[R] Efficient swapping

[Gang Chen]

Suppose that we have the following dataframe:

set.seed(1)
(tmp <- data.frame(x = 1:10, R1 = sample(LETTERS[1:5], 10, replace =
TRUE), R2 = sample(LETTERS[1:5], 10, replace = TRUE)))

    x R1 R2
1   1  B  B
2   2  B  A
3   3  C  D
4   4  E  B
5   5  B  D
6   6  E  C
7   7  E  D
8   8  D  E
9   9  D  B
10 10  A  D

I want to do the following: if the difference between the level index
of factor R1 and that of factor R2 is an odd number, the levels of the
two factors need to be switched between them, which can be performed
through the following code:

for(ii in 1:dim(tmp)[1]) {
   kk <- which(levels(tmp$R2) %in% tmp[ii,'R2'], arr.ind = TRUE) -
which(levels(tmp$R1) %in% tmp[ii,'R1'], arr.ind = TRUE)
   if(kk%%2!=0) { # swap the their levels between the two factors
      qq <- tmp[ii,]$R1
      tmp[ii,]$R1 <- tmp[ii,]$R2
      tmp[ii,]$R2 <- qq
  }
}

More concise and efficient way to do this?

Thanks,
Gang