[R] Function with 'data' parameter

[Greg Pyle (Secretary)]

Thanks, worked like a charm.

Greg.

On Feb 22, 1:58 pm, Gavin Simpson <gavin.simp... at ucl.ac.uk> wrote:
> On Sun, 2009-02-22 at 20:52 +1100, Jim Lemon wrote:
> > Greg wrote:
> > > I'm trying to write a simple function with a data parameter.
>
> > > tfun <- function(x, y, data = NULL) {
> > >       if(missing(data))
> > >               dt <- data.frame(x=x, group=y)
> > >       else {
> > >               dt <- with(data, data.frame(x=x, group=y))
> > >       }
>
> > >       return(dt)
> > > }
>
> > > If I pass variables "weight" and "grp" from a data.frame, d, like
> > > this: tfun(d$weight, d$grp), the function works.  However, if I try to
> > > do the same thing by supplying d, like this: tfun(weight, grp,
> > > data=d), I receive the following error:
>
> > > Error in data.frame(x = x, group = y) : object "weight" not found
>
> > > Can someone please tell me what I'm doing wrong?
>
> > Hi Greg,
> > In your function definition, the data argument isn't missing, it's NULL.
> > You have to test like this:
>
> > if(is.null(data)) ...
>
> > Does that fix it?
>
> No, because x and y are still being evaluated and are not present
> anywhere but within 'd'. (I converted the function definition to
> function(x, y, data) and checked that it was executing the else clause
> when data was *not* missing.)
>
> You could do:
>
> ## dummy data
> set.seed(1234)
> d <- data.frame(weight = rnorm(10),
>                 grp = rnorm(10))
>
> tfun2 <- function(x, y, data) {
>         x <- deparse(substitute(x))
>         y <- deparse(substitute(y))
>         if(missing(data))
>                 dt <- data.frame(x=x, group=y)
>         else {
>                 dt <- with(data, data.frame(x=data[[x]], group=data[[y]]))
>         }
>         return(dt)
>
> }
>
> tfun2(weight, grp, data = d)
>
> but that seems a little ugly. Alternatively, make use of a model formula
> and in-built functionality
>
> tfun3 <- function(formula, data) {
>         dat <- model.frame(formula, data)
>         names(dat) <- c("x", "group")
>         return(dat)
>
> }
>
> tfun3(weight ~ grp, data = d)
>
> HTH
>
> G
>
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