cannot combine with and functions in R

Question

Could somebody please point out to me why is that the following example does not work:

df        <- data.frame(ex =rep(1,5))
sample.fn <- function(var1) with(df, mean(var1))
sample.fn(ex)

It seems that I am using the wrong syntax to combine with inside of a function.

Thanks,

Answer 1

This is what I meant by learning to use "[" (actually"[["):

> df        <- data.frame(ex =rep(1,5))
> sample.fn <- function(var1) mean(df[[var1]])
> sample.fn('ex')
[1] 1

You cannot use an unquoted ex since there is no object named 'ex', at least not at the global environment where you are making the call to sample.fn. 'ex' is a name only inside the environment of the df-dataframe and only df itself is "visible" when the sample.fn-function is called.

Out of interest, I tried using the method that the with.default function uses to build a function taking an unquoted expression argument in the manner you were expecting:

samp.fn <- function(expr) mean( 
                  eval(substitute(expr), df, enclos = parent.frame())
                               )
samp.fn(ex)
#[1] 1

It's not a very useful function, since it would only be applicable when there was a dataframe named 'df' in the parent.frame(). And apologies for incorrectly claiming that there was a warning on the help page. As @rawr points out the warning about using functions that depend on non-standard evaluation appears on the subset page.