Scala：使用通配符映射密钥吗？

Question

Is it possible to use keys with wildcards for Scala Maps? For example tuples of the form (x,_)? Example:

scala> val t1 = ("x","y")
scala> val t2 = ("y","x")
scala> val m = Map(t1 -> "foo", t2 -> "foo")

scala> m(("x","y"))
res5: String = foo

scala> m(("x",_))
<console>:11: error: missing parameter type for expanded function ((x$1) => scala.Tuple2("x", x$1))
              m(("x",_))
                     ^

It would be great if there was a way to retrieve all (composite_key, value) pares where only some part of composite key is defined. Other ways to get the same functionality in Scala?

Answer 1

使用收集怎么样

Map( 1 -> "1" -> "11", 2 -> "2" -> "22").collect { case (k@(1, _), v ) => k -> v }

Answer 2

In general, you can do something like

m.filter(m => (m._1 == "x"))

but in your particular example it will still return only one result, because a Map has only one entry per key. If your key itself is composite then it will indeed make more sense:

scala>  Map((1,2)->"a", (1,3)->"b", (3,4)->"c").filter(m => (m._1._1 == 1))
res0: scala.collection.immutable.Map[(Int, Int),String] = Map((1,2) -> a, (1,3) -> b)

Answer 3

使用这样的理解：

for ( a @ ((k1,k2), v) <- m  if k1 == "x" ) yield a

Answer 4

Think about what is happening under the hood of the Map . The default Map in Scala is scala.collection.immutable.HashMap , which stores things based on their hash codes. Do ("x", "y") and ("x", "y2") have hash codes that relate to each other in anyway? No, they don't, and their is no efficient way to implement wildcards with this map. The other answers provide solutions, but these will iterate over key/value pair in the entire Map , which is not efficient.

If you expect you are going to want to do operations like this, use a TreeMap . This doesn't use a hash table internally, put instead puts elements into a tree based on an ordering. This is similar to the way a relational database uses B-Trees for its indices. Your wildcard query is like using a two-column index to filter on the first column in the index.

Here is an example:

import scala.collection.immutable.TreeMap

val t1 = ("x","y")
val t2 = ("x","y2")
val t3 = ("y","x")
val m = TreeMap(t1 -> "foo1", t2 -> "foo2", t3 -> "foo3")

// "" is < than all other strings
// "x\u0000" is the next > string after "x"
val submap = m.from(("x", "")).to(("x\u0000", ""))

submap.values.foreach(println) // prints foo1, foo2