Le vendredi 31 juillet 2020 20:46:08 UTC+2, Oscar a écrit :
>
> On Fri, 31 Jul 2020 at 17:26, Mikhael Myara
> <mikhae...@umontpellier.fr <javascript:>> wrote:
> >
> > Thanks for your answer.
> >
> > I don’t do it « for sport » ;-) My example is a reduced example coming
> from a practical situation I encountered.
>
> I understand that you have reduced this but it is a lot better if the
> reduction is self-contained so that others can literally copy-paste
> the code to test what is happening with an expression that
> demonstrates the issue. I could probably tell you exactly what the
> problem is in a given example if you provided minimal code for that
> example.
>
> That's what I did in my first post + screen copies :
> import sympy as sp
>
> H=-(-CB*Ve*s*(-CA*s - 1/RA)**2/(RB*(CB*s + 1/RB)*(CA*s - (-CA*s - 1/RA)**2
> /(CA*s - CB**2*s**2/(CB*s + 1/RB) + CB*s + 1/RA) + 2/RA)*(CA*s - CB**2*s**
> 2/(CB*s + 1/RB) + CB*s + 1/RA)) - CB*Ve*s/(RB*(CB*s + 1/RB)))/(Ve*(CA*s -
> CB**2*s**2/(CB*s + 1/RB) + CB*s + 1/RA))
>
> H1=H.subs({RB:1e3,CA:1e-6})
> display(H1)
> H1.simplify()
> display(H1)
>
> H2 = H.simplify()
> display(H2)
> H3=H2.subs({RB:1e3,CA:1e-6})
> H3=H3.simplify()
> display(H3)
>
> > Here it is :
> > I developed a small software that solves the voltages and currents of an
> electronic circuit described by means of a standard format (« netlist »).
> This file is parsed, equations are solved. In this netlist file, the values
> of the components are given.
>
> How exactly are they given in the file (e.g. to how many digits)?
>
> If the file has something like 0.12 then you can read that in directly
> as Rational('0.12') rather than converting the string to a float. Then
> you will have an object that represents the value from the file
> exactly with no rounding error. Alternatively you can use nsimplify to
> convert the floats to an approximate rational representation (direct
> string to Rational is better though).
>
Values can change a lot, from 1é-10 to 1e10 typically. In most cases 2 to 3
significant numbers, but can be much more.
>
> > I imagined that I could mix symbolic and numerical values in the
> description of the circuit (for example we want to study de variations of a
> single component, not of all), and it seems to be a bad idea because mixed
> expressions seem to be really difficult to handle for Sympy.
>
> Generally speaking simplification will work better with numbers rather
> than symbols. However this is only true if you give numbers in exact
> form. When you pass a float to sympy it will be treated as an
> inherently imprecise object. As a result many simplifications will be
> refused and also any arithmetic will be computed in floating point.
> The kind of simplification needed here is most likely factorisation
> and cancellation of polynomials which is poorly conditioned in
> floating point.
>
ok.
>
> > So I will have to make 100% symbolic treatment and then only replace the
> values of the components. It was to explore the possibilities for my ?
> students : this kind of question will help me giving to them good
> orientations during classroom work.
>
> If the class work involves using sympy then good orientation would be
> to avoid the use of floats in symbolic computation.
>
Indeed.
>
> --
> Oscar
>
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