Hello,
parse just parse the text into an expression.
> parse(text="a<-a*10; b<-2:6")
expression(a<-a*10, b<-2:6)
attr(,"srcfile")
<text>
If you want to evaluate the expression, you need to call eval
> y <- within(x, eval(parse(text="a<-a*10; b<-2:6")))
> y
a b
1 10 2
2 20 3
3 30 4
4 40 5
5 50 6
Or you can just do this :
> y <- within(x, { a<-a*10; b<-2:6 } )
> y
a b
1 10 2
2 20 3
3 30 4
4 40 5
5 50 6
Romain
On 01/07/2010 09:08 AM, N Klepeis wrote:
Hi,
Why can't I pass an expression to `within' by way of textual input to
the 'parse' function?
e.g.,
> x <- data.frame(a=1:5,b=LETTERS[1:5])
> x
a b
1 1 A
2 2 B
3 3 C
4 4 D
5 5 E
> within(x, parse(text="a<-a*10; b<-2:6"))
a b
1 1 A
2 2 B
3 3 C
4 4 D
5 5 E
> within(x, parse(text="a<-a*10; b<-2:6")[[1]])
a b
1 1 A
2 2 B
3 3 C
4 4 D
5 5 E
This would be very useful to allow for arbitrary evaluation of
multi-line commands at runtime.
Of course, I can edit the 'within.data.frame' function as follows, but
isn't there some way to make 'within' more generally like the 'eval'
command?
alternative:
within.data.frame <-
function (data, textCMD, ...)
{
parent <- parent.frame()
e <- evalq(environment(), data, parent)
eval(parse(text=textCMD), e) # used to be eval(substitute(expr), e)
l <- as.list(e)
l <- l[!sapply(l, is.null)]
nD <- length(del <- setdiff(names(data), (nl <- names(l))))
data[nl] <- l
if (nD)
data[del] <- if (nD == 1)
NULL
else vector("list", nD)
data
}
--Neil
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://tr.im/JFqa : R Journal, Volume 1/2, December 2009
|- http://tr.im/IW9B : C++ exceptions at the R level
`- http://tr.im/IlMh : CPP package: exposing C++ objects
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
