constructing a formula

Dear Mitchell,

Dear Listers,

I am having trouble figuring out how to build a formula using a variable
list.  For example, I have:

a _ data.frame(a=rnorm(1000))
a$b_rnorm(1000)+.5*a$a
a$c_rnorm(1000)+.5*a$b
a$d_rnorm(1000)+.5*a$b+.1*a$a
attach(a)

and I estimate,

lm(d ~ b+c+d)

Did you really mean to regress d on predictors including d? (Note that the 
data frame a shadows a$a.)

BUT,

I wish to construct a generalized solution in which,

ListOfVar _ c('b','c','d')

The question is how to leverage ListOfVar into the constuction of the
formula lm(a$d ~ a$b+a$c+a$b).  I know this must be fairly simple, but
unfortunately my readings of the various help files/books haven't been
productive yet.

Note that a$b appears twice on the RHS of the formula -- probably not what 
you meant.

Any help would be appreciated.  Thanks in advance.

If all of the variables on the RHS are numeric, as here, then something like
         lm(a$a ~ as.matrix(a[,ListOfVar]))
should work, where a is the data frame containing the variables in the list 
(actually a character vector).

If there are also factors on the RHS of the formula, then you could do 
something like
         lm(eval(parse(text=paste('a ~', paste(ListOfVar, collapse="+")))), 
data=a)
though someone will probably think of something simpler.

Does that do what you need?
   John

-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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