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

[Uwe Ligges]

Yes: "$"() has to calculate more on matching the names --- and is more 
of a hack anyway.

Although, I would not worry to much about 0.3 seconds in 1000000 
evaluations ...

Uwe


On 08.09.2010 14:31, Søren Højsgaard wrote:
> 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
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.