using a string in doBy command

I am using the summaryBy() (doBy package) function inside a function I am writing. I want to define the variable (as a string) with respect to which the summary needs to be calculated. When I define a string and pass it to summaryBy function it doesn't work. Here is an example from the help file of summaryBy function .

data(dietox)
dietox12<- subset(dietox,Time==12)  

Following co开发者_JS百科de works

summaryBy(Weight+Feed~Evit+Cu,data=dietox12,FUN=mean)

However the following doesn't work

vardef<-'Evit';
summaryBy(Weight+Feed~vardef+Cu,data=dietox12,FUN=mean)

I know why it doesn't work. I wanted to know whether there is another way. Of course I can use the ddply function in stead of the summaryBy function.

Thank you.

If you want to build formulas dynamically, the easiest way to go is to build it completely as a string.

form<-formula(paste("Weight+Feed~", vardef, "+Cu", sep=""))

summaryBy(form,data=dietox12,FUN=mean)

There are other options, but this is the easiest.