R 如何计算data.table中几个类别的平均值？

给出此数据示例：

  library(data.table)
   dat=structure(list(bin = structure(c(1L, 2L, 4L, 3L, 5L, 6L, 1L, 
  2L, 4L, 3L, 5L, 6L, 1L, 2L, 4L, 3L, 5L, 6L, 1L, 2L), .Label = c("a","b", "c", "c", "d", "e"), class = 
 "factor"), 
value = c(3, 7, 0, 
1, 1, 4, 8,4, 8, 7,2, 2, 4,6, 7, 17, 1, 2, 4, 7), nvar = c("Ecor", 
"Ecor", "Ecor", "Ecor", "Ecor", 
"Ecor", "AM", "AM", "AM", "AM", "AM", 
"AM", "SS", "SS", "SS", "SS", "SS", "SS", 
"ACC", "ACC")), row.names = c(NA, -20L), class = c("data.table", 
"data.frame"))

我想计算（使用值）的平均值

Ecor和ACC（=cat1）

nvar中除Ecor、ACC和SS（=cat2）之外的所有变量

保持SS不变

输出可以如下所示：

          bin value nvar
     1:   a     3 cat1
     2:   b     7 cat1
     3:   c     0 cat1
     4:   c     1 cat1
     5:   d     1 cat1
     6:   e     4 cat1
     7:   a     8   cat2
     8:   b     4   cat2
     9:   c     8   cat2
    10:   c     7   cat2
    11:   d     2   cat2
    12:   e     2   cat2
    13:   a     4   SS
    14:   b     6   SS
    15:   c     7   SS
    16:   c    17   SS
    17:   d     1   SS

---

您可以使用

fifelse

根据条件创建一个新列，并计算每组的平均值

library(data.table)

dat[, nvar := fifelse(nvar %in% c('Ecor', 'ACC'), 'cat1', 
                    fifelse(nvar == 'SS', 'SS', 'cat2'))]

dat[, .(value = mean(value)), .(bin, nvar)]

---

我们可以使用

data.table中的
fcase
，避免嵌套比较

library(data.table)
dat[, .(value = mean(value)), .(bin, 
       nvar = fcase(nvar %chin% c("Ecor", "ACC"), "cat1",
                    nvar == "SS", "SS", default = "cat2"))]

-输出

#      bin nvar value
# 1:   a cat1   3.5
# 2:   b cat1   7.0
# 3:   c cat1   0.0
# 4:   c cat1   1.0
# 5:   d cat1   1.0
# 6:   e cat1   4.0
# 7:   a cat2   8.0
# 8:   b cat2   4.0
# 9:   c cat2   8.0
#10:   c cat2   7.0
#11:   d cat2   2.0
#12:   e cat2   2.0
#13:   a   SS   4.0
#14:   b   SS   6.0
#15:   c   SS   7.0
#16:   c   SS  17.0
#17:   d   SS   1.0
#18:   e   SS   2.0

多谢。在我的真实数据中，我需要保持SS和MM的原样。将是fifelse（nvar==c（'SS'，'MM'），（'SS'，MM'，'cat2'））]你可以使用
dat[，nvar:=fifelse（nvar%in%c（'Ecor'，'ACC'），'cat1'，fifelse（nvar%in%c（'SS'，'MM'），nvar'，'cat2'））]