如何从现有地图派生出scala中的新不可变地图

Question

我认为这会相对简单。所以我会解释：

我有一个地图如下：这是我在 REPL 中尝试过的

scala> val entriesList = List(("tPolicyId" -> "MasterCard"), ("SSN" -> "0"), ("MasterCard" -> "3"));
entriesList: List[(String, String)] = List((tPolicyId,MasterCard), (US SSN,0), (MasterCard,3))

然后我将列表转换为地图，如下所示。

scala> val entriesMap = entriesList.toMap
entriesMap: scala.collection.immutable.Map[String,String] = Map(tPolicyId -> MasterCard,SSN -> 0, MasterCard -> 3)

到目前为止一切顺利。现在我想要完成的是一种迭代此 Map 的方法，有一个保护条件或谓词：如果我用来迭代此 Map 的函数遇到一个名为 tPolicyMap 的键，请 return 或者生成一个新的不可变映射，newMap 如下所示：

  ***newMap: scala.collection.immutable.Map[String,String] = Map(tPolicyId -> MasterCard)***

我认为这会很容易，下面是我迄今为止所有失败的尝试。

    scala> val tPolicyMap = for (entry <- entriesMap if entry.contains("tPolicyId")) yield  entry
<console>:10: error: value contains is not a member of (String, String)
       val tPolicyMap = for (entry <- entriesMap if entry.contains("tPolicyId")) yield  entry
                                                                   ^

scala> val tPolicyMap = for (entry <- entriesMap if ( entry.contains("tPolicyId"))) yield  entry
<console>:10: error: value contains is not a member of (String, String)
       val tPolicyMap = for (entry <- entriesMap if ( entry.contains("tPolicyId"))) yield  entry
                                                                     ^

scala> val tPolicyMap = for (entry <- entriesMap if ( entry.contains("tPolicyId"))) yield  (entry)
<console>:10: error: value contains is not a member of (String, String)
       val tPolicyMap = for (entry <- entriesMap if ( entry.contains("tPolicyId"))) yield  (entry)
                                                                     ^

scala> val tPolicyMap = for (entry <- entriesMap; if ( entry.contains("tPolicyId"))) yield  (entry)
<console>:10: error: value contains is not a member of (String, String)
       val tPolicyMap = for (entry <- entriesMap; if ( entry.contains("tPolicyId"))) yield  (entry)
                                                                      ^

scala> val tPolicyMap = for (entry <- entriesMap; if ( entry contains("tPolicyId"))) yield  (entry)
<console>:10: error: value contains is not a member of (String, String)
       val tPolicyMap = for (entry <- entriesMap; if ( entry contains("tPolicyId"))) yield  (entry)
                                                                      ^

scala> val tPolicyMap = for (entry <- entriesMap; if entry contains("tPolicyId"))) yield  entry
<console>:1: error: illegal start of simple expression
       val tPolicyMap = for (entry <- entriesMap; if entry contains("tPolicyId"))) yield  entry
                                                                                                   ^

scala> val tPolicyMap = for (entry <- entriesMap; if entry contains("tPolicyId")) yield  entry
<console>:10: error: value contains is not a member of (String, String)
       val tPolicyMap = for (entry <- entriesMap; if entry contains("tPolicyId")) yield  entry
                                                                    ^

scala> val tPolicyMap = for (entry <- entriesMap) yield  entry
tPolicyMap: scala.collection.immutable.Map[String,String] = Map(tPolicyId -> MasterCard/Diner's Club U
S Number, US SSN -> 0, MasterCard -> 3)

scala> val tPolicyEntry = entriesMap.contains("tPolicyId")
tPolicyEntry: Boolean = true

scala> val tPolicyEntry = if (entriesMap.contains("tPolicyId")) yield entriesMap
<console>:1: error: illegal start of simple expression
       val tPolicyEntry = if (entriesMap.contains("tPolicyId")) yield entriesMap
                                                                                  ^

scala> val tPolicyEntry = for(entry <- entriesMap; if entry == "tPolicyId" yield entry
<console>:1: error: ')' expected but 'yield' found.
       val tPolicyEntry = for(entry <- entriesMap; if entry == "tPolicyId" yield entry
                                                                                             ^

scala> val tPolicyEntry = for(entry <- entriesMap; if entry == "tPolicyId") yield entry
<console>:10: warning: comparing values of types (String, String) and String using `==' will always yield false
       val tPolicyEntry = for(entry <- entriesMap; if entry == "tPolicyId") yield entry
                                                                     ^
tPolicyEntry: scala.collection.immutable.Map[String,String] = Map()

scala> val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)) yield entry
<console>:1: error: illegal start of simple expression
       val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)) yield entry
                                                                                                      ^

scala> val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)))) yield entry
<console>:1: error: illegal start of simple expression
       val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)))) yield entry
                                                                                                     ^

scala> val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)) yield entry)
<console>:1: error: illegal start of simple expression
       val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)) yield entry)
                                                                                                      ^

scala> val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)) yield entry))
<console>:1: error: illegal start of simple expression
       val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry)) yield entry))
                                                                                                      ^

scala> val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry) yield entry))
<console>:1: error: ')' expected but 'yield' found.
       val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry) yield entry))
                                                                                                     ^

scala> val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry) yield entry)
<console>:1: error: ')' expected but 'yield' found.
       val tPolicyEntry = entriesMap.keys.foreach( (entry) => if(entriesMap.contains(entry) yield entry)

如您所见，我尝试以各种组合方式使用 filter、yield、foreach、for 等，但均未成功。我还是不明白/

所以，我已经很努力了，我可能犯了一些根本性的幼稚错误。

如果有人能帮我弄清楚如何得到我想要的地图，我将不胜感激学习经验

提前致谢

Answer 1

就像

一样简单

 entriesMap.filter { case (k, _) => k == "tPolicyId" }
 //> res0: scala.collection.immutable.Map[String,String] = Map(tPolicyId -> MasterCard)

或者干脆

 entriesMap filterKeys { _ == "tPolicyId"  }

Answer 2

您正在考虑命令式（迭代、比较，如果匹配则执行此操作），而不是 Scala 不可变集合倾向于的更高级别：

// get looks up value from a map using the key (fast)
// will give you an Option[String] back - 
// Some(value) if the key exists, None if it doesn't
tpPolicyMap.get("tPolicyId").fold[Map[String, String]](
  // result for None - no "tPolicyId"-key in the source map
  Map.empty
)(value => 
  // found, create a new map with that entry
  Map("tPolicyId" -> value)
)

Answer 3

filter的解决方案很好。您也可以执行 collect，如果您打算在 filter

之后执行 map，这可能会很方便

entriesMap.collect{ case(k, v) if k == "tPolicyId" => (k, s"test${v}") } // example instead of plain (k, v)
// scala.collection.immutable.Map[String,String] = Map(tPolicyId -> testMasterCard)

等同于：

(entriesMap filterKeys { _ == "tPolicyId"  }).map((kv) => (kv._1, s"test${kv._2}"))

// scala.collection.immutable.Map[String,String] = Map(tPolicyId -> testMasterCard)

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