r - sample rows of subgroups from dataframe with dplyr

Question

If I want to randomly select some samples from different groups I use the plyr package and the code below

require(plyr)
sampleGroup<-function(df,size) {
  df[sample(nrow(df),size=size),]
}

iris.sample<-ddply(iris,.(Species),function(df) sampleGroup(df,10))

Here 10 samples are selected from each species.

Some of my dataframes are very big and my question is can I use the same sampleGroup function with the dplyr package? Or is there another way to do the same in dplyr?

EDIT

Version 0.2 of the dplyr package introduced two new functions to select random rows from a table sample_n and sample_frac

Answer 1

Yes, you can use dplyr elegantly by the function do(). Here is an example:

mtcars %>% 
    group_by(cyl) %>%
    do(sample_n(.,2))

and the results are like this

Source: local data frame [6 x 11]
Groups: cyl

   mpg cyl  disp  hp drat    wt  qsec vs am gear carb
1 24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
2 26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2
3 21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
4 17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
5 14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
6 15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8

Update:

The do function is no longer needed for sample_n in newer versions of dplyr. Current code for taking a random sample of two rows per group:

mtcars %>% 
    group_by(cyl) %>% 
    sample_n(2)