[R] converting to a language object

[Marc Schwartz]

ripley at stats.ox.ac.uk wrote:
> In this case a simler answer is that you want a formula, so use 
> as.formula:
> 
> 
>>eq <- "y ~ x1 + x2 +x2000"
>>as.formula(eq)
> 
> y ~ x1 + x2 + x2000
> 
> once the quotation marks are in the right place.
> 
> It is much simpler to use lm(y ~ ., data=foo) !
> 
> A caveat: you may hit some limits on expression size if you try to put
> 2000 variables in a formula, and almost certainly you will hit
> computational limits if you try to do a regression on it, as you need
> n >> p = 2000, and the key computation is (I think) O(np^2).
 >
 > SNIP


Valid points of course on all accounts.

Indeed there is a good example at the end of ?as.formula that addresses 
the construction of a formula from a large number of variables with just 
the type of sequencing that Arnab is using.  To wit:

## Create a formula for a model with a large number of variables:
xnam <- paste("x", 1:25, sep="")
(fmla <- as.formula(paste("y ~ ", paste(xnam, collapse= "+"))))

which obviously results in:

y ~ x1 + x2 + x3 + x4 + x5 + x6 + x7 + x8 + x9 + x10 + x11 +
     x12 + x13 + x14 + x15 + x16 + x17 + x18 + x19 + x20 + x21 +
     x22 + x23 + x24 + x25


I had been in the habit of using the more "generic" approach of 
eval(parse(text = charvector)) when constructing R code for evaluation 
in other situations.

Regards and thanks,

Marc Schwartz