Get the current element and the list containing everything else in Clojure?

Question

What is the fastest way to get the list containing pairs of the current element with the list containing every other element? This should be fast as the list could contain a million elements or more.

For example given the list (1 2 3)

I want to get the list ((1 (2 3)) (2 (1 3)) (3 (1 2)))

Thanks for your help!

Answer 1

This will work without traversing the entire vector over and over:

(defn this-and-that [xs]
  (map-indexed (fn [i x]
                [x (concat (take i xs) 
                           (drop (inc i) xs))])
              xs))

and also works for nils:

user=> (this-and-that [1 2 nil 3])
([1 (2 nil 3)] [2 (1 nil 3)] [nil (1 2 3)] [3 (1 2 nil)])