I didn't know that reduction can be stopped with reduced. Thanks Andy!
On Sunday, June 26, 2016 at 10:08:47 PM UTC+2, Andy- wrote:
>
> To add a little variant of James Reeve's code: You can avoid the
> loop/recur by using reduce:
>
>
> (defn valid-parens? [s]
> (let [opening-brackets {\( \), \[ \], \{ \}}
> closing-brackets (clojure.set/map-invert opening-brackets)]
> (empty?
> (reduce
> (fn [stack c]
> (if (opening-brackets c)
> (conj stack c)
> (if-let [b (closing-brackets c)]
> (if (= (peek stack) b)
> (pop stack)
> (reduced [false]))
> stack)))
> []
> (seq s)))))
>
>
> HTH
>
> On Sunday, June 26, 2016 at 4:06:27 PM UTC+2, Botond Balázs wrote:
>>
>> Thanks guys.
>>
>> Very nice and simple solution, James.
>>
>> On Sunday, June 26, 2016 at 3:21:56 PM UTC+2, James Reeves wrote:
>>>
>>> The easiest way would be to factor out the bracket matching code, since
>>> that's the only thing that changes between your different case statements.
>>> For instance:
>>>
>>> (defn valid-parens? [s]
>>> (let [opening-brackets {\( \), \[ \], \{ \}}
>>> closing-brackets (clojure.set/map-invert opening-brackets)]
>>> (loop [cs (seq s), stack []]
>>> (if-not cs
>>> (empty? stack)
>>> (let [c (first cs), cs (next cs)]
>>> (if (opening-brackets c)
>>> (recur cs (conj stack c))
>>> (if-let [b (closing-brackets c)]
>>> (if (= (peek stack) b)
>>> (recur cs (pop stack))
>>> false)
>>> (recur cs stack)))))))
>>>
>>> Another way you could do it is to replace your recursive call with a
>>> continuation.
>>>
>>> - James
>>>
>>> On 26 June 2016 at 13:43, Botond Balázs <[email protected]> wrote:
>>>
>>>> Hi,
>>>>
>>>> Here is my solution to 4clojure problem #177:
>>>>
>>>> Write a function that takes in a string and returns truthy if all
>>>>> square [ ] round ( ) and curly { } brackets are properly paired and
>>>>> legally
>>>>> nested, or returns falsey otherwise.
>>>>>
>>>>
>>>> (defn valid-parens?
>>>> [s]
>>>> (loop [[ch & chs] s stack []]
>>>> (if (nil? ch)
>>>> (empty? stack)
>>>> (case ch
>>>> (\( \[ \{) (recur chs (conj stack ch))
>>>> \) (if (= (peek stack) \()
>>>> (recur chs (pop stack))
>>>> false)
>>>> \] (if (= (peek stack) \[)
>>>> (recur chs (pop stack))
>>>> false)
>>>> \} (if (= (peek stack) \{)
>>>> (recur chs (pop stack))
>>>> false)
>>>> (recur chs stack)))))
>>>>
>>>> This works but I don't like the repetition in the case branches. But
>>>> the repeated code contains a recur call so I can't simply refactor it
>>>> into a helper function. How can I achieves something like the following,
>>>> but without consuming the stack?
>>>>
>>>> (defn valid-parens?
>>>> ([s]
>>>> (valid-parens? s []))
>>>> ([[ch & chs] stack]
>>>> (if (nil? ch)
>>>> (empty? stack)
>>>> (letfn [(match? [p]
>>>> (if (= (peek stack) p)
>>>> (valid-parens? chs (pop stack))
>>>> false))]
>>>> (case ch
>>>> (\( \[ \{) (valid-parens? chs (conj stack ch))
>>>> \) (match? \()
>>>> \] (match? \[)
>>>> \} (match? \{)
>>>> (valid-parens? chs stack))))))
>>>>
>>>> Thanks!
>>>> Botond
>>>>
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>>>
>>>
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