user=> (partition 4 (range 20)) ((0 1 2 3) (4 5 6 7) (8 9 10 11) (12 13 14 15) (16 17 18 19)) user=> (partition 4 (range 22)) ((0 1 2 3) (4 5 6 7) (8 9 10 11) (12 13 14 15) (16 17 18 19)) user=> (partition 4 6 (range 20)) ((0 1 2 3) (6 7 8 9) (12 13 14 15)) user=> (partition 4 6 ["a"] (range 20)) ((0 1 2 3) (6 7 8 9) (12 13 14 15) (18 19 "a")) user=> (partition 4 6 ["a" "b" "c" "d"] (range 20)) ((0 1 2 3) (6 7 8 9) (12 13 14 15) (18 19 "a" "b"))
;; When there are less than n items in the coll, partition's behaviour ;; depends on whether there is a pad or not ;; without pad user> (partition 10 [1 2 3 4]) () ;; again, without pad user> (partition 10 10 [1 2 3 4]) () ;; with a pad this time (note: the pad is an empty sequence) user> (partition 10 10 nil [1 2 3 4]) ((1 2 3 4)) ;; or, explicit empty sequence instead of nil user> (partition 10 10 [] [1 2 3 4]) ((1 2 3 4))
Source
clojure/core.clj:2765
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(defn partition
"Returns a lazy sequence of lists of n items each, at offsets step
apart. If step is not supplied, defaults to n, i.e. the partitions
do not overlap. If a pad collection is supplied, use its elements as
necessary to complete last partition upto n items. In case there are
not enough padding elements, return a partition with less than n items."
{:added "1.0"
:static true}
([n coll]
(partition n n coll))
([n step coll]
(lazy-seq
(when-let [s (seq coll)]
(let [p (doall (take n s))]
(when (= n (count p))
(cons p (partition n step (nthrest s step))))))))
([n step pad coll]
(lazy-seq
(when-let [s (seq coll)]
(let [p (doall (take n s))]
(if (= n (count p))
(cons p (partition n step pad (nthrest s step)))
(list (take n (concat p pad)))))))))
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