DeleteDuplicates not deleting duplicates

Question

Bug introduced in 9.0 and fixed in 10.0.0

It is not surprising that DeleteDuplicates[{5,5.}] returns {5,5.} because DeleteDuplicates uses SameQ by default, and SameQ[5,5.] is False.

However, Equal[5,5.] is True, but DeleteDuplicates[{5,5.},Equal] still returns {5,5.}.

It is interesting to note that Union works properly, in that Union[{5,5.},SameTest->Equal] returns {5}, as expected.

Is this a bug, or am I missing something? I'm using version 9.

Update:

I realize that using the function #1==#2& will get it working, thanks for pointing this out.

However, my question really is "Is this a bug, or am I missing something?".

To illustrate this further, consider the following:

f[x__]:=Equal[x]
DeleteDuplicates[{5,5.},f]
(* {5} *)

In what capacity are f and Equal different? Or what is leading DeleteDuplicates to treat them differently? They both take an arbitrary number of inputs, which is what I originally thought was the problem.

@DanielLichtblau: wouldn't it make sense to add that as an answer: it looks like a bug and the reason is presumably (?) that the Equal case is treated as a special case for performance reaons. I think it would be most convincing if such an answer would come from you :-) — Albert Retey, Jan 23 '13 at 17:38
But @Albert Retey, are you saying that you won't believe me in a comment, but you will in an answer? — Daniel Lichtblau, Jan 23 '13 at 17:40
... it might also be of interest that this behavior is new to version 9, version 7 and 8 give the "expected" result... — Albert Retey, Jan 23 '13 at 17:41
@DanielLichtblau: no of course not, I think it would give the OP the chance to accept that answer and mark the question as "solved". Comments, AFAIK, are to be considered only as second class citizens here, probably not even here to stay... — Albert Retey, Jan 23 '13 at 17:43
@DanielLichtblau: sorry, I didn't get that last one. What I wanted to say is that it makes a difference whether I call it a bug or you, not so much because you live in Champaign but because of your employer... — Albert Retey, Jan 23 '13 at 18:03
Meta question: If this question ever gets a duplicate, will we be able to delete it? — Daniel Lichtblau, Jan 23 '13 at 18:27
@DanielLichtblau You're really inspired today. Go write a paper. — Dr. belisarius, Jan 23 '13 at 20:26
@belisarius I'm afraid it's one of those days where the insanity is driving me crazy. — Daniel Lichtblau, Jan 23 '13 at 20:56

score 19 · Accepted Answer · answered Jan 23 '13 at 17:42

19

As @Albert Retey remarked in a comment, a second argument of (just) Equal leads to a special case handler that, to me, seems overly ambitious. I reported it as a bug.

answered Jan 23 '13 at 17:42

Daniel Lichtblau

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+1 but... raking in rep for confirming bugs may be the wrong incentive, WRI-wise? :-P – Yves Klett Jan 24 '13 at 09:58
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My strategy has been to fix bugs/implement features, remain silent for weeks/months, and then miss the rep uptick when the new version ships because sleep started to look better than beating out intrepid explorers for reputation. Maybe I should rethink that... :) – John Fultz Jan 24 '13 at 11:37
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@Yves Klett I'm as confused as you. Meanwhile other responses take real work and get maybe 2 upvotes. – Daniel Lichtblau Jan 24 '13 at 15:13

score 9 · Answer 2 · answered Jan 23 '13 at 16:54

9

You need to use :

DeleteDuplicates[{5, 5.}, Equal[#1, #2] &]
(* {5} *)

answered Jan 23 '13 at 16:54

b.gates.you.know.what

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Something like Unevaluated@Identity@Equal will work a bit faster I think – Rojo Jan 23 '13 at 22:03

DeleteDuplicates not deleting duplicates

2 Answers2

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