dplyr issues when using group_by(multiple variables)
compound-key, dplyr, group-by, r
Solution
Taking Dickoa's answer one step further -- as Hadley says "summarise peels off a single layer of grouping". It peels off grouping from the reverse order in which you applied it so you can just use
mtcars %>%
group_by(cyl, gear) %>%
summarise(newvar = sum(wt)) %>%
summarise(newvar2 = sum(newvar) + 5)
Note that this will give a different answer if you use `group_by(gear, cyl)` in the second line.
And to get your first attempt working:
df1 <- mtcars %>%
group_by(cyl, gear) %>%
summarise(newvar = sum(wt))
df2 <- df1 %>%
group_by(cyl) %>%
summarise(newvar2 = sum(newvar)+5)
Problem
I want to start using dplyr in place of ddply but I can't get a handle on how it works (I've read the documentation). For example, why when I try to mutate() something does the "group_by" function not work as it's supposed to? Looking at mtcars: library(car) Say I make a data.frame which is a summary of mtcars, grouped by "cyl" and "gear": ``` df1 <- mtcars %.% group_by(cyl, gear) %.% summarise( newvar = sum(wt) ) ``` Then say I want to further summarise this dataframe. With ddply, it'd be straightforward, but when I try to do with with dplyr, it's not actually "grouping by": ``` df2 <- df1 %.% group_by(cyl) %.% mutate( newvar2 = newvar + 5 ) ``` Still yields an ungrouped output: ``` cyl gear newvar newvar2 1 6 3 6.675 11.675 2 4 4 19.025 24.025 3 6 4 12.375 17.375 4 6 5 2.770 7.770 5 4 3 2.465 7.465 6 8 3 49.249 54.249 7 4 5 3.653 8.653 8 8 5 6.740 11.740 ``` Am I doing something wrong with the syntax? Edit: If I were to do this with plyr and ddply: ``` df1 <- ddply(mtcars, .(cyl, gear), summarise, newvar = sum(wt)) ``` and then to get the second df: ``` df2 <- ddply(df1, .(cyl), summarise, newvar2 = sum(newvar) + 5) ``` But that same approach, with sum(newvar) + 5 in the summarise() function doesn't work with dplyr...