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

[Carrico, Paul]

Stephane pointed out on the issue : after calculation and prior to the Acos one, some values were "1.0000000000000002220446" leading to "acos = 2.107D-08i" ; in addition he suggested me to use something like "acos(max(-1,min(1,-1.0000000000000002220446)))" or "acos(max(-1,min(1,1.0000000000000002220446)))" ... very smart :)

Thanks

Paul





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De : users [mailto:users-bounces at lists.scilab.org] De la part de Stéphane Mottelet
Envoyé : mardi 29 janvier 2019 16:59
À : users at lists.scilab.org
Objet : Re: [Scilab-users] [EXTERNAL] Re: acos leads to complex values

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 8.4E-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<mailto: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







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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

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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

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