可以从.data.table（）中的浏览器查看.SD吗？ [英] Can .SD be viewed from a browser within [.data.table()?

问题描述

在构建表达式以放入 [。data.table 调用的 j 通常有助于检查和玩弄 .SD 的内容。

While constructing expressions to put in the j-slot of a [.data.table call, it would often be helpful to be able to examine and play around with the contents of .SD.

这种天真的尝试't work ...

This naive attempt doesn't work...

library(data.table)
DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)

DT[, browser(), by=x]
# Called from: `[.data.table`(DT, , browser(), by = x)
Browse[1]> 
Browse[1]> .SD
# NULL data.table

...即使一个名为 .SD 以及与当前data.table子集相关的其他几个都存在于本地环境中

... even though a variable named .SD and several others related to the current data.table subset are all present in the local environment

Browse[1]> ls(all.names = TRUE)
#  [1] ".BY"       ".GRP"      ".I"        ".iSD"      ".N"        ".SD"      
#  [7] "Cfastmean" "mean"      "print"     "x"        
Browse[1]> .N
# [1] 3
Browse[1]> .I
# [1] 4 5 6

使用。 I ，我可以查看一些+/-像 .SD ，但它是很好能够直接访问它的值：

Using .I, I can view something +/- like .SD, but it would be nice to be able to directly access its value:

Browse[1]> DT[.I]
#    x y v
# 1: b 1 4
# 2: b 3 5
# 3: b 6 6

我的问题：为什么 .SD 不能直接从浏览器（）调用（ .I ， .N ， .GRP 和 .BY are）？是否有其他方法可以访问 .SD ？

My questions: Why is the expected value of .SD not directly available from within a browser() call (while .I, .N, .GRP and .BY are)? Is there some alternative way to access the value of .SD?

推荐答案

> 根据Matthew Dowle的评论更新：

Updated in light of Matthew Dowle's comments:

结果是 .SD 是内部评估所有 j 表达式的环境，包括未明确引用 .SD的环境。为 DT 的每个子集填充所有 DT 的列不便宜，因此 [。data.table（）不会这样做，除非它真的需要。

It turns out that .SD is, internally, the environment within which all j expressions are evaluated, including those which don't explicitly reference .SD at all. Filling it with all of DT's columns for each subset of DT is not cheap, timewise, so [.data.table() won't do so unless it really needs to.

相反，充分利用R对参数的延迟评估，它预览了未评估的 j 表达式，添加到其中引用的 .SD 列。如果提及 .SD 本身，则会添加 DT 的所有列。

Instead, making great use of R's lazy-evaluation of arguments, it previews the unevaluated j expression, and only adds to .SD columns that are referenced therein. If .SD itself is mentioned, it adds all of DT's columns.

因此，要查看 .SD ，只需在 j 表达。这里是许多表达式中的一个将工作：

So, to view .SD, just include some reference to it in the j-expression. Here is one of many expressions that will work:

library(data.table)
DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)

## This works
DT[, if(nrow(.SD)) browser(), by=x]
# Called from: `[.data.table`(DT, , if (nrow(.SD)) browser(), by = x)
Browse[1]> .SD
#    y v
# 1: 1 1
# 2: 3 2
# 3: 6 3

这里有几个：

DT[,{.SD; browser()}, by=x]
DT[,{browser(); .SD}, by=x]  ## Notice that order doesn't matter

.SD 只是加载 j 表达式所需的列，依次运行这些列（键入 .SD 进入浏览器环境， Q 离开并返回正常的命令行）：

To see for yourself that .SD just loads columns needed by the j-expression, run these each in turn (typing .SD when entering the browser environment, and Q to leave it and return to the normal command-line):

DT[, {.N * y ; browser()}, by=x]
DT[, {v^2 ; browser()}, by=x]
DT[, {y*v ; browser()}, by=x]

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