On Sat, Nov 27, 2010 at 4:00 PM, rob levy <[email protected]> wrote:
> partition-by does exactly what you need.
Not quite.
user=> (take 10 (partition-by #(= 0 (rem % 3)) (iterate inc 1)))
((1 2)
(3)
(4 5)
(6)
(7 8)
(9)
(10 11)
(12)
(13 14)
(15))
At first it seems you can fix this as follows:
user=> (defn questionable-split-when [pred coll]
(let [p (partition-by pred coll)]
(cons (first p) (map #(apply concat %) (partition 2 2 []
(rest p))))))
#'user/questionable-split-when
user=> (take 10 (questionable-split-when #(= 0 (rem % 3)) (iterate inc 1)))
((1 2)
(3 4 5)
(6 7 8)
(9 10 11)
(12 13 14)
(15 16 17)
(18 19 20)
(21 22 23)
(24 25 26)
(27 28 29))
So far, so good. But:
user=> (questionable-split-when #(= 0 (rem % 3)) [1 2 3 6 7 8 9])
((1 2)
(3 6 7 8)
(9))
Whereas:
user=> (split-when #(= 0 (rem % 3)) [1 2 3 6 7 8 9])
((1 2)
(3)
(6 7 8)
(9))
The latter correctly starts a new partition each time the pred is
true; the former fails if the pred is ever true twice in a row.
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