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

Tue, 29 Jan 2019 12:37:09 -0800 


This is an improved version of the acos function with input argument check:

function  y=acos1(x)
    // Check if components are real
    is_real  =  imag(x)==0;
    // Leave real-only components
    x1  =  is_real.*x;
    // Check if components are real and belong to [-1, 1]
    in_domain  =  is_real  &  (-1<=x1  &  x1<=1);
    // Imaginary part of acos(x) is accepted only if
    // x does not belong to [-1, 1]
    y  =  in_domain.*real(acos(x))  +  (~in_domain).*acos(x);
endfunction
This works on vectors and arrays as well. Unfortunately i couldn't find a component-wise isreal() function, otherwise it would have been simpler! 

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-boun...@lists.scilab.org] *De la part de* Stéphane Mottelet 
*Envoyé :* mardi 29 janvier 2019 16:50
*À :* users@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
http://www.utc.fr/~mottelet


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