On 30/09/2011 12:26 PM, Justin Talbot wrote:
I'm interested in the difference between these two intuitively equivalent sequences that produce different results (in R version 2.13.1 (2011-07-08) 32-bit). I think R's reference counting optimization is causing this difference in behavior.> a<- 1 > a+{a[1]<- 20} [1] 21 > a<- 1 > a[1]<- 1 > a+{a[1]<- 20} [1] 40 Is one of these the "correct" answer, or is the order of side effects undefined in these statements? Section 4.3.3 of the R Language Definition just says that doing assignment in an argument to a function is "bad style", but doesn't say anything about evaluation order. In general, for primitive and internal functions, is a particular evaluation order for the arguments guaranteed?
In general, evaluation order is undefined. I was surprised by the result, but I think you're right about the explanation. In particular:
a<- 1 .Internal(inspect(a))
@4a06440 14 REALSXP g0c1 [NAM(2)] (len=1, tl=0) 1
a[1]<- 1 .Internal(inspect(a))
@4a06300 14 REALSXP g0c1 [NAM(1)] (len=1, tl=0) 1 The only difference is in the "named" value. Duncan Murdoch ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
