[R] Extracting elements from list: Is [[ always faster than $ ??

[Søren Højsgaard]

Dear list

It seems to me that extracting elements from a list using '[[' is somewhat faster than using '$'. For example:

> x        <- as.list(1:25)
> names(x) <- letters[1:length(x)]
> dput(x)
structure(list(a = 1L, b = 2L, c = 3L, d = 4L, e = 5L, f = 6L,
    g = 7L, h = 8L, i = 9L, j = 10L, k = 11L, l = 12L, m = 13L,
    n = 14L, o = 15L, p = 16L, q = 17L, r = 18L, s = 19L, t = 20L,
    u = 21L, v = 22L, w = 23L, x = 24L, y = 25L), .Names = c("a",
"b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n",
"o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y"))
>
> system.time({ for (ii in 1:1000000)  x[[21]] })
   user  system elapsed
   0.55    0.00    0.54
> system.time({ for (ii in 1:1000000)  x[["u"]]})
   user  system elapsed
   0.84    0.00    0.84
> system.time({ for (ii in 1:1000000)  x$u})
   user  system elapsed
   1.12    0.00    1.13
Does anyone know if this is always the case? (I have not found anything about speed in this connection on the help pages, apologies if I have overlooked something).

I use R.2.11.1 on Windows 7.

Regards