R help - Feb 2011 - Passing function arguments

Hi All,

Im looking for some help passing function arguments and referencing them,
I've made a replica, less complicated function to show my problem, and how
i've made a work around for this. However i suspect there is a _FAR_ better
way of doing this.

If i do:
BuildDecayModel <- function(x = "this", y = "that", data
= model.data) {
  model <- nls(y ~ SSexp(x, y0, b), data = model.data)
  return(model)
}
...

"Error in lm.fit(x, y, offset = offset, singular.ok = singular.ok, ...) :
  0 (non-NA) cases"

This function returns an error because the args are passed as "this"
and
"that" to the model, and so fails (correct?)

If  i do the following:
BuildDecayModel <- function(x = "total.reach", y =
"lift", data model.data) {
  x <- data[[x]]
  y <- data[[y]]
  model.data <- as.data.frame(cbind(x,y))
  model <- nls(y ~ SSexp(x, y0, b), data = model.data)
  return(model)
}

This works for me, but it seems that i'm missing a trick with just
manipulating the args rather than making an entire new data.frame to work
off,

Can anyone offer some advice?

Thanks in advance

Mike

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On Feb 11, 2011, at 6:14 AM, Michael Pearmain wrote:
> Hi All,
>
> Im looking for some help passing function arguments and referencing  
> them,
> I've made a replica, less complicated function to show my problem,  
> and how
> i've made a work around for this. However i suspect there is a _FAR_  
> better
> way of doing this.
>
> If i do:
> BuildDecayModel <- function(x = "this", y = "that",
data =
> model.data) {
>  model <- nls(y ~ SSexp(x, y0, b), data = model.data)
>  return(model)
> }
> ...
>
> "Error in lm.fit(x, y, offset = offset, singular.ok =  
> singular.ok, ...) :
>  0 (non-NA) cases"
>
> This function returns an error because the args are passed as
"this"
> and
> "that" to the model, and so fails (correct?)
>
> If  i do the following:
> BuildDecayModel <- function(x = "total.reach", y =
"lift", data > model.data) {
>  x <- data[[x]]
>  y <- data[[y]]
>  model.data <- as.data.frame(cbind(x,y))
>  model <- nls(y ~ SSexp(x, y0, b), data = model.data)
>  return(model)
> }
>
> This works for me, but it seems that i'm missing a trick with just
> manipulating the args rather than making an entire new data.frame to  
> work
> off,
The trick you are missing is how to build a formula from component  
character objects. The usual approach something like this:

?formula

Perhaps:
form <- as.formula( paste(y, "~ SSexp(, " , x , ", y0,
b)" ) )
(untested)
model <- nls(form, data = model.data)

paste() should result in evaluation of the argument to return "this"  
and "that" which will then be bundled into a proper language object  
which is not just a character string. I should say I hope this works,  
but there are mysteries regarding the environment of evaluation that  
continue to trip me up.

-- 
David.
>
> Can anyone offer some advice?
(Some further advice: Set your client to post in plain
text.)
>
> Thanks in advance
>
> Mike

David Winsemius, MD
West Hartford, CT