如何用purrr估算平均值

Question

我想知道为什么purrr中的cant map函数无法解决此查询

tapply(mtcars$mpg, mtcars$cyl, mean)

# 4 6 8

#26.66364 19.74286 15.10000

与

mtcars %>% split(.$cyl) %>% map(~mean(mpg))

#$4 [1] NA

#$6 [1] NA

#$8 [1] NA

#Warning messages: 1: In mean.default(mpg) : argument is not numeric or logical: returning NA 2: In mean.default(mpg) : argument is not numeric or logical: returning NA 3: In mean.default(mpg) : argument is not numeric or logical: returning NA

Answer 1

我们需要提取特定列

library(purrr)
mtcars %>%
    split(.$cyl)  %>%
    map(~mean(.$mpg))

如果我们需要获得vector

mtcars %>% 
      split(.$cyl)  %>%
      map_dbl(~mean(.$mpg))
#     4        6        8 
#26.66364 19.74286 15.10000