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<div class="moz-cite-prefix">Le 13/09/2019 à 12:45, Samuel Gougeon a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:b6cb630c-990c-99b1-5f4a-cc006a11328a@free.fr">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<div class="moz-cite-prefix">Hello,</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">To me, <a
href="https://antispam.utc.fr/proxy/2/c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy/bugzilla.scilab.org/show_bug.cgi?id=15781#c2"
moz-do-not-send="true">as already claimed there</a>, it's
clear that, for a complex-encoded number, not displaying its
null imaginary part is a bug, and the proposed patch is clearly
welcome as well.<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Another regression very close to this
one, but with real numbers display, would deserve the same care
:</div>
<div class="moz-cite-prefix">Scilab 5:</div>
<div class="moz-cite-prefix"><tt>-->[1e30 1e-30]</tt><tt><br>
</tt><tt> ans = </tt><tt><br>
</tt><tt> 1.000D+30 1.000D-30 </tt><tt><br>
</tt><br>
Scilab 6:<br>
</div>
<div class="moz-cite-prefix"><tt>--> [1e30 1e-30]</tt><tt><br>
</tt><tt> ans =</tt><tt><br>
</tt><tt> 1.000D+30 0.</tt><tt><br>
</tt></div>
</blockquote>
<tt>The patch also fixes this.</tt><br>
<blockquote type="cite"
cite="mid:b6cb630c-990c-99b1-5f4a-cc006a11328a@free.fr">
<div class="moz-cite-prefix"><tt> </tt></div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">So, very small numbers are reduced to
strict 0...<br>
</div>
<div class="moz-cite-prefix">This is a bad implementation of the
variable format mode. The Scilab 5 one was correct, at least on
this point.<br>
</div>
<p> Best regards<br>
Samuel<br>
</p>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Le 12/09/2019 à 10:26, Stéphane
Mottelet a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:3f0654df-5d03-ce02-f3e5-c1e537483d05@utc.fr">Hello
all, <br>
<br>
The subject has been already discussed a lot but I would like it
to be discussed again because I now have a real rationale to
promote a change in the way complex numbers with small imaginary
part are displayed. <br>
<br>
I don't know if some of you were aware of the clever technique
of complex-step derivative approximation, but until yesterday I
was not (see e.g. <a class="moz-txt-link-freetext"
href="https://antispam.utc.fr/proxy/1/c3RlcGhhbmUubW90dGVsZXRAdXRjLmZy/mdolab.engin.umich.edu/sites/default/files/Martins2003CSD.pdf"
moz-do-not-send="true">http://mdolab.engin.umich.edu/sites/default/files/Martins2003CSD.pdf</a>).
Roughly speaking, using the extension of a real function
x->f(x) to the complex plane allows to compute an
approximation of the derivative f'(x0) at a real x0 without
using a substraction, like in the central difference formula
(f(x0+h)-f(x0-h))/2/h which is subject to substractive
cancelation when h is small. In Scilab most operators and
elementary functions are already complex-aware so this is easy
to illustrate the technique. For example let us approximate the
derivative of x->cos(x) at x=%pi/4, first with the central
difference formula, then with the complex step technique: <br>
<br>
--> format("e",24) <br>
<br>
--> h=%eps/128, x0=%pi/4 <br>
h = <br>
<br>
1.73472347597680709D-18 <br>
<br>
x0 = <br>
<br>
7.85398163397448279D-01 <br>
<br>
<br>
--> (cos(x0+h)-cos(x0-h))/2/h <br>
ans = <br>
<br>
0.00000000000000000D+00 <br>
<br>
<br>
--> imag(cos(x0+%i*h))/h <br>
ans = <br>
<br>
-7.07106781186547462D-01 <br>
<br>
<br>
--> -sin(x0) <br>
ans = <br>
<br>
-7.07106781186547462D-01 <br>
<br>
You can see the pathological approximation with central
difference formula and the perfect (up to relative machine
precision) approximation of complex-step formula. <br>
<br>
However, the following is a pity: <br>
<br>
<br>
--> cos(x0+%i*h) <br>
ans = <br>
<br>
7.07106781186547573D-01 <br>
<br>
We cannot see the imaginary part although seeing the latter is
fundamental in the complex-step technique. We have to force the
display like this, and frankly I don't like having to do that
with my students: <br>
<br>
--> imag(cos(x0+%i*h)) <br>
ans = <br>
<br>
-1.22663473334669916D-18 <br>
<br>
I hope that you will find that this example is a good rationale
to change the default display of Scilab. To feed the discussion,
here is how Matlab displays things, without having to change the
default settings: <br>
<br>
<br>
>> h=eps/128, x0=pi/4 <br>
h = <br>
1.7347e-18 <br>
x0 = <br>
0.7854 <br>
<br>
>> (cos(x0+h)-cos(x0-h))/2/h <br>
ans = <br>
0 <br>
<br>
>> cos(x0+i*h) <br>
ans = <br>
0.7071 - 0.0000i <br>
<br>
>> imag(cos(x0+i*h))/h <br>
ans = <br>
-0.7071 <br>
<br>
>> -sin(x0) <br>
ans = <br>
-0.7071 <br>
<br>
<br>
</blockquote>
<p><br>
</p>
<br>
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</blockquote>
<pre class="moz-signature" cols="72">--
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
<a class="moz-txt-link-freetext" href="http://www.utc.fr/~mottelet">http://www.utc.fr/~mottelet</a>
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