This is clearer to read though:
(defn partition-when [f l]
(reduce #(if (f %2)
(conj %1 (vector %2))
(conj (vec (butlast %1))
(conj (vec (last %1)) %2)))
[] (vec l)))
On Sat, Nov 27, 2010 at 4:47 PM, rob levy <r.p.l...@gmail.com> wrote:
> This is more correct, because conj does not preserve the order correctly.
> I wonder if there is a more idiomatic way to cons things onto the end of
> the list.
>
>
> (defn partition-when [f l]
> (reduce #(if (f %2)
> (concat %1 (vector (vector %2)))
> (concat (butlast %1)
> (vector (concat (last %1) (vector %2)))))
> [] l))
>
>
> On Sat, Nov 27, 2010 at 4:18 PM, rob levy <r.p.l...@gmail.com> wrote:
>
>> Something like this?
>>
>> (defn partition-when [f l]
>> (reduce #(if (f %2)
>> (conj %1 (vector %2))
>> (conj (butlast %1)
>> (conj (last %1) %2)))
>> [] l))
>>
>> On Sat, Nov 27, 2010 at 4:15 PM, Ken Wesson <kwess...@gmail.com> wrote:
>>
>>> On Sat, Nov 27, 2010 at 4:00 PM, rob levy <r.p.l...@gmail.com> 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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>>
>>
>
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