arrange() not working on grouped data frame
dplyr, r
Solution
It's not working because you need to `ungroup()` the data before arranging by `cyl`. The code you are using attempts to order the `cyl` column while it's still grouped by `cyl`. Since those values are all the same (within each group), nothing changes.
To arrange the entire data by `cyl` after ranking, we need to remove the grouping first, and then we can run `arrange()` again.
library(dplyr)
group_by(mtcars, cyl) %>% ## group by cylinder
mutate(rank = row_number(mpg)) %>% ## rank by mpg
filter(rank <= 3) %>% ## top three for each cyl
arrange(rank) %>% ## arrange each group by rank
ungroup() %>% ## remove grouping
arrange(desc(cyl)) ## arrange all by cylinder (descending)
# mpg cyl disp hp drat wt qsec vs am gear carb rank
# 1 10.4 8 472.0 205 2.93 5.250 17.98 0 0 3 4 1
# 2 10.4 8 460.0 215 3.00 5.424 17.82 0 0 3 4 2
# 3 13.3 8 350.0 245 3.73 3.840 15.41 0 0 3 4 3
# 4 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4 1
# 5 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 2
# 6 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4 3
# 7 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2 1
# 8 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1 2
# 9 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1 3
As a side note, I would recommend that you consider using the `%>%` function for chaining these calls together as it will considerably cut down on assignments made with `<-`.
Problem
Assume I have the following code. In the last step, where I try arranging it, the code doesn't work and the data frame continues to be arranged in ascending order by `cyl`. ``` library(dplyr) # create a grouped data frame df <- group_by(mtcars,cyl) # rank car from best mpg to worst for every cyl df <- mutate(df,rank = row_number(mpg)) # top 3 best mpg for each cyl df <- filter(df,rank <= 3) # arrange by the number of cyl df <- arrange(df,desc(cyl), rank) ``` Any thoughts on why this is happening ?