Hi Asim,
Thank you for the code and the explanation of the context.
I noticed that partition-after could be implemented by taking my
previous code and incrementing all the 'true' indices by one. My
first stab:
(defn partition-after [pred coll]
(if (empty? coll)
'()
(let [indices (add-zero (map inc (index-filter pred coll)))
index-groups (partition-all 2 1 indices)]
(map #(apply subsequence coll %) index-groups))))
But this returns an extra list at the end if the last element is true,
so I decided to call (remove empty?) on the end result. This allowed
me to make several other simplifications. For your consideration, an
index-based implementation of partition-before and partition-after:
(use '[clojure.contrib.seq :only (indexed)])
(defn index-filter [pred coll]
(when pred
(for [[idx elt] (indexed coll) :when (pred elt)] idx)))
(defn subsequence
([coll start]
(drop start coll))
([coll start end]
(take (- end start) (drop start coll))))
(defn subsequences [coll indices]
(let [index-groups (partition-all 2 1 indices)
subseqs (map #(apply subsequence coll %) index-groups)]
(remove empty? subseqs)))
(defn partition-before [pred coll]
(let [indices (cons 0 (index-filter pred coll))]
(subsequences coll indices)))
(defn partition-after [pred coll]
(let [indices (cons 0 (map inc (index-filter pred coll)))]
(subsequences coll indices)))
On Nov 28, 12:17 pm, Asim Jalis <asimja...@gmail.com> wrote:
> Here is partition-before and partition-after -- partition-after is simpler.
>
> (defn partition-before-internal [pred coll]
> (let [first-item (first coll)]
> (if (nil? first-item) ()
> (letfn [(get-partial-partition-with-item []
> (let [partial-partition (partition-before-internal pred
> (rest coll))]
> (cons
> (cons
> first-item
> (first partial-partition))
> (rest partial-partition))))]
> (if (pred first-item)
> (cons () (lazy-seq (get-partial-partition-with-item)))
> (get-partial-partition-with-item))))))
>
> (defn partition-before [pred coll]
> (lazy-seq
> (let [partition-result (partition-before-internal pred coll)]
> (if (empty? (first partition-result))
> (rest partition-result)
> partition-result))))
>
> (deftest partitioning-before
> (are [input output] (= ',output (partition-before true? ',input))
> () ()
> (true) ((true))
> (false) ((false))
> (false false false true false true true true)
> ((false false false) (true false) (true) (true) (true))
> (true false false false true false true true true)
> ((true false false false) (true false) (true) (true) (true))))
>
> (defn partition-after [pred coll]
> (lazy-seq
> (let [first-item (first coll)]
> (if (nil? first-item) ()
> (let [partial-partition (lazy-seq (partition-after pred (rest coll)))]
> (if (pred first-item)
> (cons (list first-item) partial-partition)
> (cons
> (cons
> first-item
> (first partial-partition))
> (rest partial-partition))))))))
>
> (deftest partitioning-after
> (are [input output] (= ',output (partition-after true? ',input))
> () ()
> (true) ((true))
> (false) ((false))
> (false false false true false true true true)
> ((false false false true) (false true) (true) (true))
> (true false false false true false true true true)
> ((true) (false false false true) (false true) (true) (true))))
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