[R] using "unstack" inside my function: that old scope problem again

[Prof Brian Ripley]

The problem is the use of a formula, and hence non-standard evaluation 
(even non-standard for a formula).  Unstack contains

    res <- c(tapply(eval(form[[2]], x), eval(form[[3]], x), as.vector))

and those eval() calls are looking only in x and then along the search
path.  They should be something like

eval(form[[2]], x, parent.frame())

or

eval(form[[2]], x, environment(form))

depending on your preferences for scoping rules.

unstack.default is very simple: why not extract the part you want?


On Fri, 19 Mar 2004, Paul Johnson wrote:

> I've been reading the R mail archives and I've found  a lot of messages 
> with this same kind of problem, but I can't understand the answers.  Can 
> one of you try to explain this to me?
> 
> Here's my example. Given a regression model and a variable, I want to 
> use unstack() on the vector of residuals and make some magic with the 
> result. But unstack hates me.
>  
> 
> PCSE <- function (tmodel,groupVar) {
>   myres1 <- resid(tmodel)
>   resUnstacked <- unstack(myres1, form = myres1 ~ groupVar));
>   E <- as.matrix(resUnstacked)
>   SIGMA <- (1/nrow(E))*(t(E) %*% E)
>   OMEGA <- diag(x=1, nrow=nrow(E), ncol=nrow(E)) %x% SIGMA
> 
>   X <- model.matrix(tmodel)
>   XPRIMEXINV <- solve(t(X) %*% X)
>   PCSECOVB <- XPRIMEXINV %*%  (t(X) %*% OMEGA %*% X ) %*% XPRIMEXINV
> }
> 
> 
> The error is:
> PCSE(eld.ols1,dat2$STATEID)
> Error in eval(expr, envir, enclos) : Object "groupVar" not found
> 
> Here's what I don't understand the most.
>  
> If I hack this so that the "resUnstacked" is created by a "matrix" 
> command, then the function works. Why would matrix() work when unstack 
> does not.  And why does model.matrix() work if unstack does not.

Because the eval calls in model.matrix.default have been thought through.

> 
> Thanks in advance, as usual.
> 
> 

-- 
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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595