Q:

Random sample from a data frame in R

I have the following data frame: 
id<-c(1,1,2,3,3)
date<-c("23-01-08","01-11-07","30-11-07","17-12-07","12-12-08")
df<-data.frame(id,date)
df$date2<-as.Date(as.character(df$date), format = "%d-%m-%y")

id     date      date2
1   23-01-08 2008-01-23
1   01-11-07 2007-11-01
2   30-11-07 2007-11-30
3   17-12-07 2007-12-17
3   12-12-08 2008-12-12

Now I want to extract a random sample of ids and not the rows. In fact I am looking for a way to randomly pick two of the ids and extract all records  related to them. For instance if it randomly pick ids 2 and 3 the output data frame should look like:
id     date      date2
2   30-11-07 2007-11-30
3   17-12-07 2007-12-17
3   12-12-08 2008-12-12

Any helps would be appreciated.

A:

You can randomly pick two IDs using sample()
chosen <- sample(unique(df$id), 2)

and then extract those records
subset(df, id %in% chosen)

A:

You can use sample function.
set.seed(2)
df[match(sample(unique(df$id),2),df$id),]

sample() function will generate random indexes and then you can match them back to your df data frame rows and get the rest of the data.
For more information check ?sample

