[英]clojure filter nested map to return keys based on inner map values
For this nested map called "tables", 对于这个称为“表格”的嵌套地图,
(def tables
{:tableA {:occupied false :party nil}
:tableB {:occupied true :party nil}
:tableC {:occupied false :party nil}})
how do I filter and get back the keys where :occupied = false ? 我该如何过滤并获取:occupied = false的键?
correct result should be (:tableA :tableC)
正确的结果应该是
(:tableA :tableC)
can I do this with "filter" HOF? 我可以使用“过滤器” HOF来做到这一点吗? Should I be using a list comprehension?
我应该使用列表理解吗?
You could do it pretty easily with keep
: 您可以使用
keep
轻松完成此操作:
(keep (fn [[k v]] (if-not (:occupied v) k)) tables)
However, as you observed, using for
is often a good solution whenever you're mapping / filtering sequences, especially if you're dealing with nested sequences. 但是,正如您观察到的那样,在映射/过滤序列时,尤其是在处理嵌套序列时,使用
for
通常是一个很好的解决方案。
(for [[k v] tables :when (not (:occupied v))] k)
I usually prefer using for
, especially when I want to use destructuring on the target items. 我通常更喜欢使用
for
,尤其是当我想对目标项目使用解构时。 In this case, destructuring is nice for binding the key/value pair with [kv]
. 在这种情况下,解构非常适合将键/值对与
[kv]
绑定。
I'm not sure what your use case is, but can I suggest structuring your data as a set of maps? 我不确定您的用例是什么,但是我可以建议将数据构造为一组地图吗? Sets have certain properties that allow you to query them using relational algebra similar to the way you query tables in SQL.
集具有某些属性,这些属性使您可以使用关系代数来查询它们,类似于在SQL中查询表的方式。 The clojure.set/select function is similar to a WHERE in SQL.
clojure.set / select函数类似于SQL中的WHERE。
(use 'clojure.set)
(def tables
#{{:table "A" :occupied false :party nil}
{:table "B" :occupied true :party nil}
{:table "C" :occupied false :party nil}})
(select #(= false (:occupied %)) tables)
Hope this helps! 希望这可以帮助!
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