scala - 左连接两个列表

Question

我想左连接两个列表，而不丢失数据。下面的例子:

// two lists
val b = List("John","Alice","Gregor","Mike")                 // base list
val l = List((List(1,2,3),"Mike"), (List(3,1,2), "Alice"))  // list to left join

// sorted
val bs = b.sorted        // List("Alice","Gregor","John","Mike")
val ls = l.sortBy(_._2)  // List((List(3,1,2), "Alice"),(List(1,2,3),"Mike"))


// left join - expected result:
// Alice and Mike found in both lists, Gregor and John appear only in the base list
// List(("Alice",(List(3,1,2),"Alice"), ("Gregor",Nil), ("John",Nil), ("Mike",List("Alice","Gregor","John","Mike"))))

我找到了一些方法，但它们似乎不适合我:
- zip - 简单的“复制/粘贴”两个列表，
- flatMap - 删除没有出现在两个列表中的元素。

先感谢您。

Answer 1

尝试

b
  .map(key => key -> l.find(_._2 == key))
  .map {case (key, value) => key -> value.getOrElse(Nil) }

哪个输出

List((John,List()), (Alice,(List(3, 1, 2),Alice)), (Gregor,List()), (Mike,(List(1, 2, 3),Mike))