Summarize with dplyr “other then” groups

Question

I need to summarize in a grouped data_frame (warn: a solution with dplyr is very much appreciated but isn\'t mandatory) both something on each group (simple) and the same so

Answer 1

I don't think it is in general possible to perform operations on other groups within summarise() (i.e. I think the other groups are not "visible" when summarising a certain group). You can define your own functions and use them in mutate to apply them to a certain variable. For your updated example you can use

calc_med_other <- function(x) sapply(seq_along(x), function(i) median(x[-i]))
calc_med_before <- function(x) sapply(seq_along(x), function(i) ifelse(i == 1, NA, median(x[seq(i - 1)])))

df %>%
    group_by(group) %>%
    summarize(med = median(value)) %>%
    mutate(
        med_other = calc_med_other(med),
        med_before = calc_med_before(med)
    )
#   group   med med_other med_before
#   (chr) (dbl)     (dbl)      (dbl)
#1     a   1.5       4.5         NA
#2     b   3.5       3.5        1.5
#3     c   5.5       2.5        2.5