Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

If you use attach, it is easy to tell if a variable exists. You can simply use exists to check:

>attach(df)
>exists("varName")
[1] TRUE


However, if you don’t use attach (and I find you generally don’t want to), this simple solution doesn’t work.

> detach(df)
> exists("df\$varName")
[1] FALSE


Instead of using exists, you can use in or any from the base package to determine if a variable is defined in a data frame:

> "varName" %in% names(df)
[1] TRUE
> any(names(df) == "varName")
[1] TRUE

Or to determine if a variable is defined in a matrix:

> "varName" %in% colnames(df)
[1] TRUE
> any(colnames(df) == "varName")
[1] TRUE