R help - Apr 2011 - sweave options with variable

hello:
   Do any one know how to set sweave option by variable, for example I want
set some of my selected code chunk with:
<<eval=needRun>>= rather than <<eval=TRUE>>=, so  I can
change the action
only in the head by change the variable "needRun" one times.
I have tried use "\Def" and "\newcommand", both do not work,
so I suppose it
is related with R/Sweave its self.
thanks for any good suggestion.

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On 11-04-17 2:41 AM, Kai Ying wrote:
> hello:
>     Do any one know how to set sweave option by variable, for example I
want
> set some of my selected code chunk with:
> <<eval=needRun>>= rather than<<eval=TRUE>>=, so  I
can change the action
> only in the head by change the variable "needRun" one times.
> I have tried use "\Def" and "\newcommand", both do not
work, so I suppose it
> is related with R/Sweave its self.
> thanks for any good suggestion.
>
You can't do that, but you can get a similar effect this way:

<<echo=FALSE>>needRun <- TRUE
@

...

<<thecode,eval=FALSE>>someSlowFunction()
@

<<echo=FALSE>>if (needRun) {
<<thecode>>
}
@

Or you could use cacheSweave or weaver for caching, which may do what 
you want, or write your own Sweave driver to do exactly what you want.

Duncan Murdoch