dplyr - select first and last row from grouped data

dplyr-group-select.r

# from http://stackoverflow.com/questions/31528981/dplyr-select-first-and-last-row-from-grouped-data
#
# select first and last row from grouped data

# df adjusted for demonstration
df <- data.frame(id=c(1,1,1,2,2,2,3),
                 stopId=c("a","b","c","a","b","c","a"), 
                 stopSequence=c(1,2,3,3,1,4,3))

# way1
> df %>%
+   group_by(id) %>%
+   arrange(id,stopSequence) %>%
+   filter(row_number()==1 | row_number()==n())
Source: local data frame [5 x 3]
Groups: id [3]

     id stopId stopSequence
  <dbl> <fctr>        <dbl>
1     1      a            1
2     1      c            3
3     2      b            1
4     2      c            4
5     3      a            3
  
# way2, different from way1, select the row twice if data groups have one row only
> df %>% arrange(id, stopSequence) %>% group_by(id) %>% slice(c(1,n()))
Source: local data frame [6 x 3]
Groups: id [3]

     id stopId stopSequence
  <dbl> <fctr>        <dbl>
1     1      a            1
2     1      c            3
3     2      b            1
4     2      c            4
5     3      a            3
6     3      a            3