[scilab-Users] min/max with NaN values

[Michaël Baudin]

Ahah...

In Scilab:

-->min([1 2 3 %nan])
  ans  =
     1.
-->min(1,2,3,%nan)
  ans  =
     Nan

This is fun...

In Matlab:

 >> min([1 2 3 nan])
ans =
      1
 >> min(1,2,3,nan)
??? Error using ==> min
Too many input arguments.

So Matlab does not support the y=min(x1,x2,x3) calling sequence, but 
Scilab do. On the other hand, Scilab results are weird with nans, at least.

Regards,

Michaël

Le 24/11/2010 11:47, Mathieu Dubois a écrit :
> On 11/24/2010 11:40 AM, Michaël Baudin wrote:
>> Hi,
>>
>> The expected result is actually the matrix of Nans. If you had found 
>> another result, that would have been a bug. Even more, we have 
>> similar automatic checks that this is so - see the following unit test :
>>
>> http://gitweb.scilab.org/?p=scilab.git;a=blob;f=scilab/modules/elementary_functions/tests/unit_tests/IEEEcompatibility.tst;h=778fc00d4833f54fce6324ac917bac725d6f9e19;hb=HEAD 
>>
>>
>> Notice that this test is mainly for +,-,*,/,sqrt, the operations 
>> which are specified by the IEEE754 standard. With respect to Nan, 
>> these tests says that "Nan on input, then Nan on output". In the 
>> IEEE754 standard, the authors use the term "Nan propagation". More 
>> informations on this topic are given by "Handbook of Floating Point 
>> Arithmetic", by Muller et al.. See the sections "2.3 - Exceptions" 
>> and "3.1.5 Exceptions specified by IEEE754-1985".
> That sounds like a sane convention but then, just out of curiosity, 
> how do you explain that min([1 2 3 %nan])  returns 1?
>>
>> If you want to perform the computation without the nans, you may 
>> combine the "find" and "isnan" functions. For example, the statement:
>>
>> k = find(isnan(a))
>>
>> you get the indices where a is a nan. Now, the statement:
>>
>> a(isnan(a))=0
>>
>> sets to zero the entries where Nan is present.
>>
>> But be warned: Nans are *designed* to be propagated. That is, if your 
>> input data contains Nans, that means that some previous computations 
>> has already failed. The Nans should not be, in general, just ignored 
>> and thrown away.
>>
>> Best regards,
>>
>> Michaël Baudin
>>
>> Le 24/11/2010 10:11, Frederic Jourdin a écrit :
>>> Hi all !
>>> if I run this:
>>>
>>> a= [    1, %nan;    4,    4];
>>> b= [ %nan,    3;    3,    6];
>>> c= [    2,    2; %nan, %nan];
>>> min( a, b, c)
>>>
>>> Scilab returns the following matrix:
>>> NaN  NaN
>>> NaN  NaN
>>>
>>> while the expected matrix should be:
>>> 1  2
>>> 3  4
>>>
>>> Why?
>>>
>>> thanks
>>> Fred
>>>
>>> Scilab 5.3.0.beta3 on Linux
>>>
>>
>>
>


-- 
Michaël Baudin
Ingénieur de développement
michael.baudin at scilab.org
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