更改data.table的列位置 [英] Change column position of data.table

问题描述

我正在学习使用data.table包。我想做的事情之一是将最后一列（x）移动到第一列。这是我如何做一个数据框架：

I am learning to use the data.table package. One of the things I am trying to do is move the last column ("x") to the first column. Here is how I do it for a data frame:

DF <- cbind(x, DF[1:ncol(DF)]) #Rearrange columns

我读了setcolorder并尝试这个，但我得到一个错误

I read up on setcolorder and tried this, but I get an error

setcolorder(DT, c("x", 1: (length(DT)-1) ) )

有人知道更好的解决方案吗？

Does anyone know a better solution?

推荐答案

选项1

也许您可以使用 setdiff ：

DT <- data.table(A = 1:2, B = 3:4, X = 5:6)
DT
#    A B X
# 1: 1 3 5
# 2: 2 4 6
setcolorder(DT, c("X", setdiff(names(DT), "X")))
DT
#    X A B
# 1: 5 1 3
# 2: 6 2 4

---

选项2

使用修改版本的方法：

---

Option 2

Using a modified version of your approach:

setcolorder(DT, c("X", names(DT)[1:(length(DT)-1)]))

或

setcolorder(DT, c(length(DT), 1:(length(DT)-1)))

$ b b

为什么你的方法中的错误？您试图包括列名称和数字列索引。

Why the error in your approach? You were trying to include both column names and numeric column indices. Use one or the other, but not both.

我写了一个函数 moveme （暂时你可以找到在此Gist 或在我的博客）。您输入一个移动命令字符串，用分号分隔。这使得你可以灵活地在你的列周围移动：

I've written a function called moveme (which for the time being you can find at this Gist or at my blog). You enter a string of "move" commands, separated by semicolons. This lets you shuffle around your columns pretty flexibly:

DT <- data.table(matrix(1:20, ncol = 10, dimnames = list(NULL, LETTERS[1:10])))
DT
#    A B C D  E  F  G  H  I  J
# 1: 1 3 5 7  9 11 13 15 17 19
# 2: 2 4 6 8 10 12 14 16 18 20

setcolorder(DT, moveme(names(DT), "E, F first; B last; H, I before G"))
# DT
#     E  F A C D  H  I  G  J B
# 1:  9 11 1 5 7 15 17 13 19 3
# 2: 10 12 2 6 8 16 18 14 20 4

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