可选择将项目添加到 Scala 映射

发布于 2024-11-07 18:01:15 字数 719 浏览 0 评论 0原文

我正在寻找解决这个问题的惯用方法。

我正在构建一个 val Scala（不可变）映射，并希望有选择地添加一个或多个项目：

val aMap =
  Map(key1 -> value1,
      key2 -> value2,
      (if (condition) (key3 -> value3) else ???))

如何在不使用 var 的情况下完成此操作？什么应该取代 ???？使用 + 运算符更好吗？

val aMap =
  Map(key1 -> value1,
      key2 -> value2) +
  (if (condition) (key3 -> value3) else ???))

一种可能的解决方案是：

val aMap =
  Map(key1 -> value1,
      key2 -> value2,
      (if (condition) (key3 -> value3) else (null, null))).filter {
        case (k, v) => k != null && v != null
      }

这是最好的方法吗？

原文

I am looking for an idiomatic solution to this problem.

I am building a val Scala (immutable) Map and would like to optionally add one or more items:

val aMap =
  Map(key1 -> value1,
      key2 -> value2,
      (if (condition) (key3 -> value3) else ???))

How can this be done without using a var? What should replace the ???? Is it better to use the + operator?

val aMap =
  Map(key1 -> value1,
      key2 -> value2) +
  (if (condition) (key3 -> value3) else ???))

One possible solution is:

val aMap =
  Map(key1 -> value1,
      key2 -> value2,
      (if (condition) (key3 -> value3) else (null, null))).filter {
        case (k, v) => k != null && v != null
      }

Is this the best way?

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拒绝两难 2024-11-14 18:01:15

类似的事情怎么样

val optional = if(condition) Seq((key3 -> value3)) else Nil
val entities = Seq(key1 -> value1, key2 -> value2) ++ optional
val aMap = entities.toMap

How about something along the lines of

val optional = if(condition) Seq((key3 -> value3)) else Nil
val entities = Seq(key1 -> value1, key2 -> value2) ++ optional
val aMap = entities.toMap

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绿萝 2024-11-14 18:01:15

另一种可能性是利用 Option 的可迭代性质。

非空值o：

scala> val o = Some('z' -> 3)
scala> (Seq('x' -> 1, 'y' -> 2) ++ o).toMap
res1: scala.collection.immutable.Map[Char,Int] = Map(x -> 1, y -> 2, z -> 3)

空值o：

scala> val o = None
scala> (Seq('x' -> 1, 'y' -> 2) ++ o).toMap
res2: scala.collection.immutable.Map[Char,Int] = Map(x -> 1, y -> 2)

Another possibility is to take advantage of the iterable nature of Option.

A non-empty value o:

scala> val o = Some('z' -> 3)
scala> (Seq('x' -> 1, 'y' -> 2) ++ o).toMap
res1: scala.collection.immutable.Map[Char,Int] = Map(x -> 1, y -> 2, z -> 3)

An empty value o:

scala> val o = None
scala> (Seq('x' -> 1, 'y' -> 2) ++ o).toMap
res2: scala.collection.immutable.Map[Char,Int] = Map(x -> 1, y -> 2)

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女皇必胜 2024-11-14 18:01:15

您可以直接添加到地图：

scala> val map = Map(1 -> 2, 3 -> 4)
scala> val some = Some(5 -> 6)
scala> val none = None
scala> val combinedMap = map ++ some ++ none
combinedMap: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4, 5 -> 6)

You can add directly to the map:

scala> val map = Map(1 -> 2, 3 -> 4)
scala> val some = Some(5 -> 6)
scala> val none = None
scala> val combinedMap = map ++ some ++ none
combinedMap: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4, 5 -> 6)

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