R, dplyr - combination of group_by() and arrange() does not produce expected result?

Question

when using dplyr function group_by() and immediately afterwards arrange(), I would expect to get an output where data frame is ordered within groups that I stated in group_by(). My reading of documentation is that this combination should produce such a result, however when I tried it this is not what I get, and googling did not indicate that other people ran into the same issue. Am I wrong in expecting this result?

Here is an example, using the R built-in dataset ToothGrowth:

library(dplyr) ToothGrowth %>%   group_by(supp) %>%   arrange(len) 

Running this will produce a data frame where the whole data frame is ordered according to len and not within supp factors.

This is the code that produces the desired output:

ToothGrowth %>%   group_by(supp) %>%   do( data.frame(with(data=., .[order(len),] )) ) 

Answer 1

You can produce the expected behaviour by setting .by_group = TRUE in arrange:

library(dplyr) ToothGrowth %>%     group_by(supp) %>%     arrange(len, .by_group = TRUE)