f <- function(x, y){
  n <- length(x)
  out <- numeric(n)
  for(i in seq_along(x)){
    if(x[i] %% 2 == 0){
      out[i] <- x[i]/y
    } else {
      out[i] <- x[i]*y
    }
   }
  out
 }

f = function(x, y) {
  evens = (x %% 2) == 0
  x[evens] = x[evens] / y
  x[!evens] = x[!evens] * y
  return(x)
}