MemberQ not working?

Asked Aug 06 '17 at 12:51

Active Aug 06 '17 at 17:43

Viewed 135 times

I'm confused by the following trivial code:

{Range[0, 1, .1][[8]]} === {.7}

gives True

MemberQ[{.7}, .7]

gives True, but

MemberQ[{Range[0, 1, .1][[8]]}, .7]

gives False. Huh? What am I missing?

edited Aug 06 '17 at 17:43

asked Aug 06 '17 at 12:51

bjorne

3

{Range[0, 1, .1][[8]], .7} // InputForm ought to be enlightening. Moral lesson is that you don't do such pattern matches on inexact numbers. – J. M.'s missing motivation Aug 06 '17 at 12:53
@J.M. thanks, I thought it might be a floating point thing, but then I was surprised by the first equality, which I still don't understand – bjorne Aug 06 '17 at 12:55
1

That would be due to Internal`$SameQTolerance (or Internal`$EqualTolerance if you had used ==). See e.g. this. – J. M.'s missing motivation Aug 06 '17 at 12:59
@J.M. Great, thanks! – bjorne Aug 06 '17 at 13:00
1

Recommended reading: (5149), (5580), (21711), (66386), (86815). Please take a look at these and tell me if you feel that your question is adequately addressed in these. If so we can mark it as such. – Mr.Wizard Aug 06 '17 at 17:07

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