Remove duplicated elements from list

Question

I have a list of character vectors:

my.list <- list(e1 = c("a","b","c","k"),e2 = c("b","d","e"),e3 = c("t","d","g","a","f"))

And I'm looking for a function that for any character that appears more than once across the list's vectors (in each vector a character can only appear once), will only keep the first appearance.

The result list for this example would therefore be:

res.list <- list(e1 = c("a","b","c","k"),e2 = c("d","e"),e3 = c("t","g","f"))

Note that it is possible that an entire vector in the list is eliminated so that the number of elements in the resulting list doesn't necessarily have to be equal to the input list.

Answer 1

We can unlist the list, get a logical list using duplicated and extract the elements in 'my.list' based on the logical index

un <- unlist(my.list)
res <- Map(`[`, my.list, relist(!duplicated(un), skeleton = my.list))
identical(res, res.list)
#[1] TRUE