R-beta: ?faq example of scoping

[Thomas Lumley]

The current examples of the scoping rules of R in the faq and in R&R's
JCGS paper are clear but a bit artificial.  The following example may be
of interest as it
a) does something useful in R
b) doesn't work in S
c) was written in S independently by myself and at least three of my
fellow students over the past couple of years, causing literally hours of
confusion on each occasion.

jackknife.lm<-function (lmobj) 
{
        n <- length(resid(lmobj))
        jval <- t(apply(as.matrix(1:n), 1, function(i) coef(update(lmobj, 
                subset = -i))))
        (n - 1)* (n-1) * var(jval)/n
}

In order to make it work in S you need to explicitly pass the linear model
object into the function nested in apply(). If you don't and you are lucky
you will get  
Error: Object "lmobj" not found
If you are unlucky enough to have a linear model called lmobj in your
global environment you will get the wrong answer with no warning. 

This version works in S

jackknife.S.lm<-function (lmobj) 
{
        n <- length(resid(lmobj))
        jval <- t(apply(as.matrix(1:n), 1, function(i,lmobj)
 coef(update(lmobj, subset = -i)),lmobj=lmobj))
        (n - 1)* (n-1) * var(jval)/n
}


Thomas Lumley
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