如何判断地图是否具有默认值? [英] How to tell if a Map has a default value?
问题描述
有没有办法检查 Map
是否具有定义的默认值?我想要的是一些相当于 myMap.getOrElse(x,y)
的地方,如果密钥 x
不在如果 myMap
具有默认值,则返回该值
-
- else return
y
问题的例子:
scala> def f(m:Map [String,String])= m.getOrElse(hello,world)
f:(m:Map [String,String])String
scala> ; val myMap = Map(a - >A)withDefaultValue(Z)
myMap:scala.collection.immutable.Map [String,String] = Map(a - > A)
scala> f(myMap)
res0:String = world
在这种情况下,我想要code> res0 为Z
而不是world
,因为 myMap
被定义为默认值。但是 getOrElse
不能这样工作。
我可以使用 m.apply
而不是 m.getOrElse
,但是地图不能保证具有默认值,所以它可能会抛出异常(我可以抓到异常,但这是非常的)。
scala> def f(m:Map [String,String])= try {
| m(你好)
| catch {
| case e:java.util.NoSuchElementException => world
| }
f:(m:Map [String,String])String
scala> val myMap = Map(a - >A)withDefaultValue(Z)
myMap:scala.collection.immutable.Map [String,String] = Map(a - > A)
scala> f(myMap)
res0:String = Z
scala> val mapWithNoDefault = Map(a - >A)
mapWithNoDefault:scala.collection.immutable.Map [String,String] = Map(a - > A)
阶> f(mapWithNoDefault)
res1:String = world
以上产生了预期值,但似乎乱。根据地图是否具有默认值,我无法模式匹配,并调用应用
或 getOrElse
类型是相同的( scala.collection.immutable.Map [String,String]
),不管默认值。
你可以检查地图是否是一个实例 Map.WithDefault
:
隐式类EnrichedMap [K, V](m:Map [K,V]){
def getOrDefaultOrElse(k:K,v:=> V)=
if(m.isInstanceOf [Map.WithDefault [K,V] ])m(k)else m.getOrElse(k,v)
}
然后:
scala> val myMap = Map(a - >A)withDefaultValue(Z)
myMap:scala.collection.immutable.Map [String,String] = Map(a - > A)
scala> myMap.getOrDefaultOrElse(hello,world)
res11:String = Z
scala> val myDefaultlessMap = Map(a - >A)
myDefaultlessMap:scala.collection.immutable.Map [String,String] = Map(a - > A)
阶> myDefaultlessMap.getOrDefaultOrElse(hello,world)
res12:String = world
这种反思是否比使用非特殊控制流程的异常更好是一个悬而未决的问题。
Is there a way to check if a Map
has a defined default value? What I would like is some equivalent of myMap.getOrElse(x, y)
where if the key x
is not in the map,
- if
myMap
has a default value, return that value - else return
y
A contrived example of the issue:
scala> def f(m: Map[String, String]) = m.getOrElse("hello", "world")
f: (m: Map[String,String])String
scala> val myMap = Map("a" -> "A").withDefaultValue("Z")
myMap: scala.collection.immutable.Map[String,String] = Map(a -> A)
scala> f(myMap)
res0: String = world
In this case, I want res0
to be "Z"
instead of "world"
, because myMap
was defined with that as a default value. But getOrElse
doesn't work that way.
I could use m.apply
instead of m.getOrElse
, but the map is not guaranteed to have a default value, so it could throw an exception (I could catch the exception, but this is nonideal).
scala> def f(m: Map[String, String]) = try {
| m("hello")
| } catch {
| case e: java.util.NoSuchElementException => "world"
| }
f: (m: Map[String,String])String
scala> val myMap = Map("a" -> "A").withDefaultValue("Z")
myMap: scala.collection.immutable.Map[String,String] = Map(a -> A)
scala> f(myMap)
res0: String = Z
scala> val mapWithNoDefault = Map("a" -> "A")
mapWithNoDefault: scala.collection.immutable.Map[String,String] = Map(a -> A)
scala> f(mapWithNoDefault)
res1: String = world
The above yields the expected value but seems messy. I can't pattern match and call apply
or getOrElse
based on whether or not the map had a default value, because the type is the same (scala.collection.immutable.Map[String,String]
) regardless of default-ness.
Is there a way to do this that doesn't involve catching exceptions?
You can check whether the map is an instance of Map.WithDefault
:
implicit class EnrichedMap[K, V](m: Map[K, V]) {
def getOrDefaultOrElse(k: K, v: => V) =
if (m.isInstanceOf[Map.WithDefault[K, V]]) m(k) else m.getOrElse(k, v)
}
And then:
scala> val myMap = Map("a" -> "A").withDefaultValue("Z")
myMap: scala.collection.immutable.Map[String,String] = Map(a -> A)
scala> myMap.getOrDefaultOrElse("hello", "world")
res11: String = Z
scala> val myDefaultlessMap = Map("a" -> "A")
myDefaultlessMap: scala.collection.immutable.Map[String,String] = Map(a -> A)
scala> myDefaultlessMap.getOrDefaultOrElse("hello", "world")
res12: String = world
Whether this kind of reflection is any better than using exceptions for non-exceptional control flow is an open question.
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