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

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),] )) )

Solution 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)

Solution 2:

I think you want

ToothGrowth %>%
  arrange(supp,len)

The chaining system just replaces nested commands, so first you are grouping, then ordering that grouped result, which breaks the original ordering.

Solution 3:

Another way to fix this unexpected order issue while still using the group_by() statement is to convert the grouped_df back to a data frame. group_by is needed for summaries for example:

ToothGrowthMeanLen <-  ToothGrowth %>%
    group_by(supp, dose) %>%
    summarise(meanlen = mean(len)) 

This summary table is not arranged in the order of meanlen

ToothGrowthMeanLen %>%
    arrange(meanlen)

This summary table is arranged in the order of meanlen

ToothGrowthMeanLen %>%
    data.frame() %>%   # Convert to a simple data frame
    arrange(meanlen)

Converting grouped_df back to a data frame is the first way I found to sort a summarised data.frame. But in fact dplyr::ungroup exists for that purpose.

ToothGrowthMeanLen %>%
    ungroup() %>%   # Remove grouping
    arrange(meanlen)