让 Dickoa 的答案更进一步——正如 Hadley 所说的总结剥离了单层分组".它从您应用它的相反顺序剥离分组，以便您可以使用

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)

请注意，如果您在第二行中使用 group_by(gear, cyl)，这将给出不同的答案.

Note that this will give a different answer if you use group_by(gear, cyl) in the second line.

为了让您的第一次尝试成功:

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)