我試圖創建一個可變的Map
,默認情況下,當一個元素被請求時,它不會在地圖中創建一個新的ListBuffer
。但是,當新地圖返回爲默認地圖時,它不會保留在地圖中。也許這就是它的工作方式,我想,但是當我用Int
而不是ListBuffer
進行測試時,它的確如我所願。這裏有一些代碼來解釋我的意思 - 我做錯了什麼?默認情況下在斯卡拉映射一個空的ListBuffer
首先,這裏正在與Map[Int]
:
scala> val a = collection.mutable.Map(1 -> 1).withDefault(i => 0)
a: scala.collection.mutable.Map[Int,Int] = Map(1 -> 1)
scala> a(1) += 1 // adding to an existing element works as expected
scala> a
res48: scala.collection.mutable.Map[Int,Int] = Map(1 -> 2)
scala> a(2) += 1 // what about adding to a non-existing element?
scala> a // the new element has been added to the map
res50: scala.collection.mutable.Map[Int,Int] = Map(1 -> 2, 2 -> 1)
現在有Map[ListBuffer[Int]]
:
scala> val b = collection.mutable.Map(1 -> collection.mutable.ListBuffer[Int]()).withDefault(i => collection.mutable.ListBuffer.empty[Int])
b: scala.collection.mutable.Map[Int,scala.collection.mutable.ListBuffer[Int]] = Map(1 -> ListBuffer())
scala> b(1) += 1 // appending to an existing element works as expected
res51: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1)
scala> b
res52: scala.collection.mutable.Map[Int,scala.collection.mutable.ListBuffer[Int]] = Map(1 -> ListBuffer(1))
scala> b(2) += 1 // but appending to a non-existing element...
res53: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1)
scala> b // leaves the map unchanged
res54: scala.collection.mutable.Map[Int,scala.collection.mutable.ListBuffer[Int]] = Map(1 -> ListBuffer(1))
偉大的,這是有道理的 - 那麼我該如何解決它?我是否需要通過三行來完成,獲取元素,追加到它,然後設置它?因爲如果可能的話我寧願避免! – Russell 2012-03-22 19:02:26
我想我可以做地圖(我)=地圖(我)+ = 5,但它看起來很蹩腳。 – Russell 2012-03-22 19:05:35
是的,我認爲是這樣的(當然,你總是可以將它解釋爲一個好幫手方法)。 – 2012-03-22 19:17:40