[英]ScalaCheck generate BST
我正在嘗試使用ScalaCheck為BST創建一個Gen,但是當我調用.sample方法時,它會給我java.lang.NullPointerException。 我哪里錯了?
sealed trait Tree
case class Node(left: Tree, right: Tree, v: Int) extends Tree
case object Leaf extends Tree
import org.scalacheck._
import Gen._
import Arbitrary.arbitrary
case class GenerateBST() {
def genValue(left: Tree, right: Tree): Gen[Int] = (left, right) match {
case (Node(_, _, min), Node(_, _, max)) => arbitrary[Int].suchThat(x => x > min && x < max)
case (Node(_, _, min), Leaf) => arbitrary[Int].suchThat(x => x > min)
case (Leaf, Node(_, _, max)) => arbitrary[Int].suchThat(x => x < max)
case (Leaf, Leaf) => arbitrary[Int]
}
val genNode = for {
left <- genTree
right <- genTree
v <- genValue(left, right)
} yield Node(left, right, v)
def genTree: Gen[Tree] = oneOf(const(Leaf), genNode)
}
GenerateBST().genTree.sample
由於您為遞歸數據類型遞歸定義生成器的方式,您需要在頂部使用Gen.lzy
:
def genTree: Gen[Tree] = Gen.lzy(oneOf(const(Leaf), genNode))
作為一個不相關的附注,在您的生成器定義中使用suchThat
通常應該只是最后的手段。 這意味着sample
經常會失敗(大約三分之一的時間都是代碼的固定版本),更重要的是,如果你有一天想要創建導致Tree
任意函數,你會看到很多可怕的org.scalacheck.Gen$RetrievalError: couldn't generate value
異常。
在這種情況下,你可以通過使用Gen.chooseNum
輕松避免suchThat
Gen.chooseNum
,如果它們的順序錯誤,則交換左側和右側:
sealed trait Tree
case class Node(left: Tree, right: Tree, v: Int) extends Tree
case object Leaf extends Tree
import org.scalacheck.{ Arbitrary, Gen }
object GenerateBST {
def swapIfNeeded(l: Tree, r: Tree): (Tree, Tree) = (l, r) match {
// If the two trees don't have space between them, we bump one and recheck:
case (Node(_, _, x), n @ Node(_, _, y)) if math.abs(x - y) <= 1 =>
swapIfNeeded(l, n.copy(v = y + 1))
// If the values are in the wrong order, swap:
case (Node(_, _, x), Node(_, _, y)) if x > y => (r, l)
// Otherwise do nothing:
case (_, _) => (l, r)
}
def genValue(left: Tree, right: Tree): Gen[Int] = (left, right) match {
case (Node(_, _, min), Node(_, _, max)) => Gen.chooseNum(min + 1, max - 1)
case (Node(_, _, min), Leaf) => Gen.chooseNum(min + 1, Int.MaxValue)
case (Leaf, Node(_, _, max)) => Gen.chooseNum(Int.MinValue, max - 1)
case (Leaf, Leaf) => Arbitrary.arbitrary[Int]
}
val genNode = for {
l0 <- genTree
r0 <- genTree
(left, right) = swapIfNeeded(l0, r0)
v <- genValue(left, right)
} yield Node(left, right, v)
def genTree: Gen[Tree] = Gen.lzy(Gen.oneOf(Gen.const(Leaf), genNode))
}
現在你可以使用Arbitrary[Whatever => Tree]
而不用擔心發電機故障:
scala> implicit val arbTree: Arbitrary[Tree] = Arbitrary(GenerateBST.genTree)
arbTree: org.scalacheck.Arbitrary[Tree] = org.scalacheck.ArbitraryLowPriority$$anon$1@606abb0e
scala> val f = Arbitrary.arbitrary[Int => Tree].sample.get
f: Int => Tree = org.scalacheck.GenArities$$Lambda$7109/289518656@13eefeaf
scala> f(1)
res0: Tree = Leaf
scala> f(2)
res1: Tree = Node(Leaf,Leaf,-20313200)
scala> f(3)
res2: Tree = Leaf
scala> f(4)
res3: Tree = Node(Node(Leaf,Leaf,-850041807),Leaf,-1)
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.