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
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|- http://tr.im/IW9B : C++ exceptions at the R level
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