define $ right parameter with a variable in R [duplicate]

Use [[ instead of $. x$y is short hand for x[["y"]].

my_helper <- function (my_param) {
  levels(diamond[[my_param]])
}
my_helper("cut")

You cannot access components of an object without having access to the object itself, which is why your my_helper() fails.

It seems that you are a little confused about R objects, and I would strongly recommend a decent introductory texts. Some good free ones at the CRAN sites, and there are also several decent books. And SO had a number of threads on this as e.g.

• https://stackoverflow.com/questions/192369/books-for-learning-the-r-language
• What are some good books, web resources, and projects for learning R?
• https://stackoverflow.com/questions/1204043/good-intro-books-for-r

Try something like this:

dat = data.frame(one=rep(1,10), two=rep(2,10), three=rep(3,10))
myvar="one"
dat[,names(dat)==myvar]

This should return the first column/variable of the data frame dat

dat$one --> [1] 1 1 1 1 1 1 1 1 1 1