在 Scala 中平面映射嵌套的 Map

Question

Suppose I have val someMap = Map[String -> Map[String -> String]] defined as such:

val someMap = 
Map( 
    ("a1" -> Map( ("b1" -> "c1"), ("b2" -> "c2") ) ),
    ("a2" -> Map( ("b3" -> "c3"), ("b4" -> "c4") ) ),
    ("a3" -> Map( ("b5" -> "c5"), ("b6" -> "c6") ) )   
)

and I would like to flatten it to something that looks like

List(   
    ("a1","b1","c1"),("a1","b2","c2"),
    ("a2","b3","c3"),("a2","b4","c4"),
    ("a3","b5","c5"),("a3","b6","c6")
)

What is the most efficient way of doing this? I was thinking about creating some helper function that processes each (a_i -> Map(String,String)) key value pair and return

def helper(key: String, values: Map[String -> String]): (String,String,String) 
= {val sublist = values.map(x => (key,x._1,x._2))
return sublist
}

then flatmap this function over someMap . But this seems somewhat unnecessary to my novice scala eyes, so I was wondering if there was a more efficient way to parse this Map.

Answer 1

No need to create helper function just write nested lambda:

val result = someMap.flatMap { case (k, v) => v.map { case (k1, v1) => (k, k1, v1) } }

Or

val y = someMap.flatMap(x => x._2.map(y => (x._1, y._1, y._2)))

Answer 2

Since you're asking about efficiency, the most efficient yet functional approach I can think of is using foldLeft and foldRight .

You need foldRight since :: constructs the immutable list in reverse.

someMap.foldRight(List.empty[(String, String, String)]) { case ((a, m), acc)  =>
  m.foldRight(acc) {
    case ((b, c), acc) => (a, b, c) :: acc
  }
}

Here, assuming Map.iterator.reverse is implemented efficiently, no intermediate collections are created.

Alternatively, you can use foldLeft and then reverse the result:

someMap.foldLeft(List.empty[(String, String, String)]) { case (acc, (a, m))  =>
  m.foldLeft(acc) {
    case (acc, (b, c)) => (a, b, c) :: acc
  }
}.reverse

This way a single intermediate List is created, but you don't rely on the implementation of the reversed iterator ( foldLeft uses forward iterator).

---

Note: one liners, such as someMap.flatMap(x => x._2.map(y => (x._1, y._1, y._2))) are less efficient, as, in addition to the temporary buffer to hold intermediate results of flatMap , they create and discard additional intermediate collections for each inner map .

---

UPD

Since there seems to be some confusion, I'll clarify what I mean. Here is an implementation of map , flatMap , foldLeft and foldRight from TraversibleLike :

  def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    def builder = { // extracted to keep method size under 35 bytes, so that it can be JIT-inlined
      val b = bf(repr)
      b.sizeHint(this)
      b
    }
    val b = builder
    for (x <- this) b += f(x)
    b.result
  }

  def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    def builder = bf(repr) // extracted to keep method size under 35 bytes, so that it can be JIT-inlined
    val b = builder
    for (x <- this) b ++= f(x).seq
    b.result
  }

  def foldLeft[B](z: B)(op: (B, A) => B): B = {
    var result = z
    this foreach (x => result = op(result, x))
    result
  }

  def foldRight[B](z: B)(op: (A, B) => B): B =
    reversed.foldLeft(z)((x, y) => op(y, x))

It's clear that map and flatMap create intermediate buffer using corresponding builder , while foldLeft and foldRight reuse the same user-supplied accumulator object, and only use iterators.