Changing map behaviour in Clojure

Question

I need to modify map function behavior to provide mapping not with minimum collection size but with maximum and use zero for missing elements.

Standard behavior:

Answer 1

Your method is concise, but inefficient (it calls count). A more efficient solution, which does not require the entirety of its input sequences to be stored in memory follows:

(defn map-pad [f pad & colls]
  (lazy-seq
   (let [seqs (map seq colls)]
     (when (some identity seqs)
       (cons (apply f (map #(or (first %) pad) seqs))
             (apply map-pad f pad (map rest seqs)))))))

Used like this:

user=> (map-pad + 0 [] [1] [1 1] (range 1 10))
(3 3 3 4 5 6 7 8 9)

Edit: Generalized map-pad to arbitrary arity.