如何在两列之间交换值 [英] How to swap values between two columns

问题描述

我有一个包含三个变量和250K记录的数据框。例如

I have a data frame with three variables and 250K records. As an example consider

df <- data.frame(V1=c(1,2,4), V2=c("a","a","b"), V3=c(2,3,1))
V1 V2 V3
1  a  2
2  a  3
4  b  1

，并希望基于V2的值在V1和V3之间交换值，如下所示：
如果 V2 =='b'，则 V1 <-V3 和 V3<-V1
导致

and want to swap values between V1 and V3 based on the value of V2 as follows: if V2 == 'b' then V1 <- V3 and V3 <- V1 resulting in

V1 V2 V3
1  a  2
2  a  3
1  b  4

我尝试了do循环但这需要永远。如果我使用Perl，则需要几秒钟。我相信在R中也可以有效地完成此任务。谢谢您的任何建议。

I tried a do loop but it takes forever. If I use Perl, it takes seconds. I believe this task can be done efficiently in R as well. Any suggestions are appreciated.

推荐答案

尝试一下

 df <- data.frame(V1=c(1,2,4), V2=c("a","a","b"), V3=c(2,3,1))
 df[df$V2 == "b", c("V1", "V3")] <- df[df$V2 == "b", c("V3", "V1")] 

其结果是：

> df
  V1 V2 V3
1  1  a  2
2  2  a  3
3  1  b  4

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