r - 如何重新排序 data.table 列？

Question

如何置换 a 中的列data.table？我可以为 a 做到这一点data.frame，但data.table会覆盖该方法：

> df <- data.frame(a=1:3,b=4:6)
> df
  a b
1 1 4
2 2 5
3 3 6
> df[c("b","a")]
  b a
1 4 1
2 5 2
3 6 3
> dt <- as.data.table(df)
> dt
   a b
1: 1 4
2: 2 5
3: 3 6
> dt[c("b","a")]
Error in `[.data.table`(dt, c("b", "a")) : 
  When i is a data.table (or character vector), x must be keyed (i.e. sorted, and, marked as sorted) so data.table knows which columns to join to and take advantage of x being sorted. Call setkey(x,...) first, see ?setkey.
Calls: [ -> [.data.table

请注意，这不是对如何在 R 中重新排序列？.

Answer 1

使用setcolorder：

> library(data.table)
> dt <- data.table(a=1:3,b=4:6)
> setcolorder(dt, c("b", "a"))
> dt
   b a
1: 4 1
2: 5 2
3: 6 3

Answer 2

这就是你的做法data.table（不修改原始表）：

dt[, list(b, a)]

或者

dt[, c("b", "a")]

或者

dt[, c(2, 1)]