潮间带生态系统中by的当量

group_by %>% ... 所以它变得和我的 by(...) 命令?

library(MASS)
library(tidyverse)
set.seed(0)

dd <- data.frame(mvrnorm(40, 0:2, matrix(c(3,1,1,1,3,1,1,1,3),3,3)), k=rep(1:2, each=20))
by(dd[-4], dd$k, function(i) det(cov(i)))

#dd$k: 1
#[1] 16.62934
#--------------------------------------------------------------- 
#dd$k: 2
#[1] 3.849242

# Equivalent of above in tidyverse:
group_by(dd, k) %>% cov() %>% det()  # doesn't match above
#[1] 2.43671