Understanding tracemem

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

But that should be

system.time(replicate(100, x[1e7 + 1L] <- 1L)) / 100
# ~0.10s
system.time(replicate(100, z$b <- 1L)) / 100
# ~ 0.04s

which suggests that it's not a deep copy.

But

x <- 1:1e6
z <- list(a = x)
system.time(replicate(100, z$b <- 1L)) / 100
# ~0.005s

which suggests it's not a shallow copy either.


But then neither of those are probably good tests because they modify
in place.  I'll think more.

Hadley

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