使用点号点(...)表示从data.table对象的自定义函数返回的列

问题描述 投票:3回答:3

我想使用...表示我想从data.table对象的自定义函数返回的变量。这是一个最小的可复制示例:

library(data.table)
d = data.table(mtcars)

getvar = function(...){
  return(d[,.(xyz = mean(hp), ...), cyl])
}

getvar(mpg, cyl, disp)

[.data.table(d,,。(N = .N,...),cyl)中的错误:找不到对象'cyl'

我希望得到的是:

d[,.(xyz = mean(hp), mpg, cyl, disp), cyl]

 #    cyl       xyz  mpg cyl  disp
 # 1:   6 122.28571 21.0   6 160.0
 # 2:   6 122.28571 21.0   6 160.0
 # 3:   6 122.28571 21.4   6 258.0
 # 4:   6 122.28571 18.1   6 225.0
 # 5:   6 122.28571 19.2   6 167.6

任何人都可以分享他们的解决方案吗?

r function data.table ellipsis
3个回答
3
投票

[一种可能的解决方案是在函数中使用mget,但返回一个列表,然后将xyz与带有c的列表合并。要进行此工作,需要将要添加的列指定为字符向量:

getvar = function(...){
  return(d[, c(xyz = mean(hp), mget(...)), cyl])
}

getvar(c("mpg", "cyl", "disp"))

给出:

> getvar(c("mpg", "cyl", "disp"))
    cyl       xyz  mpg cyl  disp
 1:   6 122.28571 21.0   6 160.0
 2:   6 122.28571 21.0   6 160.0
 3:   6 122.28571 21.4   6 258.0
 4:   6 122.28571 18.1   6 225.0
 5:   6 122.28571 19.2   6 167.6
 6:   6 122.28571 17.8   6 167.6
 7:   6 122.28571 19.7   6 145.0
 8:   4  82.63636 22.8   4 108.0
 9:   4  82.63636 24.4   4 146.7
10:   4  82.63636 22.8   4 140.8
....

0
投票

要使它在不引用列名的情况下起作用,您必须使用一些非标准的评估策略:

getvar = function(...){
  vars <- substitute(list(xyz = mean(hp), ...))
  return(d[, eval(vars), cyl])
}

getvar(mpg, cyl, disp)
    cyl       xyz  mpg cyl  disp
 1:   6 122.28571 21.0   6 160.0
 2:   6 122.28571 21.0   6 160.0
 3:   6 122.28571 21.4   6 258.0
 4:   6 122.28571 18.1   6 225.0
 5:   6 122.28571 19.2   6 167.6
...etc...

0
投票

基于@Konrad Rudolph here的答案,我们可以编写函数

getvar = function(...){
   dots = match.call(expand.dots = FALSE)$...
   cols = sapply(dots, deparse)
   d[, c(xyz = mean(hp), mget(cols)), cyl]
   #thanks to @Jaap for simplified version
}

getvar(mpg, cyl, disp)
#    cyl    xyz  mpg cyl  disp
# 1:   6 122.29 21.0   6 160.0
# 2:   6 122.29 21.0   6 160.0
# 3:   6 122.29 21.4   6 258.0
# 4:   6 122.29 18.1   6 225.0
# 5:   6 122.29 19.2   6 167.6
# 6:   6 122.29 17.8   6 167.6
#....
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