如何使用从group_by（dplyr）淘汰的值创建列

Question

Suppose we have the following data.frame:

df <- data.frame( a=c( 1 , 1 , 1 , 1 , 2 , 2 , 2 , 2 , 3 , 3 , 3 , 3),
                  b=c('x','x','y','y','v','v','w','w','x','x','y','y')
                 )

What I want to do is to create a column 'd' such that d would be a operation on each group added to the value of a. For example, suppose 'd' would be equal to each value of 'a' minus the mean of 'a' for each group. We'd have:

new_df <- data.frame(a=c( 1 , 1 , 1 , 1 , 2 , 2 , 2 , 2 , 3 , 3 , 3 , 3),
                     b=c('x','x','y','y','v','v','w','w','x','x','y','y'),
                     d=c(-1 , -1, -1, -1, 0 , 0 , 0 , 0 , 1 , 1 , 1 , 1)

Is there a way to do that using dplyr?

Answer 1

Try

library(dplyr)
df %>%
   group_by(b) %>% 
   mutate(d= a-mean(a))