[scilab-Users] min/max with NaN values

[Michaël Baudin]

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".

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