计算多个变量的多重聚合

Question

我用这种方式计算一个简短的汇总表：

library(data.table)
library(tidyverse)

mtcars.dt <- data.table(mtcars)
mtcars_grby <- mtcars.dt %>% 
  # group by cyl
  group_by(cyl) %>% 
  # summarise
  summarise(
    # number observ
    quantity = n(), 
    # average
    mpg_avg = mean(mpg, na.rm = TRUE), 
    # min and max weight
    wt_min = min(wt), 
    wt_max = max(wt))

mtcars_grby
# A tibble: 3 x 5
    cyl quantity mpg_avg wt_min wt_max
  <dbl>    <int>   <dbl>  <dbl>  <dbl>
1     4       11    26.7   1.51   3.19
2     6        7    19.7   2.62   3.46
3     8       14    15.1   3.17   5.42

仅使用 data.table 执行此操作的最短方法是什么？我只能想象合并四个不同的聚合，但这确实不优雅。

Answer 1

像这样使用 .(...)、keyby= 和 .N：

library(data.table)
mtcars.dt <- data.table(mtcars)

mtcars.dt[, .(quantity = .N,
  mpg_avg = mean(mpg, na.rm = TRUE),
  wt_min = min(wt),
  wt_max = max(wt)),
  keyby = cyl]

给予

Key: <cyl>
     cyl quantity  mpg_avg wt_min wt_max
   <num>    <int>    <num>  <num>  <num>
1:     4       11 26.66364  1.513  3.190
2:     6        7 19.74286  2.620  3.460
3:     8       14 15.10000  3.170  5.424