hi list,
used versions: 2.12.1 and 2.14.0 under ubuntu and macosx.
I recently stumbled over a problem with `nls', which occurs if the model
is not specified explicitly but via an evaluation of a 'call' object.
simple example:
8<--------------------------------------------------------------------------------------
nlsProblem <- function (len = 5) {
#=======================================================================
# purpose: to demonstrate an apparent problem with `nls' which occurs,
# if the model is specified by passing th lhs as an evaled 'call'
# object. The problem is related to the way `nls' tries to compute
# its internal variable `varIndex' which rests on the assumption that
# the dependent ("y") and, possibly, the independent ("x") variable
# are identified by having a length equal to the `nls' variable
# `respLength'. the problem arises when there are `varNames'
# components accidentally having this length, too.
# in the present example, setting the number of data points to
# len=2 triggers the error since the `call' object `fifu' has this
# length, too and `nls' constructs an erroneous `mf$formula' internally.
#=======================================================================
#generate some data
x <- seq(0, 4, len = len)
y <- exp(-x)
y <- rnorm(y, y, .001*y)
data <- list(x = x, y = y)
#define suitable model
model <- y ~ exp(-k*x)
fifu <- model[[3]]
#this fit is fine:
fit1 <- nls(model, data = data, start = c(k = 1))
print(summary(fit1))
#this fit crashes `nls' if len = 2:
fit2 <- nls(y ~ eval(fifu), data = data, start = c(k = 1))
print(summary(fit2))
}
8<--------------------------------------------------------------------------------------
to see the problem call `nlsProblem(2)'.
as explained in the above comments in the example function, I tracked it
down to the way
`nls' identifies x and y in the model expression. the problem surfaces in
the line
varIndex <- n%%respLength == 0
(line 70 in the function listing from within R) which, in the case of
`fit2' in the above
example always returns a single TRUE index as long as `len != 2' (which
seems fine for the
further processing) but returns a TRUE value for the index of `fifu' as
well if `len == 2'.
question1: I'm rather sure it worked about 6 months ago with (ca.) 2.11.x
under ubuntu. have there been changes in this area?
question2: is something like the `fit2' line in the example expected to
work or not?
qeustion3: if it is not expected to work, should not the manpage include a
corresponding caveat?
question4: is there a a substitute/workaround for the `fit2' line which
still allows to specify the rhs of the model via a variable instead of a
constant (explicit) expression or function call?
the above example is of course construed but in my use case I actually
need this sort of thing. is there any chance that
the way `nls' analyzes its `model' argument can be changed to parse the
`eval(fifu)' construct correctly in all cases?
since I'm currently not subscribed to the list I'd appreciate if responses
could be Cc'ed to me directly.
thanks in advance,
joerg
______________________________________________
R-help@r-project.org mailing list
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.
