Relative frequencies / proportions with dplyr

Question

Suppose I want to calculate the proportion of different values within each group. For example, using the mtcars data, how do I calculate the relative f

Answer 1

Despite the many answers, one more approach which uses prop.table in combination with dplyr or data.table.

library("dplyr")
mtcars %>%
    group_by(am, gear) %>%
    summarise(n = n()) %>%
    mutate(freq = prop.table(n))

library("data.table")
cars_dt <- as.data.table(mtcars)
cars_dt[, .(n = .N), keyby = .(am, gear)][, freq := prop.table(n) , by = "am"]