Is Hash ordered based on when the entry was inserted?

[xqyww123]

Hi, everybody.

I just want to make sure whether Hash#keys or Hash#each returns entries in order based on when they were inserted. I found it's mentioned in Hash#key_index in document, and it looks like it's ordered viewing the code of key_index. But nothing mentioned in any other place about this, and the document of Set say Set based on Hash is unordered.

[xqyww123]

And more is it safe to insert new entry into Hash during the iteration of it, and will the new inserted entry be visited in the iteration?

[straight-shoota]

1. Key-value pairs in a Hash are sorted by insertion order.
2. A Set by definition has no order. In practice, the items will be sorted by insertion order because of the implementation. But it's not guaranteed. If we change the implementation, this might not longer be true. Sets should always be considered unordered.
3. It should be safe to add elements to any stdlib collection while enumerating it and newly added items should be enumerated as well.

We should use this issue to improve the documentation.

[asterite]

For 2, a Set has an order. In theory it doesn't have one, but in practice it does. Same with Hash: in theory a mapping of keys from values isn't ordered, but in practice it is. And for example Ruby guarantees this (because they guarantee this through the implementation, and they won't suddenly change the implementation to unordered). We should do the same (guarantee that Set will stay ordered).

[straight-shoota]

@asterite I can't find such a guarantee for Set in Ruby. The API docs explicitly state that it's an unordered collection and that the order of elements returned by #to_a is uncertain.

[asterite]

@straight-shoota Oh, I guess it's ordered then because it's built on top of Hash. Nevermind.

[ysbaddaden]

BTW: Hash wasn't ordered in Ruby 1.8.