Created
January 7, 2011 17:14
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Clojure - Split a collection into partitions on boundaries specified by supplied function.
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(defn partition-between | |
"Splits coll into a lazy sequence of lists, with partition | |
boundaries between items where (f item1 item2) is true. | |
(partition-between = '(1 2 2 3 4 4 4 5)) => | |
((1 2) (2 3 4) (4) (4 5))" | |
[f coll] | |
(lazy-seq | |
(when-let [s (seq coll)] | |
(let [fst (first s)] | |
(if-let [rest-seq (next s)] | |
(if (f fst (first rest-seq)) | |
(cons (list fst) (partition-between f rest-seq)) | |
(let [rest-part (partition-between f rest-seq)] | |
(cons (cons fst (first rest-part)) (rest rest-part)))) | |
(list (list fst))))))) | |
; more idiomatic perhaps, but 3 times slower than the above | |
(defn partition-between | |
"Splits coll into a lazy sequence of lists, with partition | |
boundaries between items where (f item1 item2) is true. | |
(partition-between = '(1 2 2 3 4 4 4 5)) => | |
((1 2) (2 3 4) (4) (4 5))" | |
[f coll] | |
(lazy-seq | |
(if-not (seq coll) | |
'() | |
(let [pairs (map list coll (rest coll)) | |
[h t] (map #(map last %) (split-with #(not (apply f %)) pairs))] | |
(cons (cons (first coll) h) (partition-between f t)))))) |
Shorter one from StackOverflow:
(defn partition-between [pred? coll]
(let [switch (reductions not= true (map pred? coll (rest coll)))]
(map (partial map first) (partition-by second (map list coll switch)))))
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It's the third time I used it in 4clojure excersises. Can I use something similar from stdlib?
Ok. I for that last one I could use (partition-by identity _).