On 12-04-28 10:37 AM, Ista Zahn wrote:
Hi JN,
You can use
eval(parse(text = cstr))
for this. I've been told to avoid this when possible, though I'm not sure why.
I would say to avoid it because it's hard to catch errors easy errors,
and it can introduce difficult errors. Presumably cstr isn't a constant
string (or you wouldn't have bothered with this form), so you need to
make sure that cstr contains valid code. That requires lots of checks
that really belong in the parser at install time, not at execution time,
when errors are harder to fix.
For an example of a hard error, I think you might use
deparse(substitute(arg)) to get an expression passed as an argument.
But deparse() is not guaranteed to reproduce the original expression.
So why not just use substitute(arg), which is guaranteed to give the
original expression, and plunk it into your function using a trick like
Gabor's, e.g.
body(f) <- substitute(arg)
if that's what you're intending? (This example is nearly as silly as
John's, but the point is that once something is parsed, just keep it
parsed, don't deparse and reparse it.)
Duncan Murdoch
Best,
Ista
On Sat, Apr 28, 2012 at 10:27 AM, John C Nash<nas...@uottawa.ca> wrote:
I've been creating some R tools that manipulate objective functions for
optimization. In so doing, I create a character string with R code, and then
want to have it in my workspace. Currently -- and this works fine -- I write
the code out, then use source() to bring it in again. Example:
cstr<-"jack<-function(x){\n cat(\"Silly x:\")\n print(x) \n }\n"
write(cstr, file='tfile.txt')
jack<-source('tfile.txt')$value # You need the value element!
print(jack)
However, I feel it would be more elegant if I could avoid the file, and am
sure I must have missed some way to pipe the cstr through the source()
function. Also, if the file cannot be written (directory permissions?), then
my approach won't work.
JN
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