[Scilab-users] [EXTERNAL] Re: acos leads to complex values

[Federico Miyara]

I think it would be interesting to include an input argument check in 
some functions prone to give complex results in limiting cases (such as 
acos or asin) so that if inside the domain (to within 1 %eps) where the 
output argument should be real the imaginary part is canceled. Something 
like this for scalars:

function  y=acos1(x)
if   isreal(x)
     if  -1<=x  &  x<=1
         y  =  real(acos(x))
     else
         y  =  acos(x)
     end
end
endfunction


However, I must say that acos(1-%eps) gives on my Scilab 6.0.1 (Windows 
7, i7 processor) a result of

0.000000021073424255447,

being %eps

0.0000000000000002220446

I get no imaginary part. I guess there must be some dependency on the 
arithmetic engine.


Federico Miyara


On 29/01/2019 12:58, Stéphane Mottelet wrote:
> It is the same if x is slightly > 1:
>
> --> x=1+%eps
>  x  =
>
>    1.
>
>
> --> acos(x)
>  ans  =
>
>    2.107D-08i
>
> --> format(25); x
>  x  =
>
>    1.0000000000000002220446
>
> Le 29/01/2019 à 16:55, Carrico, Paul a écrit :
>>
>> When I scroll to the list, the lowest (positive) value is 8.4^E -08 
>> (works fine) and no %eps .
>>
>> How Can I check if %eps is in?
>>
>> *De :*users [mailto:users-bounces at lists.scilab.org] *De la part de* 
>> Stéphane Mottelet
>> *Envoyé :* mardi 29 janvier 2019 16:50
>> *À :* users at lists.scilab.org
>> *Objet :* [EXTERNAL] Re: [Scilab-users] acos leads to complex values
>>
>> Le 29/01/2019 à 16:45, Carrico, Paul a écrit :
>>
>>     Dear All
>>
>>     I spent some time in looking for a mistake in my code ; finally
>>     I’ve found that the ACOS of a real vector leads to some complex
>>     values (???)
>>
>>     acos(Scar_P(:,1)./CM_x_CN(:,1))
>>
>> Are your really sure, because we may have
>>
>> --> x=-1-%eps
>>  x  =
>>
>>   -1.
>>
>>
>> --> acos(x)
>>  ans  =
>>
>>    3.1415927 - 2.107D-08i
>>
>> S.
>>
>>     (the formula worked so far)
>>
>>     I checked that the input values are correct:
>>
>>     -Comprised between [-1; 1] using MIN and MAX
>>
>>     -Composed only of real values using ISREAL (all the vectors are
>>     correct)
>>
>>     Thus I do not understand why complex values appear ?
>>
>>     May it come from the vectorization ?
>>
>>     Paul
>>
>>
>>
>>     _______________________________________________
>>
>>     users mailing list
>>
>>     users at lists.scilab.org <mailto:users at lists.scilab.org>
>>
>>     https://antispam.utc.fr/proxy/1/c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy/lists.scilab.org/mailman/listinfo/users
>>     <https://antispam.utc.fr/proxy/2/c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy/urldefense.proofpoint.com/v2/url?u=https-3A__antispam.utc.fr_proxy_1_c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy_lists.scilab.org_mailman_listinfo_users&d=DwMG-g&c=0hKVUfnuoBozYN8UvxPA-w&r=4TCz--8bXfJhZZvIxJAemAJyz7Vfx78XvgYu3LN7eLo&m=rN8DsnGalih7CBHModSb9evOi3rmZRFFYcBFHTC71gU&s=RgKyyESZ7uSTxlU7V0nR42XTJybjC0Ar5fDcrouQThE&e=>
>>
>> -- 
>> 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 
>> <https://antispam.utc.fr/proxy/2/c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy/urldefense.proofpoint.com/v2/url?u=http-3A__www.utc.fr_-7Emottelet&d=DwMG-g&c=0hKVUfnuoBozYN8UvxPA-w&r=4TCz--8bXfJhZZvIxJAemAJyz7Vfx78XvgYu3LN7eLo&m=rN8DsnGalih7CBHModSb9evOi3rmZRFFYcBFHTC71gU&s=34gC0H3RYWxwcKp7fr4bR5XFfy1acxO72YI9AYJJFhA&e=>
>>
>> _______________________________________________
>> users mailing list
>> users at lists.scilab.org
>> https://antispam.utc.fr/proxy/1/c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy/lists.scilab.org/mailman/listinfo/users
>
>
> -- 
> 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
>
>
> _______________________________________________
> users mailing list
> users at lists.scilab.org
> http://lists.scilab.org/mailman/listinfo/users



---
El software de antivirus Avast ha analizado este correo electrónico en busca de virus.
https://www.avast.com/antivirus
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.scilab.org/pipermail/users/attachments/20190129/83e43d85/attachment.htm>