如何根据R中data.table中具有对称值的两列删除某些行？ [英] How can I delete certain rows according to two columns which have symmetricl values in data.table in R?

问题描述

例如，我有一张表，如下所示：

For example, I have a table as follows:

DT <- data.table(
  A = c(1,1,1,2,2,2,3,3,3), 
  B = c(1,2,3,1,2,3,1,2,3),
  key = "A"
)

我想删除下面的行条件，例如 A == 2和 B == 1，因为已经存在 A == 1和 B == 2的行。

I wand to delete the rows under the conditon such as "A" == 2 and "B" == 1, since there is already the row that "A" == 1 and "B" == 2.

简而言之，我想删除在前几行中已经对称出现的行，我该如何实现？

In short, I want to delete the rows that already appears symmetrically in the previous rows, how can I realize it?

推荐答案

另一种选择：

DT[, g := paste(B, A, sep="_")][A < B, g := paste(A, B, sep="_")][!duplicated(g), !"g"]

   A B
1: 1 1
2: 1 2
3: 1 3
4: 2 2
5: 2 3
6: 3 3

所以...

1. 将分组变量设为A + B ，


2. 将子集A<上的订单翻转为B +A。 B或A> B


3. 对分组变量进行重复

最后一步可以是 unique（DT，by = g）。

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