[R] system.time gives error when "=" is used for assignment(R-2.6.0)

[Bill.Venables at csiro.au]

Kyeongmi asks:

> Hello list, 
> 
> I found that system.time works correctly when I used "<-" to assign
> a value to a variable but when I happened to use "=" instead of
> "<-", R gave an error message: "Error in system.time(your argument
> here...)". It happened with a few functions I tried. Is this a bug
> or is there any circumstances that "=" cannot be used for
> assignment? Here is a real simple example.
> 
> fn1 <- function(x) x+1
> 
> r1 <- system.time(res1=fn1(2))
> r2 <- system.time(res2 <- fn1(2))
> 

No, it is not a bug.  It is a syntactic trap that using '=' for
assignment will lure you into.  This is one reason why I suggest you
just do not do it.

Think about it.  If the function system.time() had an argument res1,
how would you call it giving a value for this argument?  As you have
done.  So it has nothing to do with the function system.time iteslf,
it applies generally.

Ways around it include

r1 <- system.time((res1 = fn1(2)))  ## if you must!
r1 <- system.time({res1 = fn1(2)})  ## ditto
r1 <- system.time(res1 <- fn1(2))   ## as you discovered.

Bill Venables.