Scala系列,单键多重值 [英] Scala collections, single key multiple values
问题描述
我正在使用 Map [Int,List [String]]
/ p>
我宣布地图为
var nodes = new HashMap [ Int,List [String]]
然后我有两种方法来处理添加新元素。第一个是添加新密钥 addNode
,第二个是添加新值 addValue
。最初,密钥不会有与之相关的任何值。后来,在执行期间,新值将被关联。
def addNode(key:Int)= nodes + =(key - >)
def addValue(key:Int,value:String)= ???
我不知道如何实现 addValues
更新:
为了回应@ oxbow-lakes答案,这是我收到的错误。请注意,密钥不需要与其相关联的值。
scala> var nodes = Map.empty [Int,List [String]]
nodes:scala.collection.immutable.Map [Int,List [String]] = Map()
scala>节点+ =(1-> null)
scala> node + =(1 - >(one::(nodes get 1 getOrElse Nil)))
java.lang.NullPointerException
at。< init>(< console>:9) $&$ {$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $
at $ print(< console>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
在java.lang.reflect.Method.invoke(Method.java:597)
at scala.tools.nsc.interpreter .IMain $ ReadEvalPrint.call(IMain.scala:704)
at scala.tools.nsc.interpreter.IMain $ Request $$ anonfun $ 14.apply(IMain.scala:920)
at scala.tools .nsc.interpreter.Line $$ anonfun $ 1.apply $ mcV $ sp(Line.scala:43)
at scala.tools.nsc.io.package $$ anon $ 2.run(package.scala:25)
在java.lang.Thread.run(Thread.java :680)
更新2:
上述代码的问题是 nodes + =(1-> null)
的关键字应该与 Nil
而不是。以下是工作代码。
scala> var nodes = Map.empty [Int,List [String]]
nodes:scala.collection.immutable.Map [Int,List [String]] = Map()
scala>节点+ =(1> Nil)
scala> nodes + =(1 - >(one::(nodes get 1 getOrElse Nil)))
scala>节点
res27:scala.collection.immutable.Map [Int,List [String]] = Map(1 - > List(one))
使用MultiMap
您可能想使用 MultiMap
,这是一个与 Map [K,Set [V]]
同构的可变集合。使用如下:
import collection.mutable
val mm = new mutable.HashMap [Int,mutable.Set [ String]] with mutable.MultiMap [Int,String]
然后添加您的节点:
mm addBinding(key,value)
没有MultiMap
另一种选择是坚持不变的值。假设您希望避免使用镜头(请参阅 scalaz ),可以按如下方式添加节点:
nodes + =(key - >(value ::(nodes get key getOrElse Nil)))
pre>
这里正在工作(响应您的评论):
阶> var nodes = Map.empty [Int,List [String]]
nodes:scala.collection.immutable.Map [Int,List [String]] = Map()
scala> def addNode(key:Int,value:String)=
| node + =(key - >(value ::(nodes get key getOrElse Nil)))
addNode:(key:Int,value:String)单位
scala> addNode(1,嗨)
scala> addNode(1,Bye)
scala>节点
res2:scala.collection.immutable.Map [Int,List [String]] = Map(1 - > List(Bye,Hi))
使用Scalaz
使用scalaz库,您可以意识到这只是使用
空
pattern:nodes + =(key - >(value ::〜节点获取密钥)))
或者您可以利用
地图
是单一:nodes = nodes | + |地图(键 - >列表(值))
I have a list of parent keys, each of which could possibly have zero or more associated values. I am not sure which collection to use.
I am using
Map[Int,List[String]]
I am declaring the Map as
var nodes = new HashMap[Int, List[String]]
Then I have two methods to handle adding new elements. The first is to add new keys
addNode
and the second is to add new valuesaddValue
. Initially, the key will not have any values associated with it. Later on, during execution, new values will be associated.def addNode(key: Int) = nodes += (key -> "") def addValue(key: Int, value: String) = ???
I am not sure how to implement
addValues
Update:
In response to @oxbow-lakes answer, This is the error I am receiving. Please note that keys need not have values associated with them.
scala> var nodes = Map.empty[Int, List[String]] nodes: scala.collection.immutable.Map[Int,List[String]] = Map() scala> nodes += (1->null) scala> nodes += (1 -> ("one" :: (nodes get 1 getOrElse Nil))) java.lang.NullPointerException at .<init>(<console>:9) at .<clinit>(<console>) at .<init>(<console>:11) at .<clinit>(<console>) at $print(<console>) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704) at scala.tools.nsc.interpreter.IMain$Request$$anonfun$14.apply(IMain.scala:920) at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43) at scala.tools.nsc.io.package$$anon$2.run(package.scala:25) at java.lang.Thread.run(Thread.java:680)
Update 2:
The problem with the code above is the line
nodes += (1->null)
the key should be associated withNil
instead. Below is the working code.scala> var nodes = Map.empty[Int, List[String]] nodes: scala.collection.immutable.Map[Int,List[String]] = Map() scala> nodes += (1->Nil) scala> nodes += (1 -> ("one" :: (nodes get 1 getOrElse Nil))) scala> nodes res27: scala.collection.immutable.Map[Int,List[String]] = Map(1 -> List(one))
解决方案Using MultiMap
You possibly want to use
MultiMap
, which is a mutable collection isomorphic toMap[K, Set[V]]
. Use as follows:import collection.mutable val mm = new mutable.HashMap[Int, mutable.Set[String]] with mutable.MultiMap[Int, String]
Then you add your nodes:
mm addBinding (key, value)
Without MultiMap
The alternative is to stick with immutable values. Assuming you want to avoid using lenses (see scalaz), you can add nodes as follows:
nodes += (key -> (value :: (nodes get key getOrElse Nil)))
Here it is working (in response to your comment):
scala> var nodes = Map.empty[Int, List[String]] nodes: scala.collection.immutable.Map[Int,List[String]] = Map() scala> def addNode(key: Int, value: String) = | nodes += (key -> (value :: (nodes get key getOrElse Nil))) addNode: (key: Int, value: String)Unit scala> addNode(1, "Hi") scala> addNode(1, "Bye") scala> nodes res2: scala.collection.immutable.Map[Int,List[String]] = Map(1 -> List(Bye, Hi))
Using Scalaz
Using the scalaz library, you can realize that this is simply using the
Empty
pattern:nodes += (key -> (value :: ~(nodes get key)))
Or you could take advantage of the fact that
Map
is a monoid:nodes = nodes |+| Map(key -> List(value))
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