Returns a lazy sequence of the non-nil results of (f item). Note,
this means false return values will be included. f must be free of
side-effects.
;; comparisons among keep, map and for. user> (keep #(if (odd? %) %) (range 10)) (1 3 5 7 9) user> (map #(if (odd? %) %) (range 10)) (nil 1 nil 3 nil 5 nil 7 nil 9) user> (for [ x (range 10) :when (odd? x)] x) (1 3 5 7 9) user> (keep #(if(even? %) %) (range 10)) (0 2 4 6 8) user> (map #(if (even? %) %) (range 10)) (0 nil 2 nil 4 nil 6 nil 8 nil) user> (for [ x (range 10) :when (even? x)] x) (0 2 4 6 8)
;; Sieve of Eratosthenes by using 'keep'.
(defn keep-mcdr [f coll]
(lazy-seq
(when-let [x (first coll)]
(cons x (keep-mcdr f (f x (rest coll)))))))
(defn prime-number [n]
(cons 1
(keep-mcdr
(fn[x xs] (if (not-empty xs)
(keep #(if-not (zero? (rem % x)) %)
xs)))
(range 2 n))))
user> (prime-number 100)
(1 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97)
Source
clojure/core.clj:5638
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(defn keep
"Returns a lazy sequence of the non-nil results of (f item). Note,
this means false return values will be included. f must be free of
side-effects."
{:added "1.2"}
([f coll]
(lazy-seq
(when-let [s (seq coll)]
(if (chunked-seq? s)
(let [c (chunk-first s)
size (count c)
b (chunk-buffer size)]
(dotimes [i size]
(let [x (f (.nth c i))]
(when-not (nil? x)
(chunk-append b x))))
(chunk-cons (chunk b) (keep f (chunk-rest s))))
(let [x (f (first s))]
(if (nil? x)
(keep f (rest s))
(cons x (keep f (rest s))))))))))
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