[Rd] Questions about calls and formulas
Douglas Bates
dmbates at gmail.com
Mon Aug 22 23:52:08 CEST 2005
On 8/22/05, Erich Neuwirth <erich.neuwirth at univie.ac.at> wrote:
> I am trying to adapt boxplot.formula (in graphics) to accept an
> additional parameter, weights.
> I already managed to adapt boxplot.default to do this.
>
> boxplot.formula prepares the data for a call to boxplot.default and to
> achieve that does the following: It takes a formula like
>
> x~g*h
>
> as the first argument, and then by using
>
> m <- match.call(expand.dots = FALSE)
>
> saves the call. It transforms the call
>
> m$na.action <- na.action # force use of default for this method
> m[[1]] <- as.name("model.frame")
>
> and then evaluates the modified call
> mf <- eval(m, parent.frame())
>
> print(m)
> gives
> model.frame(formula = x ~ g * h)
>
> Then it uses components of mf for the call to boxplot.default.
>
> m has a component m$formula containing the parsed model formula.
> mode(m$formula) is "call".
> In our case, deparse(m$formula) gives a string representation of the
> formula: "x~g*h".
> I want to replace the response variable (in our case x) by the weights
> variable, which in the string expression can be done easily with
> strsplit and paste. Then I need to reconvert the modified string to a call.
>
> So I create newmodelstring<-"weights~g*h" and try
>
> m$formula<-as.call(parse(newmodelstring))
>
> print(m)
> gives
> model.frame(formula = weights ~ g * h())
>
>
> When I try to evaluate the modified m this does not work. When I try to
> evaluate m with this modification I get
>
> Error in model.frame(formula = weights ~ g * h()) :
> attempt to apply non-function
>
> Is there a way to get rid of the empty parentheses at the
> end of the formula? I think then my code could work.
>
> --
> Erich Neuwirth, Didactic Center for Computer Science
> University of Vienna
> Visit our SunSITE at http://sunsite.univie.ac.at
> Phone: +43-1-4277-39902 Fax: +43-1-4277-9399
>
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>
I think the preferred way to do this is using substitute although
formulas are a bit tricky in that you need to eval them after the
substitution to make sure that the object has class "formula".
> (foo <- eval(substitute(x ~ g * h, list(x = as.name("weights")))))
weights ~ g * h
> class(foo)
[1] "formula"
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