how may I remove elements in a string vector by elements in another string vector

Question

setdiff(c("a","c"),c("a","b","c","d"))
 #character(0)

This code is supposed to return a vector of c("b","d"), what's wrong with it?

Answer 1

setdiff is asymmetric, as the help page warns about (though subtly).

This works as you expect,

> setdiff(c("a","b","c","d"),c("a","c"))
[1] "b" "d"

A simple function works either way,

setdiff2 <- function(x,y){
  d1 <- setdiff(x,y)
  d2 <- setdiff(y,x)
  if(length(d2) > length(d1))
    return(d2)
  else
    return(d1)
}

> setdiff2(c("a","c"), c("a","b","c","d"))
[1] "b" "d"