[Rd] Questions about calls and formulas
erich.neuwirth at univie.ac.at
Mon Aug 22 23:39:53 CEST 2005
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
as the first argument, and then by using
m <- match.call(expand.dots = FALSE)
saves the call. It transforms the call
m$na.action <- na.action # force use of default for this method
m[] <- as.name("model.frame")
and then evaluates the modified call
mf <- eval(m, parent.frame())
model.frame(formula = x ~ g * h)
Then it uses components of mf for the call to boxplot.default.
m has a component m$formula containing the parsed model formula.
mode(m$formula) is "call".
In our case, deparse(m$formula) gives a string representation of the
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
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
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