R help - Jul 2008 - function to transform response of a formula

Hi,

I am trying to write a function which takes a formula as input and 
outputs a new formula with a different response while keeping the rest 
of the formula the same.

I.e.

respapply : ( y ~ a+b ) -> ( f(y) ~ a + b )

I have tried the following but it doesn't work.  The terms become 
invalid as shown below.

respapply <- function(fm, f) {

     fm[[2]] <- f(eval(fm[[2]]))
     fm;

}

 > fm <- formula(y ~ a + b)
 > y <- runif(5)
 > newfm <- respapply(fm,identity)
 > newfm

c(0.552921097259969, 0.939932722365484, 0.62522904924117, 
0.899310540175065,
0.877736972644925) ~ a + b

 > terms(newfm)
Error in terms.formula(fmapply(fm, identity)) :
   invalid term in model formula

Could someone put me in the right direction of how to correctly write a 
respapply function as described.

Regards,

Paul Emberson

Its ok.

I've just read about update.formula in another message.

Paul

Paul Emberson wrote:
> Hi,
> 
> I am trying to write a function which takes a formula as input and 
> outputs a new formula with a different response while keeping the rest 
> of the formula the same.
> 
> I.e.
> 
> respapply : ( y ~ a+b ) -> ( f(y) ~ a + b )
> 
> I have tried the following but it doesn't work.  The terms become 
> invalid as shown below.
> 
> respapply <- function(fm, f) {
> 
>     fm[[2]] <- f(eval(fm[[2]]))
>     fm;
> 
> }
> 
>  > fm <- formula(y ~ a + b)
>  > y <- runif(5)
>  > newfm <- respapply(fm,identity)
>  > newfm
> 
> c(0.552921097259969, 0.939932722365484, 0.62522904924117, 
> 0.899310540175065,
> 0.877736972644925) ~ a + b
> 
>  > terms(newfm)
> Error in terms.formula(fmapply(fm, identity)) :
>   invalid term in model formula
> 
> Could someone put me in the right direction of how to correctly write a 
> respapply function as described.
> 
> Regards,
> 
> Paul Emberson
> 
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