from a hash at O(1) cost, at least with a pure ruby implementation.


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