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 <balazs...@gmail.com <javascript:>
> > 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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