在Clojure中有条件地初始化映射的元素
我正在寻找一种最好的方法,有条件地避免在初始化/定义映射时将元素添加到映射中。在这种情况下,如果键的值为nil,我希望避免向映射中添加元素在Clojure中有条件地初始化映射的元素,clojure,Clojure,我正在寻找一种最好的方法,有条件地避免在初始化/定义映射时将元素添加到映射中。在这种情况下,如果键的值为nil,我希望避免向映射中添加元素 (defn create-record [data] (let [res { :username (data :username) :first-name (get-in data [:user-info :name :first]) :last-name (get-in data [:user-info :name :last]
(defn create-record [data]
(let [res {
:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])
:gender (get-in data [:user-info :sex])
}])
)
如果进入结果为零(数据中的性别字段不存在),我不想将性别添加到地图中。当我创建地图时,有没有办法做到这一点?创建映射后,我可以删除所有值为零的键,但在某些情况下,我希望某些键具有零值,而其他键如果具有零值,则根本不在映射中。以下是一个尝试:
(defn exclude-nils-for [m kw-set]
(apply hash-map (apply concat (remove (fn [[k v]] (and (kw-set k) (nil? v))) m))))
测试:
user> (exclude-nils-for {:gender "m" :name "Thomas" :age "24"} #{})
{:age "21", :gender "m", :name "Thomas"}
user> (exclude-nils-for {:gender "m" :name "Thomas" :age "24"} #{:name})
{:age "21", :gender "m", :name "Thomas"}
user> (exclude-nils-for {:gender "m" :name nil :age "24"} #{:name})
{:age "21", :gender "m"}
user> (exclude-nils-for {:gender "m" :name nil :age nil} #{:age})
{:gender "m", :name nil}
构建映射并基于谓词(此处--
nil?
)分解要施加条件的键可能是最简单的方法(注意,此函数仅测试作为参数明确提及的键;未提及的键永远不会删除,无论附加到它们的值是否满足谓词):
在REPL上:
user> (dissoc-when nil? {:foo nil :bar true :quux nil} :foo :bar)
{:quux nil, :bar true}
虽然一般来说,如果您希望使用大量表示某一特定类型的真实世界实体的映射,您可能希望使用记录,然后您可以在从输入映射提取值的阶段跳过所有nil
s,因为记录作为映射查看时,似乎总是包含与其字段对应的键。例如
(defrecord Person [username first-name last-name])
然后,您可以考虑映射之间的“模式转换”逻辑:
(defn translate-map
"Transforms the data map in accordance with the spec in table.
Skips nil-valued entries."
[data table]
(->> table
(keep (fn [[to from]]
(when-let [from-val (get-in data from)]
[to from-val])))
(into {})))
现在,您的create record
函数变成了translate map
和map->Person
的组合:
(defn create-person [data]
(map->Person
(translate-map data {:username [:username]
:first-name [:user-info :name :first]
:last-name [:user-info :name :last]
:gender [:user-info :sex]})))
如果您确实更喜欢使用常规贴图,则可以使用类似以下内容的内容来实现等效输出:
(defn create-person [data]
(merge (zipmap [:username :first-name :last-name] (repeat nil))
(translate-map data {:username [:username]
:first-name [:user-info :name :first]
:last-name [:user-info :name :last]
:gender [:user-info :sex]})))
在REPL(Clojure 1.3中的记录版本):
您可以定义字段以及哪些字段是可选的:
(def fields
[[:username [:username]]
[:first-name [:user-info :name :first]]
[:sex [:user-info :sex] true]])
然后编写一个函数来使用该信息:
(defn create-record [data keys]
(->>
(for [[n k ignore-nil?] keys
:let [v (get-in data k)]
:when (or (not ignore-nil?) v)]
[n v])
(into {})))
它的工作原理如下:
; If :sex is missing don't create a field
user=> (create-record {:username "dr" :user-info { :name {:first "Dave"} }} fields)
{:username "dr", :first-name "Dave"}
user=> (create-record {:username "dr" :user-info { :name {:first "Dave"} :sex :m }} fields)
{:username "dr", :first-name "Dave", :sex :m}
; If :first is missing, create a nil field
user=> (create-record {:username "dr" :user-info { :name {} :sex :m }} fields)
{:username "dr", :first-name nil, :sex :m}
根据需要修改:)您可以执行以下操作
(let [not-nils #{:gender}]
(defn create-record [data]
(into {} (for [[k v] {:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])
:gender (get-in data [:user-info :sex])}
:when (not (and (nil? v) (not-nils k)))]
[k v]))))
对于这些可选参数,我将使用
merge
和when let
的组合
核心思想是为每个可选参数合并一个元素映射或nil。在nil中合并将不起任何作用,因此您不会在地图中看到nil
(defn create-record [data]
(let [res (merge {:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])}
(when-let [gender (get-in data [:user-info :sex])]
{:gender gender}))]
res))
根据您需要执行此操作的频率,我建议您围绕when let编写一个简短的宏或函数,以使代码更简洁。您可能希望将
展平
替换为应用concat
,否则更复杂的值会导致问题(在第一个示例中,尝试将:foo[12]
添加到映射中). (或者打开一个级别的paren并使其应用(comp hash map concat))此外,我还将为不排除nil
值的情况添加一个测试(尽管这确实有效)。完成。谢谢你的评论!谢谢,这很好-但是为什么要使用多余的(让[res…res]
wrapper?@Alex我一直在尽可能地保持代码与他的代码相似。我假设他可能在作业后对结果做了其他事情,所以我把它放在那里了。你是对的,虽然没有必要。啊,谢谢,有道理@deterb。我担心我遗漏了什么。旁白:“性别”!=“性”
(defn create-record [data]
(let [gender (get-in data [:user-info :sex])]
(->> {:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])}
(#(if gender (assoc % :gender gender) %)))))
(defn create-record [data]
(let [res (merge {:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])}
(when-let [gender (get-in data [:user-info :sex])]
{:gender gender}))]
res))
(defn create-record [data]
(let [gender (get-in data [:user-info :sex])]
(->> {:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])}
(#(if gender (assoc % :gender gender) %)))))
(cond-> {:username (data :username)
:first-name (get-in data [:user-info :name :first])
:last-name (get-in data [:user-info :name :last])}
(get-in data [:user-info :sex]) (assoc :gender (get-in data [:user-info :sex])))