Re: [R] symbolic manipulations

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From: Thomas Lumley (tlumley@u.washington.edu)
Date: Sat 07 Apr 2001 - 10:07:29 EST


Message-ID: <Pine.A41.4.33.0104061701460.89396-100000@homer01.u.washington.edu>

On Fri, 6 Apr 2001, Jeff Miller wrote:

>
>
> Hi All,
>
>
> I copied code from an Splus manual to take symbolic derivatives:
>
> my.deriv <- function( mathfunc, var )
> {
> tmp <- substitute(mathfunc)
> name <- deparse(substitute(var))
> D(tmp, name)
> }
>
> (The code also works in R).
>
>
> When I try this on x^2 I get
>
> > my.deriv(x^2, x)
> 2 * x
>
> Suppose I assign the output of my.deriv(x^2, x) to deriv.xsqr :
>
> > deriv.xsqr <- my.deriv(x^2, x)
> > deriv.xsqr
> 2 * x
> >
>
> My question is, how do I take the derivative of deriv.xsqr
> (I want the answer to be 2) ?

FAQ 7.6 explains that this is precisely why the D() function doesn't work
like my.deriv().

You can do it with
R> eval(substitute(my.deriv(expr,x),list(expr=deriv.xsqr)))
 [1] 2

but this is making life unnecessarily difficult for yourself.

R> D(D(expression(x^2),"x"),"x")
 [1] 2

is easier

        -thomas

Thomas Lumley Asst. Professor, Biostatistics
tlumley@u.washington.edu University of Washington, Seattle

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