On Fri, Dec 12, 2008 at 12:11 PM, Vitalie Spinu <vitosm...@rambler.ru> wrote:
> On Fri, 12 Dec 2008 17:38:13 +0100, hadley wickham <h.wick...@gmail.com>
> wrote:
>
>> You could also imagine similar iterators for random sampling, like
>> samp(0.2) to choose 20% of the indices, or boot(0.8) to choose 80%
>> with replacement. first(n) could also be useful, selecting the first
>> min(n, length(vector)) observations. An iterator version of rev()
>> would also be handy.
>>
>> Maybe selector would be a better name than iterator though, as these
>> don't have the same feel as iterators in other languages.
>
> That is really something!! Real high level language!!
> "Selectors" could depend on named variables in data frame as well:
>
> mtcars[sel(cyl>3)&last(5)]
> mtcars[sel(cyl>3)&boot(80%)]
>
> or may be just
> mtcars[cyl>3&last(20)]
>
You can do this (and quite a bit more) in data.table:
> library(data.table)
> mtcars.dt <- as.data.table(mtcars)
> tail(mtcars.dt[cyl < 5], 4)
mpg cyl disp hp drat wt qsec vs am gear carb
[1,] 27.3 4 79.0 66 4.08 1.935 18.9 1 1 4 1
[2,] 26.0 4 120.3 91 4.43 2.140 16.7 0 1 5 2
[3,] 30.4 4 95.1 113 3.77 1.513 16.9 1 1 5 2
[4,] 21.4 4 121.0 109 4.11 2.780 18.6 1 1 4 2
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