[R] equalize function with zero and convert it

[Gabor Grothendieck]

On Thu, Aug 26, 2010 at 9:45 AM, Immanuel Seeger <jimmy-seeger at web.de> wrote:
> Thanks, but my problem is that I have my formulas in functions.
> How can I do that for functions or e.g. for the body(Df) below?

Try this:

> library(Ryacas)
> Solve(deparse(body(f)), "x")
[1] "Starting Yacas!"
expression(list(x == -((10 - (100 - y) + root((100 - y - 10)^2,
    2))/2), x == -((10 - (100 - y) - root((100 - y - 10)^2, 2))/2)))
>
> out <- yacas(Solve(deparse(body(f)), "x"))
> out$text[[1]][[2]]
x == -((10 - (100 - y) + root((100 - y - 10)^2, 2))/2)


> And: How can I refer to the first solution of expression?
> Thanks a lot!
>
> -----Ursprüngliche Nachricht-----
> Von: Gabor Grothendieck [mailto:ggrothendieck at gmail.com]
> Gesendet: Donnerstag, 26. August 2010 14:56
> An: Immanuel Seeger
> Cc: r-help at r-project.org
> Betreff: Re: [R] equalize function with zero and convert it
>
> On Thu, Aug 26, 2010 at 6:57 AM, Immanuel Seeger <jimmy-seeger at web.de>
> wrote:
>> Dear all,
>> I want to equalize a symbolic derivative (of a function with two
>> variables) with zero and convert it to a variable, e.g. x.
>>
>> I'm computing the derivative by: (found it in the archive)
>> library(Ryacas)
>> f <- function(x,y) (100-x-y)*x-10*x
>> yacas(f) # register f with yacas
>> Df <- f
>> body(Df) <- yacas(expression(deriv(f(x,y),x)))[[1]]
>> Df
>>
>> R-Output:
>> function (x, y)
>> 100 - x - y - x - 10
>>
>> Questions :
>> 1.      How can I equalize the derivative(140-x-y-x-20) with zero?
>> 2.      How can I convert it to x?
>>
>> I want something like:
>> 45-0.5*y=x
>>
>> With only one variable or two non-symbolic variables it's working with
>> uniroot(f, c(-10, 10))$root.
>>
>
> This solves for x:
>
>> library(Ryacas)
> Loading required package: XML
>> x <- Sym("x")
>> y <- Sym("y")
>> Solve((100-x-y)*x-10*x == 0, "x")
> [1] "Starting Yacas!"
> expression(list(x == -((10 - (100 - y) + root((100 - y - 10)^2,
>    2))/2), x == -((10 - (100 - y) - root((100 - y - 10)^2, 2))/2)))
>
> --
> GKX Group
> GKX Associates Inc.
> 1-877-GKX-GROUP
> ggrothendieck at gmail.com
>
>



-- 
GKX Group, GKX Associates Inc.
Statistics & Software Consulting
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com