Function that either takes a variable from the data argument or from the parent frame.

fun.r

fun <- function(x, data) {
  if (missing(data))
    data <- NULL
  if (is.null(data))
    data <- sys.frame(sys.parent())
  mf <- match.call()
  mf.x <- mf[[match("x", names(mf))]]
  x <- eval(mf.x, data, enclos=sys.frame(sys.parent()))
  temp <- x^2 + 5
  return(temp)
}

dat <- data.frame(x=c(2,4))
fun(x, dat)
fun(dat$x)
fun(c(2,4))
fun(x)
x <- c(2,4)
fun(x)