[R] turning R expressions into functions?

[Jochen Voß]

[ 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
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http://seehuhn.de/