[R] turning R expressions into functions?

[Greg Snow]

Look at the replicate function, it takes an expression (does not need
a function) and runs that expression the specified number of times.
Will that accomplish what you want without needing to worry about
substitute, quote, eval, etc.?

On Fri, Jun 29, 2012 at 11:36 AM, Jochen Voß <voss at seehuhn.de> wrote:
> [ please copy me on answers, since I am not subscribed to the list ]
>
> Dear all,
>
> I am trying to write an R function which uses system.time
> to determine which of a given list of R expressions executes
> fastest.  To work around the limited resolution of system.time,
> I want to convert the given expressions into functions which
> execute the given expressions a fixed number of times.
> My current attempt is as follows:
>
>  FuncIt <- function(k, expr) {
>    k <- as.numeric(k)
>    expr <- eval.parent(substitute(expr))
>    eval(substitute(function() { for (funcit.i in 1:k) { expr } }))
>  }
>
> This works, but seems not very robust.
> My question: is there a better way of doing this?
>
> Here are some experiments.
>
> 1) good: If I run the following using "Rscript"
>
>  test1 <- function(e1) {
>    e1 <- substitute(e1)
>    FuncIt(100, e1)
>  }
>
>  f <- test1(rnorm(1))
>  print(f)
>
> then I get the following output:
>
>  function ()
>  {
>      for (funcit.i in 1:100) {
>          rnorm(1)
>      }
>  }
>  <environment: 0x102260c28>
>
> This is what I want.  But why do I need the extra "substitute"
> in test1?  I only found by experiment that this is needed.
>
>
> 2) bad: If I try to call FuncIt directly, it fails:
>
>  f <- FuncIt(100, rnorm(1))
>  print(f)
>
> has the output:
>
>  function ()
>  {
>      for (funcit.i in 1:100) {
>          -0.763894772833099
>      }
>  }
>  <environment: 0x102265790>
>
> This is bad, since now 'rnorm(1)' already has been
> evaluated.  How do I prevent this from happening,
> without breaking the good case 1 above?
>
>
> 3) ugly: If I run the same commands in the R gui on MacOS
> (R 2.15.1 released on 2012/06/22), I get different output:
>
>> source("/Users/voss/project/statcomp/test.R")
>  function() { for (funcit.i in 1:k) { expr } }
>  <environment: 0x19cc040>
>  function() { for (funcit.i in 1:k) { expr } }
>  <environment: 0x19bc884>
>
> This is on the same machine using (as far as I can tell) the
> same R engine.  So why is the output different?
>
>
> Many thanks,
> Jochen
> --
> http://seehuhn.de/
>
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-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com