[18915] in s-news-athena
[S] Splus symbolic manipulation
daemon@ATHENA.MIT.EDU (alan9@carpal.ucsf.edu)
Tue Aug 10 21:28:11 1999
From: alan9@carpal.ucsf.edu
Date: Tue, 10 Aug 1999 18:23:56 -0700 (PDT)
Message-Id: <199908110123.SAA04320@carpal.ucsf.edu>
To: s-news@wubios.wustl.edu
I'm using Splus to fit data to some non-linear models derived from a base
function. The fit formula itself is made up of a composite function consisting
of the sum of the partials of the base function. The form of the base function
is the only thing I want to change directly as the partials and resulting
composite function get pretty long.
I've got a function of three independents, M(x,y,z). I want to calculate
partials with respect to the independents:
DxM <- partial of M with respect to x
DyM <- partial of M with respect to y
DzM <- partial of M with respect to z
And then build a composite function that I can then use in a formula to
a non-linear fitting function such as nlme or nlsList:
GrM <- m1 * DxM + m2*DyM + m3*DzM
nlme(formula=s~GrM(. . . ),data= . . . .)
I wanted to use deriv and managed to get the partials, but I couldn't see
how to separate and combine the elements from the three different calls.
I then tried using D() and paste() as follows:
deriv.f <- function( M )
{
DxM <- D(M,c("x"))
DyM <- D(M,c("y"))
DzM <- D(M,c("z"))
t1 <- paste("m1 * ( ", deparse(DxM), " ) ")
t2 <- paste("+ m2 * ( ", deparse(DyM), " ) ")
t3 <- paste("+ m3 * ( ", deparse(DzM), " ) ")
paste(t1,t2,t3)
}
Calling deriv.f with M <- a*x*y*z
> deriv.f(M)
[1] "m1 * ( . . . rest of stuff looks ok . . . "
>
How can I now get this vector to be the body of a function?
Or, is there a much better approach that I've missed?
Any help appreciated. Will post summary of replies.
Thanks.
Frank Ashford
Assistant Development Engineer
UCSF Orthopaedic Bioengineering Laboratory
(alan9@carpal.ucsf.edu)
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