[R] nls() syntax

[Rolf Turner]

I want to fit a model y = x/(x-a) where the value of a depends
on the level of a factor z.  I cannot figure out an appropriate
syntax for nls().  The "parameter" a (to be estimated) should be a
vector of length equal to the number of levels of z.

I tried:

strt <- rep(3,length(levels(z))
names(strt=levels(z)
fit <- nls(y ~ x/(x - a[z]),start=strt,data=xxx)

but of course got an error:

> Error in nls(y ~ x/(x - a[z]), start = strt, data = xxx) : 
>   parameters without starting value in 'data': a

I keep thinking that there is something obvious that I should
be doing, but I can't work out what it is.

Does there *exist* an appropriate syntax for doing what I want
to do?  Can anyone enlighten me?  The data set "xxx" is given
in dput() form at the end of this message.

cheers,

Rolf Turner

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

Data set "xxx":

structure(list(x = c(30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 
130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 
30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 
170, 180, 190, 200, 210, 220, 230, 240, 250, 30, 40, 50, 60, 
70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 
200, 210, 220, 230, 240, 250, 30, 40, 50, 60, 70, 80, 90, 100, 
110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 
240, 250, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 
150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250), y = c(1.27, 
1.16, 1.19, 1.15, 1.09, 1.07, 1.07, 1.05, 1.07, 1.03, 1.05, 1.07, 
1.06, 1.03, 1.05, 1.04, 1.03, 1.03, 1.03, 1.02, 1.02, 1.01, 1.01, 
1.21, 1.15, 1.1, 1.1, 1.06, 1.06, 1.05, 1.03, 1.07, 1.04, 1.04, 
1.02, 1.04, 1.02, 1.04, 1.03, 1.01, 1.03, 1.01, 1, 1.02, 1.03, 
1.02, 1.42, 1.27, 1.23, 1.14, 1.17, 1.08, 1.11, 1.06, 1.07, 1.08, 
1.06, 1.07, 1.04, 1.03, 1.07, 1.04, 1.03, 1.03, 1.03, 1.04, 1.03, 
1.03, 1.04, 1.85, 1.41, 1.35, 1.21, 1.22, 1.15, 1.14, 1.07, 1.1, 
1.09, 1.1, 1.09, 1.08, 1.08, 1.09, 1.09, 1.07, 1.06, 1.03, 1.08, 
1.05, 1.02, 1.05, 1.99, 1.6, 1.44, 1.4, 1.24, 1.3, 1.21, 1.23, 
1.18, 1.18, 1.12, 1.15, 1.09, 1.07, 1.13, 1.1, 1.05, 1.13, 1.09, 
1.03, 1.11, 1.07, 1.05), z = structure(c(1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 
4L, 4L, 4L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 
5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L), .Label = c("p1", 
"p2", "p3", "p4", "p5"), class = "factor")), class = "data.frame", row.names = c(NA, 
-115L))