[Rd] eval functions... (PR#668)

[Peter Dalgaard BSA]

aprasad@fs.fed.us writes:

> Eg., the foll. extract from a function runs fine in S-PLUS but gives the error:
> Error in x[[j]] : subscript out of bounds
> in R
> 
> code snippet .....
> 
>  attach(xd)
> 	 resp <- "iv802"
> 	pickedlist <- "BD + CEC + CLAY + ERODFAC + INCEPTSL"
> 	lmexp <- paste(resp, " ~ ", pickedlist,sep="")
>         tmpzx <- eval(lm(lmexp,data=xd))
> 
> Essentially, I want to do:
>  lm(iv802 ~ BD + CEC + CLAY + ERODFAC + INCEPTSL, data=xd)
> thru a function that has the parameters passed thru TclTk and Perl......

Um, I'm somewhat surprised that this gives sensible results in
Splus... It does, since Splus accepts a text string for a formula
which R does not. 

[The use of eval is also unnecessary, and the sep="" is peculiar in
view of the spaces around the ~...]

In R you need to jump through an extra hoop to create a formula object
from the string representation:

lmexp <- formula(paste(resp, "~", pickedlist))
tmpzx <- lm(lmexp,data=xd)

The Splus docs explicitly say that the first argument to lm should be
a formula object, and R's behaviour is consistent with that.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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