[Scilab-users] Certain operations are not expressed as floating point

[Stéphane Mottelet]

Hello,

This problem is fixed in Scilab 6.1

S.

Le 06/02/2020 à 08:30, Federico Miyara a écrit :
>
> Dear all,
>
> Trying to implement sine integral from the series, my first attempt
>
>         y = 0
>         N = 30;
>         for n=0:N
>             y = y + (-1)^n * x.^(2*n+1)./(2*n+1)./factorial(2*n+1);
>         end
>
> works fine for x<20. Now I want to vectorize it so the plan is to 
> create a polynomial and then apply horner. The coefficients are
>
> coe = (-1).^(0:N)./(1:2:2*N+1)./factorial(1:2:2*N+1)
>
> I get
>
> coe  =
>
>          column 1 to 6
>
>    1.  -0.055555555556   0.001666666667  -0.000028344671 
> 0.000000306192  -0.000000002277
>
>          column 7 to 19
>
>    0.000000000012  -5.09810915D-14   1.65379838D-16   0.   0. 0.   
> 0.   0.   0.   0.   0.   0.   0.
>
>          column 20 to 31
>
>    0.   0.   0.   0.   0.   0.   0.   0.   0.   0.   0.   0.
>
> From the 10-th coefficient on, the value seems to be 0. However, 
> coe(10) yields  9.32044813D-22
>
> Actually this is a documented behavior (it is described in hhe format 
> help page). But I wonder if it is advisable, since it may be confusing.
>
> Indeed, it took me some time to discover that the coefficients were 
> stored as floating point. At the beginning I "believed" what I saw and 
> started to think which would be the best order of the operations to 
> avoid too small intermediate values.
>
> Is there a reason for that behavior that outweighs the inconvenience 
> I've mentioned?
>
> Regards,
>
> Federico Miyra
>
>
>
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-- 
Stéphane Mottelet
Ingénieur de recherche
EA 4297 Transformations Intégrées de la Matière Renouvelable
Département Génie des Procédés Industriels
Sorbonne Universités - Université de Technologie de Compiègne
CS 60319, 60203 Compiègne cedex
Tel : +33(0)344234688
http://www.utc.fr/~mottelet

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