[英]recursively following values in a hash map clojure
I have a problem providing an edge case for the following recursive function that searches hash maps and other similar key,value storage.我在为以下递归函数提供边缘情况时遇到问题,该函数搜索哈希映射和其他类似的键值存储。
(def hashbrownies
{"Mary","Dave"
"Dave","Anne"
"Anne","Tim"})
current approach目前的方法
(defn recursive-lookup
[key-lst search-func conditional]
(let [next-key (search-func (first key-lst))]
(if (conditional next-key)
(reverse key-lst)
(recur (cons next-key key-lst) search-func conditional))))
examples that are working有效的例子
>> (recursive-lookup ["Mary"] #(hashbrownies %) (partial = nil))
=> ("Mary" "Dave" "Anne" "Tim")
>> (recursive-lookup ["Mary"] #(hashbrownies %) #(< (.length %) 4))
=> ("Mary" "Dave" "Anne")
Problematic:有问题:
>> (recursive-lookup ["Mary"] #(hashbrownies %) #(> (.length %) 4))
=> NullPointerException clojure.lang.Reflector.invokeNoArgInstanceMember (Reflector.java:296)
I can see what the problem is: as the condition can not be met, the function #(> (.length %) 4)
takes nil
(the last possible return value) as an argument.我可以看到问题是什么:由于无法满足条件,函数#(> (.length %) 4)
将nil
(最后一个可能的返回值)作为参数。 But being new to Clojure I am not sure how to cover for this.但是作为 Clojure 的新手,我不确定如何解决这个问题。 Is there an idiomatic way?有没有惯用的方法?
solution:解决方案:
(defn recursive-lookup
[key-lst search-func conditional]
(let [next-key (search-func (first key-lst))]
(if (or (nil? next-key)
(conditional next-key))
(reverse key-lst)
(recur (cons next-key key-lst) search-func conditional))))
You need to handle nil
in your conditional function.您需要在条件函数中处理nil
。 You can use fnil for that.你可以使用fnil 。 fnil
replaces nil with some default value. fnil
用一些默认值替换 nil。 So you can try:所以你可以试试:
(fnil #(> (.length %) 4) "")
If this conditional function receives nil
it replaces nil
with empty string "" and then call your function #(> (.length %) 4)
.如果此条件函数收到nil
它将用空字符串 "" 替换nil
,然后调用您的函数#(> (.length %) 4)
。
Also you can use count
instead .length
.您也可以使用count
代替.length
。 Count returns 0 for nil: Count 为 nil 返回 0:
(count "123") => 3
(count nil) => 0
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