Unexpected behavior of KeyTake

Question

list = {{a, b, c, e}, {1, 2, b, h}, {"a", "h", 3, 5}};
asso = GroupBy[list, {#[[1]], #[[2]]} &]

<|{a, b} -> {{a, b, c, e}}, 
  {1, 2} -> {{1, 2, b, h}}, 
  {"a","h"} -> {{"a", "h", 3, 5}}|>

There is a problem in this code, since "a" and "h" might be two keys. However, KeyExistsQ is not Listable, but KeyTake is.

KeyExistsQ[asso, {"a", "h"}]

True

KeyTake[asso, {"a", "h"}]

<||>

So I wrap Key around {"a", "h"} to eliminate ambiguity.

KeyTake[asso, Key[{"a", "h"}]]

<||>

Contrast the above with

KeyDrop[asso, Key[{"a", "h"}]]

<|{a, b} -> {{a, b, c, e}}, {1, 2} -> {{1, 2, b, h}}|>

Have I found a bug? Tested on V10.4

I would have expected Key to work. I think you should report it to WRI. — Edmund, Apr 11 '16 at 09:50
@ciao glad to see that, where do you see such examples in Help Page？ or You just tried it out? — HyperGroups, Apr 12 '16 at 12:51
@HyperGroups - it is how it's documented, not sure what you're asking: {x,y,...} is an argument with multiple keys, {{x,y,...}} is an argument with a single, compound key. — ciao, Apr 12 '16 at 22:28
I think there is a real issue here and the question should not be closed. — m_goldberg, Jul 02 '16 at 23:58
You have found a bug in KeyDrop. See update to my answer below. — m_goldberg, Jul 08 '16 at 10:14

score 5 · Answer 1 · edited Jun 16 '20 at 09:23

The behavioral discrepancy can be shown with a very simple example.

assoc = <|1 -> 1, 2 -> 2|>;
{KeyTake[assoc, 1], KeyTake[assoc, Key[1]], KeyTake[assoc, Key[1][assoc]]}

{<|1 -> 1|>, <||>, <|1 -> 1|>}

{KeyDrop[assoc, 1], KeyDrop[assoc, Key[1]], KeyDrop[assoc, Key[1][assoc]]}

{<|2 -> 2|>, <|2 -> 2|>, <|2 -> 2|>}

Surely the behavior of KeyTake[assoc, Key[1]] and KeyDrop[assoc, Key[1]] should be more consistent. Personally, I feel it is the behavior of KeyDrop that is wrong.

Update

I reported this issue to Wolfram tech support. Yesterday, 2016-Jul-7, I received a reply from which I quote the relevant section.

It does appear that KeyDrop is not behaving consistently. I have forwarded an incident report to our developers with the information you provided. If I receive any progress on this issue, you will be posted regarding the same.

This confirms my suspicion that it is KeyDrop and not KeyTake that is behaving badly.

In version 11, the line of code with KeyDrop now returns {<|2 -> 2|>, <|1 -> 1, 2 -> 2|>, <|2 -> 2|>} — , Aug 23 '16 at 23:00
@Xavier. I don't have V11, but if that's the case, KeyDrop has been fixed in V11. I suggest you update the question to mention the fact. — m_goldberg, Aug 24 '16 at 05:12

Unexpected behavior of KeyTake

1 Answers1

Update

Linked