data.table:当列名未知时创建条件变量的正确方法? [英] data.table: Proper way to do create a conditional variable when column names are not known?

问题描述

我的问题与创建一个变量有关，该变量依赖于 data.table 中的其他列，而事先不知道任何变量名称.

My question relates to the creation of a variable which depends upon other columns within a data.table when none of the variable names are known in advance.

下面是一个玩具示例，其中我有 5 行，当条件等于 A 时新变量应为 1，否则为 4.

Below is a toy example where I have 5 rows and the new variable should be 1 when the condition is equal to A and 4 elsewise.

library(data.table)
DT <- data.table(Con = c("A","A","B","A","B"),
                 Eval_A = rep(1,5),
                 Eval_B = rep(4,5))
Col1 <- "Con"
Col2 <- "Eval_A"
Col3 <- "Eval_B"
Col4 <- "Ans"

下面的代码有效，但感觉就像我在滥用包！

The code below works but feels like I'm misusing the package!

DT[,Col4:=ifelse(DT[[Col1]]=="A",
                 DT[[Col2]],
                 DT[[Col3]]),with=FALSE]

更新:谢谢，我对下面的答案做了一些快速的计时.一次在具有 500 万行且只有相关列的 data.table 上，然后在添加 10 个不相关列后，结果如下:

Update: Thanks, I did some quick timing of the answers below. Once on a data.table with 5 million rows and only the relevant columns and again after adding 10 non relevant columns, below are the results:

+-------------------------+---------------------+------------------+
|         Method          | Only relevant cols. | With extra cols. |
+-------------------------+---------------------+------------------+
| List method             | 1.8                 | 1.91             |
| Grothendieck - get/if   | 26.79               | 30.04            |
| Grothendieck - get/join | 0.48                | 1.56             |
| Grothendieck - .SDCols  | 0.38                | 0.79             |
| agstudy - Substitute    | 2.03                | 1.9              |
+-------------------------+---------------------+------------------+

看起来像 .SDCols 最适合速度和使用易于阅读的代码的替代品.

Look's like .SDCols is best for speed and using substitute for easy to read code.

推荐答案

1.获取/如果尝试使用 get :

DT[, (Col4) := if (get(Col1) == "A") get(Col2) else get(Col3), by = 1:nrow(DT)]

<强>2.获取/加入或尝试这种方法:

setkeyv(DT, Col1)
DT[, (Col4):=get(Col3)]["A", (Col4):=get(Col2)]

3..SDCols或者这个:

setkeyv(DT, Col1)
DT[, (Col4):=.SD, .SDcols = Col3]["A", (Col4):=.SD, .SDcols = Col2]

更新:添加了一些额外的方法.

UPDATE: Added some additional approaches.

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