Am 17.01.2012 22:17, schrieb Cedric Greevey:
> On Tue, Jan 17, 2012 at 3:46 PM, Dennis Haupt <d.haup...@googlemail.com>
> wrote:
>> after the "wtf"s have worn off a bit, go on reading.
>> imagine a simple problem: you have a collection of numbers and you have
>> to write a function which collects all the numbers that are contained
>> uneven times. for example, for a collection (1,2,3,2,3,4) the correct
>> result is (1,4)
>>
>> ask a child:
>> "you just make pairs and return the leftovers"
>>
>> ask a non-child human:
>> "count how often a number is in the list and pick the ones that are
>> contained uneven times"
>>
>> i also asked some oo programmers. one of the answers involved a
>> multimap, building it, iterating over it again and putting the result in
>> a new list. one answer was "i don't see any purpose in this".
>> there was no FP coder around at that time.
>
> FWIW,
>
> (defn odd-occurrences [x]
> (map first
> (filter
> (fn [[_ v]] (odd? v))
> (frequencies x))))
>
> Signed,
> An FP coder
>
> :)
>
it's interesting that the FP solutions translate almost 1:1 to the
explained task
//non child's solution
def odds(numbers:Seq[Int]) =
numbers.distinct.filter(e => numbers.count(_ == e) % 2 =! 0)
//child's solution - more efficient
def oddsViaFold(numbers: Seq[Int]) = {
numbers.foldLeft(HashSet.empty[Int])((acc, e) => {
// store those which are waiting for a second number in a set
if (acc.contains(e)) acc - e else acc + e
})
}
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