Using unique.m function on NaNs
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Can someone please clarify if the result of unique([1 nan; 1 nan],'rows') should be
a) [1 nan; 1 nan] OR b) [1 nan]
Basically, in my version of matlab (2008b), I get a) as the answer and I think it's a bug as the answer should be b). Can someone please confirm if this is a feature or an issue. My mathworks support is limited because of $$ :(
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Sean de Wolski
2011-5-17
No. It's supposed to be a.
By definition nans are not equal to themselves so they're not equal in rows either.
isequal(nan,nan)
or
nan==nan
更多回答（3 个）
Jan
2011-5-17
3 个投票
NaNs compared to anything replies FALSE, even NaN==NaN. This is defined by the IEEE754 conventions.
Therefore [1, NaN] compared to [1, NaN] is FALSE also. In consequence UNIQUE([[1, NaN; 1, NaN], 'rows') must reply two rows.
Matt Fig
2011-5-17
In addition to what others have said, note this horribly named function:
A = [1 nan; 1 nan];
isequalwithequalnans(A(1,:),A(2,:))
5 个评论
Matt Fig
2011-5-17
Where are:
ismemberwithequalnans
intersectwithequalnans
setdiffwithequalnans
etc.
Sean de Wolski
2011-5-17
Some are on the FEX, though I think the authors took more care in naming them :-)
Andrew Newell
2011-5-17
What's so bad about the name? It's long, but it says what it does.
Sean de Wolski
2011-5-17
nanisequal does too and sticks in line with: nanmean, nanmedian, nancumsum etc.
Matt Fig
2011-5-17
@Andrew, I think you would agree that there is more to a great function name that it being descriptive! How often do you name your functions, for instance:
performstatisticsonmydatathenplottheresults(A)
I just think it is a monstrosity (IMO) to have a 20 character function name, that's all. Why not, for example, use:
isequal(A,B,'nansequal') % Or similar
or a name like Sean de recommends?
Andy
2011-5-17
If you have some application for which you need to ignore NaNs rather than take them into account, you could replace them with some other value which is easily identifiable as non-data. For example:
A; % your data
A(isnan(A)) = Inf; % replace NaN with Inf
A = unique(A,'rows'); % unique rows, where [1,Inf] == [1,Inf]
A(isinf(A)) = NaN; % convert back to NaN
It's a little hackish. I know there are various toolbox functions, like nanmean and nanstd, which calculate other statistics of a dataset while ignoring NaNs. But there is no nanunique or nansort. Also, the work in unique is done by calling sortrows which calls a MEX file, so you can't just edit it to take NaNs into account. So the above code seems like the cleanest quick solution.
4 个评论
Sean de Wolski
2011-5-17
What if there are infs in there though? You don't want those incorrectly turned to nans or lost.
Matt Fig
2011-5-17
I think the point is for the user to substitute a value which is not in the array. Although I have used automatic replacement software before, I usually use something like:
A(isnan(A)) = eps/(pi*exp(1))
Rahul Chander
2011-5-17
Andy
2011-5-17
@Sean de, Matt Fig is correct. I intended him to replace Inf with any "value which is easily identifiable as non-data". I was just using Inf as an example, since, for my purposes, Inf never comes up data. But Rahul should choose whatever flag value is appropriate for the application.
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