What does "[?]" represent in a grouped data frame?

[Bob]

Hi All,

Below is one of the demos from dplyr's Introduction vignette (https://cran.rstudio.com/web/packages/dplyr/vignettes/introduction.html). What is the meaning of the question mark?

Thanks,

Bob

daily <- group_by(flights, year, month, day)
(per_day   <- summarise(daily, flights = n()))
#> Source: local data frame [365 x 4]
#> Groups: year, month [?]  <=============What does this mean?
#> 
#> # A tibble: 365 x 4
#>    year month   day flights
#>   <int> <int> <int>   <int>
#> 1  2013     1     1     842
#> 2  2013     1     2     943
#> 3  2013     1     3     914
#> 4  2013     1     4     915
#> ... with 361 more rows

[Hadley Wickham]

It basically means that dplyr doesn't know (currently) how many groups
there are.
Hadley

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