替代(m)获取data.table函数 [英] Alternative to (m)get in data.table functions

问题描述

假设我具有以下 data.table ，并希望通过引用存储在向量中的变量来获取以下输出:

Let's say I have the following data.table and would like to get the output below by referring to variables stored in a vector:

dt <- data.table(a = rep(1, 3),
                 b = rep(2, 3))

x <- 'a'
y <- 'b'

dt[, .(sum(get(x)), mean(get(y)))]

   V1 V2
1:  3  2

很酷，它有效.但是现在我想做一个函数，然后做类似的事情:

Cool, it works. But now I'd like to make a function, and then do something like:

foo <- function(arg1, arg2) {

  dt[, .(sum(get(arg1)), mean(get(arg2)))]

}

foo(x, y)

意识到它的工作原理，我想避免调用所有这些 gets ，并执行以下操作:

Realizing it works, I'd like to avoid calling all those gets, and do something like:

foo <- function(arg1, arg2) {

  eval(substitute(dt[, .(sum(arg1), mean(arg2))]))

}

foo(x, y) # or foo('x', 'y')

但是这失败了.是否有关于如何像多次调用 get 一样一次评估所有参数的想法?

But this fails. Any idea on how to evaluate all the arguments at once in a way similar to calling get multiple times?

推荐答案

我们可以使用 as.symbol 或 as.name 和 eval uate

We can convert to symbol with as.symbol or as.nameand evaluate

foo <- function(arg1, arg2) {

    dt[, .(sum(eval(as.name(arg1))), mean(eval(as.name(arg2))))]

 }

foo(x, y)
#   V1 V2
#1:  3  2

或使用 [[]来对数据表的列进行子集化

Or use [[ to subset the columns of the data.table

foo <- function(arg1, arg2) {

  dt[, .(sum(.SD[[arg1]]), mean(.SD[[arg2]]))]

 }

foo(x, y)
#   V1 V2
#1:  3  2

或者另一个选择是粘贴和 eval/parse

 foo <- function(arg1, arg2) {

     eval(parse(text = paste0("dt[, .(sum(", arg1, "), mean(", arg2, "))]")))

   }

foo(x, y)
#   V1 V2
#1:  3  2

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