Question

I have numeric vectors, such as c(1, 2, 3, 3, 2, 1, 3) or c(1, 4, 1, 4, 4, 1), and I would like to keep individual element’s position, but swap/reverse the value, so that we get c(3, 2, 1, 1, 2, 3, 1), c(4, 1, 4, 1, 1, 4) respectively.

To achieve that, I came up with a rather rough and ugly code below with lots of debugging and patching…

blah <- c(1, 4, 1, 4, 4, 1, 3)
blah.uniq <- sort(unique(blah))
blah.uniq.len <- length(blah.uniq)
j <- 1
end <- ceiling(blah.uniq.len / 2)
if(end == 1) {end <- 2} # special case like c(1,4,1), should get c(4,1,4) 
for(i in blah.uniq.len:end) {
  x <- blah == blah.uniq[i]
  y <- blah == blah.uniq[j]
  blah[x] <- blah.uniq[j]
  blah[y] <- blah.uniq[i]
  j = j + 1
}
blah

Is there an easier way to do this?

Answer 1

Congratulations! You might have finally found a use for factors , I was still looking for one 🙂

x <- c(1, 2, 3, 3, 2, 1, 3)
# [1] 1 2 3 3 2 1 3
y <- factor(x)
# [1] 1 2 3 3 2 1 3
# Levels: 1 2 3
levels(y) <- rev(levels(y))
# [1] 3 2 1 1 2 3 1
# Levels: 3 2 1

Built on that idea, here is a function that returns an object with the same class as the input:

swap <- function(x) {
    f <- factor(x)
    y <- rev(levels(f))[f]
    class(y) <- class(x)
    return(y)
}
swap(c(1, 2, 3, 3, 2, 1, 3))
# [1] 3 2 1 1 2 3 1
swap(c(1, 4, 1, 4, 4, 1))
# [1] 4 1 4 1 1 4