[R] symbolic computing example with Ryacas

[Gabor Grothendieck]

Here are some more examples:

library(Ryacas)

x <- Sym("x")
yacas("x:=2")
Eval(x*x)
## [1] 4

# vignette has similar example
y <- Sym("y")
Eval(Subst(y*y, y, 3))
## [1] 9

# demo("Ryacas-Function") has similar example to this
f <- function(z) {}
body(f) <- yacas(expression(z*z))[[1]]
f(4)
## [1] 16



On Tue, Sep 19, 2017 at 2:08 PM, Vivek Sutradhara <viveksutra at gmail.com> wrote:
> 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]]
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posti
>>> ng-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.



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