title: Random sample from a data frame in R
body: <p>I have the following data frame: </p>
  
  <pre><code>id&lt;-c(1,1,2,3,3)
  date&lt;-c("23-01-08","01-11-07","30-11-07","17-12-07","12-12-08")
  df&lt;-data.frame(id,date)
  df$date2&lt;-as.Date(as.character(df$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
  </code></pre>
  
  <p>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:</p>
  
  <pre><code>id     date      date2
  2   30-11-07 2007-11-30
  3   17-12-07 2007-12-17
  3   12-12-08 2008-12-12
  </code></pre>
  
  <p>Any helps would be appreciated.</p>