“Adding missing grouping variables” message in dplyr in R

Question

I have a portion of my script that was running fine before, but recently has been producing an odd statement after which many of my other functions do not work properly. I a

Answer 1

For consistency sake the grouping variables should be always present when defined earlier and thus are added when select(value) is executed. ungroup should resolve it:

qu25 <- mydata %>% 
  group_by(month, day, station_number) %>%
  arrange(desc(value)) %>% 
  slice(2) %>% 
  ungroup() %>%
  select(value)

The requested result is without warnings:

> mydata %>% 
+   group_by(month, day, station_number) %>%
+   arrange(desc(value)) %>% 
+   slice(2) %>% 
+   ungroup() %>%
+   select(value)
# A tibble: 1 x 1
  value
  <dbl>
1   113

Answer 2

Did you update dplyr recently by chance? I wonder if your dplyr::arrange call has been adversely effected by https://blog.rstudio.org/2016/06/27/dplyr-0-5-0/

Breaking changes arrange() once again ignores grouping, reverting back to the behaviour of dplyr 0.3 and earlier. This makes arrange() inconsistent with other dplyr verbs, but I think this behaviour is generally more useful. Regardless, it’s not going to change again, as more changes will just cause more confusion.

Answer 3

You can also convert your tibble to a data frame before your select statement using data.frame(). Then dplyr loses track of your grouping variables and isn't worried about them anymore.

qu25 <- mydata %>% 
      group_by(month, day, station_number) %>% 
      arrange(desc(value)) %>% 
      slice(3) %>% 
      data.frame() %>%
      select(value)