data.table：j中的匿名函数 [英] data.table: anonymous function in j

问题描述

我试图让一个匿名函数在 data.table 的 j 参数中返回多个列。这是一个示例：

I'm trying to have an anonymous function return multiple columns in the j argument of a data.table. Here's an example:

## sample data
tmpdt <- data.table(a = c(rep("a", 5), rep("b", 5)),
                    b = c(rep("f", 3), rep("r", 7)),
                    c = 1:10,
                    d = 21:30)
tmpdt[c %in% c(2,4), c := NA]

## this works fine
tmpdt[ , list(testout =
                (function(x) {
                    model <- lm(c ~ d, x)
                    residuals(model)
                })(.SD)),
      by = a]

## but I want to return a data.frame from the
## anonymous function

tmpdt[ , list(testout =
                (function(x) {
                    model <- lm(c ~ d, x)
                    tmpresid <- residuals(model)
                    tmpvalue <- x$b[as.numeric(names(tmpresid))]
                    data.frame(tmpvalue, tmpresid)
                })(.SD)),
      by = a]

Th第二个版本不起作用，因为该函数返回 data.frame 而不是向量。是否有任何方法可以在不编写data.table j 参数之外编写函数调用的情况下完成这项工作？

The second version doesn't work because the function returns a data.frame instead of just a vector. Is there any way to make this work without writing the function call outside of the data.table j argument?

推荐答案

您不需要匿名函数-您可以将要包装在 {} （匿名 body ）in j ：

You don't need anonymous functions - you can have whatever expression you want wrapped in { } (anonymous body) in j:

tmpdt[, {
          model <- lm(c ~ d, .SD)
          tmpresid <- residuals(model)
          tmpvalue <- b[as.numeric(names(tmpresid))]
          list(tmpvalue, tmpresid) # every element of the list becomes a column in result
        }
      , by = a]

---

在 {} 的一些文档> j ：

---

Some documentation on the use of anonymous body { } in j:

1. 在中的示例中添加注释？ data.table ：

1. Comment in Examples in ?data.table:

中的匿名lambda j ： j 接受任何有效的表达式。要记住：列表的每个元素都会成为结果列。

anonymous lambda in j: j accepts any valid expression. TO REMEMBER: every element of the list becomes a column in result.

1. data.table 常见问题解答2.8 j 表达式？

1. data.table FAQ 2.8 What are the scoping rules for j expressions?

没有匿名函数传递给 j 。而是将匿名 body [ {} ]传递给 j [... ]某些编程语言将其称为 lambda 。

No anonymous function is passed to j. Instead, an anonymous body [{ }] is passed to j [...] Some programming languages call this a lambda.

1. 安德鲁·布鲁克斯（Andrew Brooks）关于在 j 中使用 {} 的博客文章：使用{}
$ b $抑制中间输出b

1. Blog post by Andrew Brooks on the use of { } in j: Suppressing intermediate output with {}

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