trh-seq-ideas.clj

;; the missing drop-nth, versions of which, similar to this, have been floating around the Clojure forum:
;;
(defn drop-nth [n coll]
  (lazy-seq
    (when-let [xs (seq coll)]
      (concat (take (dec n) xs) (drop-nth n (drop n xs)))
    )
  )
)

;; A sub sequence function based on pretending the sequence is indexed (ala 'subvec').
;; Authors: Tom Hicks and Tim Pratley.
;;
(defn indexed-subseq
   "Returns a lazy sequence of the items in the given sequence from
    start (inclusive) to end (exclusive). If end is not supplied,
    defaults to (count sequence)."
  ([coll start]
    (when (not (neg? start))
      (drop start coll))
  )
  ([coll start end]
    (when (and (not (neg? start)) (<= start end))
      (take (- end start) (drop start coll)))
  )
)

;; some tests
; (def v (vec (range 1 8)))
; (indexed-subseq v 4)
; = (5 6 7)
; (map #(indexed-subseq v %) (range 8))
; = ((1 2 3 4 5 6 7) (2 3 4 5 6 7) (3 4 5 6 7) (4 5 6 7) (5 6 7) (6 7) (7) ())
; (map #(indexed-subseq v %1 %2) (repeat 0) (range 9))
; = (() (1) (1 2) (1 2 3) (1 2 3 4) (1 2 3 4 5) (1 2 3 4 5 6) (1 2 3 4 5 6 7) (1 2 3 4 5 6 7))
; (indexed-subseq [0 1 2 3 4] 1 3)
; = (1 2)
; (indexed-subseq [0 1 2 3 4] -5 3)
; = nil
; (indexed-subseq [0 1 2 3 4] 4 3)
; = nil
; (indexed-subseq [0 1 2 3 4] 2)
; = (2 3 4)