[scilab-Users] min/max with NaN values

[Michaël Baudin]

Noticed the small line at the end of the Matlab/min: "The min function 
ignores NaNs.". This is not written in the help page Scilab/min, but I 
guess that this is also true.

Now, Matlab is more consistent :

 >> min([1 nan])
ans =
      1
 >> min(1,nan)
ans =
      1

while Scilab is weird :

Scilab:

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

I think that you have found a 2 more bugs (that is, added with the bug 
#7608), i.e. :
1. nans are ignored by Scilab inside a matrix, but not across the input 
arguments,
2. this is not explicitely written in the help page.
I guess that the particular processing of Nans in the loop over the 
arguments has just been forgotten.

Would you mind to submit these bug report please ?

Best regards,

Michaël Baudin

Le 24/11/2010 11:54, Michaël Baudin a écrit :
> 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
-------------------------
Consortium Scilab - Digiteo
Domaine de Voluceau - Rocquencourt
B.P. 105 - 78153 Le Chesnay Cedex
Tel. : 01 39 63 56 87 - Fax : 01 39 63 55 94


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.scilab.org/pipermail/users/attachments/20101124/54fc99f3/attachment.htm>