> df <- data.frame(x = c(1,1,2,2,2,3,3,3,4,4, NA))
>
> df
    x
1   1
2   1
3   2
4   2
5   2
6   3
7   3
8   3
9   4
10  4
11 NA
>
> df %>%
+   group_by(x) %>%
+   summarise(sum(x))
# A tibble: 5 x 2
      x `sum(x)`
  <dbl>    <dbl>
1     1        2
2     2        6
3     3        9
4     4        8
5    NA       NA