如何重新排序 data.table 列(不复制) [英] How to reorder data.table columns (without copying)

问题描述

我想重新排序我的 data.table x 中的列，给定列名的字符向量 neworder:

I'd like to reorder columns in my data.table x, given a character vector of column names, neworder:

library(data.table)
x <- data.table(a = 1:3, b = 3:1, c = runif(3))
neworder <- c("c", "b", "a")

显然我可以做到:

x[ , neworder, with = FALSE]
# or
x[ , ..neworder]
#            c b a
# 1: 0.8476623 3 1
# 2: 0.4787768 2 2
# 3: 0.3570803 1 3

但这需要再次复制整个数据集.有没有其他方法可以做到这一点?

but that would require copying the entire dataset again. Is there another way to do this?

推荐答案

使用setcolorder():

library(data.table)
x <- data.table(a = 1:3, b = 3:1, c = runif(3))
x
#      a b         c
# [1,] 1 3 0.2880365
# [2,] 2 2 0.7785115
# [3,] 3 1 0.3297416
setcolorder(x, c("c", "b", "a"))
x
#              c b a
# [1,] 0.2880365 3 1
# [2,] 0.7785115 2 2
# [3,] 0.3297416 1 3

来自?setcolorder:

用 data.table 的说法，所有 set* 函数都通过引用改变它们的输入.也就是说，除了一列大小的临时工作内存之外，根本不进行任何复制.

In data.table parlance, all set* functions change their input by reference. That is, no copy is made at all, other than temporary working memory, which is as large as one column.

所以应该非常有效.详见?setcolorder.

so should be pretty efficient. See ?setcolorder for details.

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