[R] symbolic computing example with Ryacas

Vivek Sutradhara
2017-09-19T18:08:18Z

Thanks for the response. Yes, I did study the vignette but did not
understand it fully. Anyway, I have tried once again now. I am happy to say
that I have got what I wanted.

library(Ryacas)
x <- Sym("x");U <- Sym("U");x0 <-
Sym("x0");C <- Sym("C")
my_func <- function(x,U,x0,C) {
  return (U/(1+exp(-(x-x0)/C)))}
FirstDeriv <- deriv(my_func(x,U,x0,C), x)
PrettyForm(FirstDeriv)
#slope <- yacas("Subst(x,x0),deriv(my_func(x,U,x0,C), x)")
slope <- Subst(FirstDeriv,x,x0)
#PrettyForm(slope) - gives errors
PrettyForm(Simplify(slope))

I was confused by the references to the yacas command.  Now, I have chosen
to omit it. Then I get what I want.
Thanks,
Vivek

2017-09-19 16:04 GMT+02:00 Bert Gunter <bgunter.4567 at gmail.com>:

> Have you studied the "Introduction to Ryacas" vignette that come
with the
> package?
>
> Cheers,
> Bert
>
>
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along
and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip
)
>
> On Tue, Sep 19, 2017 at 2:37 AM, Vivek Sutradhara <viveksutra at
gmail.com>
> wrote:
>
>> Hi all,
>> I am trying to implement the following matlab code with Ryacas :
>>
>> syms U x x0 C
>>
>> d1=diff(U/(1+exp(-(x-x0)/C)),x);
>>
>> pretty(d1)
>>
>> d2=diff(U/(1+exp(-(x-x0)/C)),x,2);
>>
>> pretty(d2)
>>
>> solx2 = solve(d2 == 0, x, 'Real', true)
>>
>> pretty(solx2)
>>
>> slope2=subs(d1,solx2)
>>
>>
>> I have tried the following :
>>
>> library(Ryacas)
>>
>> x <- Sym("x");U <- Sym("U");x0 <-
Sym("x0");C <- Sym("C")
>>
>> my_func <- function(x,U,x0,C) {
>>
>>   return (U/(1+exp(-(x-x0)/C)))}
>>
>> FirstDeriv <- deriv(my_func(x,U,x0,C), x)
>>
>> PrettyForm(FirstDeriv)
>>
>> slope <- yacas("Subst(x,x0),deriv(my_func(x,U,x0,C), x)")
>>
>> PrettyForm(slope)
>>
>>
>> I don't understand how I should use the Subst command. I want the
slope of
>> the first derivative at x=x0. How do I implement that?
>>
>> I would appreciate any help that I can get.
>>
>> Thanks,
>>
>> Vivek
>>
>>         [[alternative HTML version deleted]]
>>
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>>
>
>