is there an equivalent to Stata's egen functio

问题:

This question already has answers here:

Calculate group mean (or other summary stats) and assign to original data (4 answers)

Closed 7 months ago.

Stata has a very nice command, egen, which makes it easy to compute statistics over group of observation. For instance, it is possible to compute the max, the mean and the min for each group and add them as a variable in the detailed data set. The Stata command is one line of code :

by group : egen max = max(x)

I've never found the same command in R. summarise in the dplyr package makes it easy to compute statistics for each group but then I have to run a loop to associate the statistic to each observation :

library("dplyr")
N  <- 1000
tf  <- data.frame(group = sample(1:100, size = N, replace = TRUE), x = rnorm(N))
table(tf$group)
mtf  <- summarise(group_by(tbl_df(tf), group), max = max(x))
tf$max  <- NA
for (i in 1:nrow(mtf)) {
  tf$max[tf$group == mtf$group[i]]  <- mtf$max[i]
}

Does any one has a better solution ?

回答1:

Here are a few approaches:

dplyr

library(dplyr)

tf %>% group_by(group) %>% mutate(max = max(x))

ave

This uses only the base of R:

transform(tf, max = ave(x, group, FUN = max))

data.table

library(data.table)

dt <- data.table(tf)
dt[, max:=max(x), by=group]