R help - Jan 2004 - Problem with lme, ns and df (variable scoping problem?)

Hi,

I'm experimenting with random effect natural splines, and I've 
encountered an odd problem.

library(nlme); library(splines)
a <- data.frame(x = 1:10, y = 1:10 + runif(10, min=-3, max=3), c = 
rep(c(1,2),5))
df <- 10   
lml <- lmList(y ~ ns(x,df=df) | c, a)

Error in df - 1 : non-numeric argument to binary operator

I presume this is because the formula isn't evaluated until it's deep in
the bowels of lmList, where a local df exists.  So I try again with a 
variable name that probably doesn't exist elsewhere:

d.f <- 10
lml <- lmList(y ~ ns(x,df=d.f) | c, a)

This works, but now when I calculate the mixed model, I get a different 
error.

lme <- lme(lml, random=pdIdent(~c))
Error in model.frame(formula, rownames, variables, varnames, extras, 
extranames,  :
        variable lengths differ

Neither of these problems occur if I specify df directly (eg. df=10)

Can anyone offer a work around?

Thanks.

Hadley

Use substitute, something like

lml <- eval(substitute(lmList(y ~ ns(x,df=d.f) | c, a), list(d.f.=df)))

This sort of thing is in any case necessary to get a sensible call
component recorded in the fitted object.


On Thu, 22 Jan 2004, Hadley Wickham wrote:
> Hi,
> 
> I'm experimenting with random effect natural splines, and I've 
> encountered an odd problem.
> 
> library(nlme); library(splines)
> a <- data.frame(x = 1:10, y = 1:10 + runif(10, min=-3, max=3), c = 
> rep(c(1,2),5))
> df <- 10   
> lml <- lmList(y ~ ns(x,df=df) | c, a)
> 
> Error in df - 1 : non-numeric argument to binary operator
> 
> I presume this is because the formula isn't evaluated until it's
deep in
> the bowels of lmList, where a local df exists.  So I try again with a 
> variable name that probably doesn't exist elsewhere:
> 
> d.f <- 10
> lml <- lmList(y ~ ns(x,df=d.f) | c, a)
> 
> This works, but now when I calculate the mixed model, I get a different 
> error.
> 
> lme <- lme(lml, random=pdIdent(~c))
> Error in model.frame(formula, rownames, variables, varnames, extras, 
> extranames,  :
>         variable lengths differ
> 
> Neither of these problems occur if I specify df directly (eg. df=10)
> 
> Can anyone offer a work around?
> 
> Thanks.
> 
> Hadley
> 
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> 
> 
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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