[R] Change global env variables from within a function

[Jon Zadra]

  Hi,

I've looked all over for a solution to this, but haven't had much look 
in specifying what I want to do with appropriate search terms.  Thus I'm 
turning to R-help.

In the process of trying to write a simple function to rename individual 
column names in a data frame, I ran into the following problem: When I 
rename the columns within my function, I can't seem to get it to apply 
to the data frame in the global environment in a simple manner.

Given:

tempdf <- data.frame("a" = 1:6, "b" = 7:12)

#I can rename a to g this way:
names(tempdf)[names(tempdf)=="a"] <- "g"

#Wanting to simplify this for the future, I have the function:
colrename <- function(dframe, oldname, newname) {
     names(dframe)[names(dframe)==oldname] <- newname
     }

colrename(tempdf, "a", "g")

#However of course the change to tempdf stays within colrename().  I 
could add "return(names(dframe))" to the function and then call the 
function like:
#names(tempdf) <- colrename(tempdf, "a", "g")
#but at that point its not much simpler than the original:
#names(tempdf)[names(tempdf)=="a"] <- "g"

So, i'm wondering, is there a way to write code within that function so 
that the tempdf in the global environment gets changed?
I thought of doing "names(dframe)[names(dframe)==oldname] <<- newname" 
but then of course it doesn't replace "dframe" with "tempdf" in the 
evaluation, so it seems to be a problem of looking at some variables 
within the function environment but others in the parent environment.

Any help is greatly appreciated.

Thanks,

Jon
-- 
Jon Zadra
Department of Psychology
University of Virginia
P.O. Box 400400
Charlottesville VA 22904
(434) 982-4744
email: zadra at virginia.edu
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