What is the reasoning behind the fact that min(0,NaN) is 0?
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I know that Mathworks pays a lot of attention to this stuff, so I am wondering why the expression
>> min(0,NaN)
is 0. Returning a NaN here seems more logical to me.
4 个评论
Bryan
2020-4-25
编辑:Bryan
2020-4-25
yes i noticed that very quickly (hence deleted that bit of the post). but you were in there very quick with helpful feedback, thanks for the explanation.
As a small bit of feedback, perhaps the min() documentation could mention in the very first line of the documentation that NaN values will be ignored by default, since this is unusal behaviour in the Matlab environment. The documentation here: https://se.mathworks.com/help/matlab/ref/min.html and in 'help min' states
M = min(X) is the smallest element in the vector X.
perhaps it could state
M = min(X) is the smallest non-NaN element in the vector X.
or alternatively
M = min(X) is the smallest element in the vector X, whereby NaN elements are ignored by default.
cheers
Stephen23
2020-4-25
编辑:Stephen23
2020-4-26
@Bryan: you should make that as an enhancement request.
Another option is to stop relying on inconsistent "default" behavior and always specify any flags, dimensions, etc. for any function that has these kind of options. Although it requires a little more typing, it has the following advantages:
- makes the intention clear
- avoids bugs, e.g. when a matrix ony has one row (and thus min returns a scalar, not a row vector)
- throws an error on versions that do not support that option, rather than silently continuing...
回答(5 个)
Walter Roberson
2012-5-21
If you initialize the result to inf, and then loop testing whether the current value is less than the result and replace the result if it is, then since NaN < any number is false, the result will never get replaced with NaN. You would have to add special code to return NaN in such a case.
0 个评论
Sean de Wolski
2012-5-21
2 个评论
Jan
2012-5-22
It depends on how you understand the MIN function. 0 < NaN replies FALSE, but NaN < 0 replies FALSE also. As long as it is well documented, both values are reasonable.
Daniel Shub
2012-5-22
Given the behavior of MIN, I find it odd that there is a NANMIN function.
2 个评论
Jonathan Sullivan
2012-6-21
That is really interesting. If you look inside nanmin, it has one line.:
[varargout{1:nargout}] = min(varargin{:});
per isakson
2012-6-21
MIN and MAX ignores NaN. MEAN and SUM does not. I guess NANMIN (in stat toolbox) is for people like me who cannot remember all the details when we cannot see the underlying logic.
M Sohrabinia
2012-6-21
NaN is considered undefined, so undefined is ignored by most functions (0/0 will be resulted in NaN which is basically undefined but any number divided by 0, say 4/0, will result in inf). However, the question is why Matlab has decided to treat NaNs in a certain way in some functions, e.g., sort function will always arrange NaNs at the top end (A to Z mode). I guess Matlab has just decided to adopt some rules to handle exceptions.
0 个评论
Mark vanRossum
2021-6-3
I encountered this when working on arrays.
x=[1 NaN 10];
y=[5 5 5];
m=min(x,y) and m=nanmin(x,y) return [1,5,5]
In V2020, min(x,y,'includenan') returns [1, NaN,5]
Here is an ugly workaround to get the desired behaviour in older versions.
q=isnan(x)
m=min(x,y)
m(q)=NaN
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