On Sat, Nov 27, 2010 at 5:53 PM, Benny Tsai <benny.t...@gmail.com> wrote:
> If you don't mind, I would love to see your version with lazy-seq and
> recursion. Seems like that's the idiomatic way of solving problems
> like this, judging by the source for the partition functions.
Hi Benny,
Certainly. My solution is appended at the bottom.
Here is some background on this. I frequently run into this problem
where I have lines and I have to group them based on some marker. A
simple example is a file containing mail messages in which lines that
begin with "^From " mark the start of each new message. To parse this
requires a way to start with a sequence of undifferentiated lines and
turn that into a sequence of collections of lines each representing a
message.
And there are really two versions of this partition: partition-before
and partition-after: partition-before would put the true with the
falses following it, and partition-after will group it with the falses
preceding it.
Here is the solution I have for partition-before. The function
partition-before-internal is almost like partition-before except that
it always puts a list of items preceding the first true as its first
element. If the first element of coll is true then it sticks an empty
list here.
(defn partition-before-internal [pred coll]
(let [first-item (first coll)
second-item (second coll)
rest-items (rest coll)]
(if (nil? first-item) ()
(if (nil? second-item) (list () (list first-item))
(letfn [(get-partial-partition-with-item []
(let [partial-partition (partition-before-internal
pred rest-items)]
(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-using-predicate
(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))))
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