[Rd] model.matrix evaluation challenges

[Felix Andrews]

how about...

nrow(with(cc, model.matrix(params, data=environment())))

cheers
-Felix

2009/8/10 Ben Bolker <bolker at ufl.edu>:
>  I am having difficulty with evaluation/environment construction
> for a formula to be evaluated by model.matrix().  Basically, I
> want to construct a model matrix that first looks in "newdata"
> for the values of the model parameters, then in "object at data".
> Here's what I've tried:
>
>  1. model.matrix(~f,data=c(newdata,object at data)) -- fails because
> something (terms()?) within model.matrix() tries to coerce "data"
> to a data frame -- but newdata and object at data have different
> numbers of rows.
>
>  2. with(c(newdata,object at data),model.matrix(~f))   works --
> BUT not when ~f is assigned to another object (as it will be
> when it is being dug out of a previously fit model).
>
>  3. If "f" exists as a column within newdata and/or object at data,
> but the formula is assigned to another object (see below), then
> with(c(newdata,object at data),model.matrix(z))  fails -- because
> model.matrix() is explicitly evaluating the variables in the formula
> in the environment of z (i.e., ignoring the first argument of "with" ...)
>
>
>  Any advice on how to solve this without making a bigger mess?
>
>  sincerely
>    Ben Bolker
>
>
> ## set up a data frame for prediction
>
> set.seed(1001)
> f = factor(rep(letters[1:4],each=20))
> x = runif(80)
> u = rnorm(4)
> y = rnorm(80,mean=2+x*(3+u[f]),sd=0.1)
> dat = data.frame(f,x,y)
>
> ## fit a model ... could easily do by lm() but want to
> ##   demonstrate the problem
>
> library(bbmle)
> m1 = mle2(y~dnorm(a+b*x,sd=exp(logs)),parameters=list(b~f),data=dat,
>  start=list(a=0,b=2,logs=-3))
>
> ## data frame for prediction
> pp0 = expand.grid(x=seq(0,1,length=11),
>  f=levels(dat$f))
>
> ## combine frame and model data: have to keep the model data
> ##  around, because it contain other information needed for
> ##  prediction.
>
> cc = c(pp0,m1 at data)
> try(mm <- model.matrix(~f,cc))  ## fails -- different number of rows
>  ## (at some point R tries to coerce cc to a data frame)
> nrow(with(cc,model.matrix(~f))) ## works ... but ...
>
> ## here's what happens within predict.mle2()
> ## extract information about parameter models
> params <- eval(m1 at call$parameters)
> params <- params[[1]]
> params[[2]] <- NULL
>
> nrow(with(cc,model.matrix(params)))
> rm(f)
> ## object "f" not found -- because model.matrix
> ##  evaluates in the parent environment of params ...
> try(nrow(with(cc,model.matrix(params))))
>
>
>
> --
> Ben Bolker
> Associate professor, Biology Dep't, Univ. of Florida
> bolker at ufl.edu / www.zoology.ufl.edu/bolker
> GPG key: www.zoology.ufl.edu/bolker/benbolker-publickey.asc
>
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>



-- 
Felix Andrews / 安福立
Postdoctoral Fellow
Integrated Catchment Assessment and Management (iCAM) Centre
Fenner School of Environment and Society [Bldg 48a]
The Australian National University
Canberra ACT 0200 Australia
M: +61 410 400 963
T: + 61 2 6125 1670
E: felix.andrews at anu.edu.au
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-- 
http://www.neurofractal.org/felix/