I'm perfectly aware of that, and if you read my comment you'll see that I acknowledge that fact. However, I also said: > they're obviously keeping track of th

I'm perfectly aware of that, and if you read my comment you'll see that I acknowledge that fact. However, I also said: > they're obviously keeping track of the keys internally in an array to support the ordered hashes. Which means that they can return them in an O(1) operation, but they choose not to for some reason. Edit: I'm wrong- I just remembered why they can't return them, and it is because they're not storing the keys in an array. D'oh. They're using a doubly-linked list. So to return a list of keys you'd need to walk the linked list- an O(n) operation. See: http://www.igvita.com/2009/02/04/ruby-19-internals-ordered-hash/ http://www.igvita.com/2009/02/04/ruby-19-internals-ordered-h... for more info.