为什么总结功能只给出一个结果？

Question

smalldat <- data.frame(group1 = rep(1:2, c(5,5)),
                       group2 = rep(c("a","b"), 5),
                       x = rnorm(10))

smalldat
#    group1 group2          x
# 1       1      a -1.2173399
# 2       1      b  0.2601609
# 3       1      a -1.9955389
# 4       1      b -0.7949134
# 5       1      a  0.9655160
# 6       2      b -1.2307946
# 7       2      a  0.3562118
# 8       2      b  0.7674343
# 9       2      a -0.2472418
# 10      2      b -1.2653220
 a<-group_by(smalldat,group1)
 summarize(a,mm=mean(x))
 #      mm
 # 1 -0.1690133

那么，为什么我得到所有x的平均值，而不是1和2的平均值？ 谢谢

Answer 1

你需要打破管道。

smalldat %>% group_by(group1) %>% summarize(mm = mean(x))

# Source: local data frame [2 x 2]
# 
#   group1         mm
#    (int)      (dbl)
# 1      1 -0.5564231
# 2      2 -0.3239425

（必备data.table插件：我觉得这更具可读性）：

library(data.table); setDT(smalldat)

smalldat[ , mean(x), by = group1]

#or, named:
smalldat[ , .(mean(x)), by = group1]

Answer 2

作为替代方案，我们可以使用aggregate

中的base R

aggregate(x~group1, smalldat, mean)
# group1         x
#1      1 0.2487354
#2      2 0.2275124