dplyr group_by and mutate, how to access the data frame?

Question

When using dplyr\'s \"group_by\" and \"mutate\", if I understand correctly, the data frame is split in different sub-dataframes according to the group_by argument. For examp

Answer 1

We can use within do

data %>%
    group_by(let ) %>% 
    do(mutate(., mean.by.letter = mean(.$x)))