Understanding tracemem

But does this?

z <- as.list(x)
z$a <- 11

Yes of course, as z is now of length 11.  There is no provision in R to
extend a vector except by creating a new one.  (Well, there is at C level
but I think it is not currently used.)

I guess a better example is

z <- list(a = 1:1e6, b = runif(1e6))
z$c <- 1

The list gets copied, but do a and b, or does the new list point to
the existing locations?  The following test suggests that it's a deep
copy.

x <- 1:1e7
z <- list(a = x)

system.time(replicate(100, z$b <- 1L)) / 100
# ~ 0.05s
system.time(replicate(100, x[1e6 + 1L] <- 1L)) / 100
# ~ 0.04s

Hadley

Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/