r - 根据逻辑向量选择 data.table 中的列

Question

假设我有以下 data.frame 和以下 data.table:

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

使用 data.frame，我可以根据逻辑向量选择列，如下所示:

DF[,c(TRUE,TRUE,FALSE)]

结果是:

  x y
1 a 1
2 a 3
3 a 6
4 b 1
5 b 3
6 b 6
7 c 1
8 c 3
9 c 6

然而

DT[,c(TRUE,TRUE,FALSE)]

造成:

[1]  TRUE  TRUE FALSE

怎么做到呢？

Answer 1

更新 2020-04-22

在 data.table 的当前 CRAN 版本中, DT[ , c(TRUE, TRUE, FALSE)]会工作 - 不需要 with=FALSE .将这个较旧的答案留在这里供后代使用:

我们需要with=FALSE

DT[, c(TRUE, TRUE, FALSE), with=FALSE]

基于 ?data.table 中的文档

By default with=TRUE and j is evaluated within the frame of x; column names can be used as variables. When with=FALSE j is a character vector of column names or a numeric vector of column positions to select, and the value returned is always a data.table. with=FALSE is often useful in data.table to select columns dynamically.