r - How to swap values between two columns

Question

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

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

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.

Answer 1

Try this

 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")] 

which yields:

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