[R] Multiple left hand side variables in a formula

[William Dunlap]

I don't know how much of the information that model.frame supplies you need,
but you could make a data.frame containing all the variables on both sides of them
formula by changing lhs~rhs into ~lhs+rsh before calling model.frame.  E.g.,

f <- function (formula)  {
    if (length(formula) == 3) { # has left hand side
        envir <- environment(formula)
        formula <- formula(call("~", call("+", formula[[2]],  formula[[3]])))
        environment(formula) <- envir
    }
    formula
}

This doesn't quite take care of the wild-card dot in the formula: straight
variables are omitted from dot's expansion but functions of variables are
not:
> colnames(model.frame(f(log(mpg)+hp ~ .), data=mtcars))
 [1] "log(mpg)" "hp"       "mpg"      "cyl"     
 [5] "disp"     "drat"     "wt"       "qsec"    
 [9] "vs"       "am"       "gear"     "carb"    

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Frank Harrell
> Sent: Friday, March 01, 2013 4:17 PM
> To: r-help at r-project.org
> Subject: [R] Multiple left hand side variables in a formula
> 
> The lattice package uses special logic to allow for multiple left-hand-side
> variables in a formula, e.g. y1 + y2 ~ x.  Is there an elegant way to do
> this outside of lattice?  I'm trying to implement a data summarization
> function that logically takes multiple dependent variables.  The usual
> invocation of model.frame( ) causes R to try to do arithmetic addition to
> create a single dependent variable.
> 
> Thanks
> Frank
> 
> 
> 
> -----
> Frank Harrell
> Department of Biostatistics, Vanderbilt University
> --
> View this message in context: http://r.789695.n4.nabble.com/Multiple-left-hand-side-
> variables-in-a-formula-tp4660060.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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