從Scala中的列表返回元素 (Returning an element from a List in Scala)

問題描述

從 Scala 中的 List 返回一個元素 (Returning an element from a List in Scala)

我最近一直在從事 Scala 的初學者項目，並且有一個關於 Scala 列表的初學者問題。

假設我有一個元組列表（ List[Tuple2[String, String ]]，例如）。是否有一種方便的方法可以從 List 中返回指定元組的第一次出現，還是需要手動遍歷列表？

參考解法

方法 1:

scala> val list = List(("A", "B", 1), ("C", "D", 1), ("E", "F", 1), ("C", "D", 2), ("G", "H", 1))

list: List[(java.lang.String, java.lang.String, Int)] = List((A,B,1), (C,D,1), (E,F,1), (C,D,2), (G,H,1))

scala> list find {e => e._1 == "C" && e._2 == "D"}

res0: Option[(java.lang.String, java.lang.String, Int)] = Some((C,D,1))

</pre>
方法 2:
 
You could try using find. (Updated scala‑doc location of find)

方法 3:
 
As mentioned in a previous comment, find is probably the easiest way to do this.  There are actually three different "linear search" methods in Scala's collections, each returning a slightly different value.  Which one you use depends upon what you need the data for.  For example, do you need an index, or do you just need a boolean true/false?

方法 4:
 
If you're learning scala, I'd take a good look at the Seq trait. It provides the basis for much of scala's functional goodness.

方法 5:
 
You could also do this, which doesn't require knowing the field names in the Tuple2 class‑‑it uses pattern matching instead:

list find { case (x,y,_) => x == "C" && y == "D" }
"find" is good when you know you only need one; if you want to find all matching elements you could either use "filter" or the equivalent sugary for comprehension:
for ( (x,y,z) <‑ list if x == "C" && y == "D") yield (x,y,z)
(by Max Pagels、Binil Thomas、Tim Sullivan、Daniel Spiewak、sblundy、Alex Cruise)
參考文件


Returning an element from a List in Scala (CC BY‑SA 2.5/3.0/4.0)