Remove elements in a component of a list that already appeared in earlier components in R

I have a list with each component being a numeric vector. There are some common numbers in the components. Here is an example:

ls <- list(c(7, 4, 9), c(5, 9, 2, 19), c(3, 13, 4, 2))

Component 2 has its second element already appeared in component 1. Component 3 has its 3rd and 4th element already appeared in component 1 and 2, respectively. I want to remove the elements in later component that already appeared in earlier component. For the specific example, I want ls2 <- list(c(7, 4, 9), c(5, 2, 19), c(3, 13)).

I wish to write an R script that automatically executes the above task, which generalizes to other lists.

>Solution :

Use setdiff:

ls <- list(c(7, 4, 9), c(5, 9, 2, 19), c(3, 13, 4, 2))

foo <- function(x) {
  for (i in seq_along(x)[-1]) {
    x[[i]] <- setdiff(ls[[i]], unlist(ls[seq_len(i - 1)]))
  }
  x
}

ls2 <- foo(ls)
#[[1]]
#[1] 7 4 9
#
#[[2]]
#[1]  5  2 19
#
#[[3]]
#[1]  3 13