[R] limited formula length in tsls

[John Fox]

Dear Brian,

At 07:53 AM 23/08/2001 +0100, Prof Brian D Ripley wrote:

>On Wed, 22 Aug 2001, John Fox wrote:
>. . .
> >
> > Clearly, the current situation is unsatisfactory, but it's unclear to me
> > what the best course of action is. A more elegant approach to treating the
> > model formula and instruments consistently would solve the problem, but
> > as.character should probably not truncate formulas.
>
>as.character is not the best tool for formulae.  Use deparse:
>
> > deparse(mod)
>[1] "this ~ is + a + very + long + formula + with + a + very + large + "
>[2] "    number + of + characters"
>
>say via
>
>paste(deparse(mod, 500), collapse="")
>
>which will cope with arbitrary long formulae.
>
>The cutoff limit is 500, and a trivial change to the internals of
>as.character (which I have just made for 1.3.1) increases the cutoff from
>60 to 500. Nevertheless. paste(deparse(mod, 500), collapse="") remains
>better.

Thank you for the suggestion. I've attached a modified tsls.formula, which 
I'll include in the next version of the sem package. A few functions in my 
car package do similar formula manipulation, and these too need to be 
changed to avoid problems with long formulas.

Regards,
  John

------------- snip ------------------

tsls.formula <- function(model, instruments, data, subset,
     na.action, contrasts=NULL){
     if (missing(na.action))
         na.action <- options()$na.action
     m <- match.call(expand.dots = FALSE)
     if (is.matrix(eval(m$data, sys.frame(sys.parent()))))
         m$data <- as.data.frame(data)
     c1 <- unlist(strsplit(paste(deparse(model, width.cutoff=500),
         collapse=""),'~'))
     c2 <- unlist(strsplit(paste(deparse(instruments, width.cutoff=500),
         collapse=""),'~'))
     formula <- as.formula(paste(c1[1], '~', c1[2], '+', c2[2]))
     m$formula <- formula
     m$instruments <- m$model <- m$contrasts <- NULL
     m[[1]] <- as.name("model.frame")
     mf <- eval(m, sys.frame(sys.parent()))
     na.act <- attr(mf, "na.action")
     Z <- model.matrix(instruments, data = mf, contrasts)
     response <- attr(attr(mf, "terms"), "response")
     y <- mf[,response]
     X <- model.matrix(model, data=mf, contrasts)
     result <- tsls(y, X, Z, colnames(X))
     result$response.name <- c1[1]
     result$formula <- model
     result$instruments <- instruments
     if (!is.null(na.act))
         result$na.action <- na.act
     class(result) <- "tsls"
     result
     }



-----------------------------------------------------
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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