ScalaCheck:Gen.choose 输出超出指定范围的值

Question

作为作业的一部分，我正在使用 Scalacheck 来查找缺陷。令人难以置信的是，我被卡住了，因为它正在生成一对非零整数。

从我的 IntelliJ 工作表中，逐字逐句:

import org.scalacheck._
    import Arbitrary._
    import Gen._
    import Prop._

    implicit lazy val genUnequalIntPairs = for {
      i <- Gen.choose(1,1000)
      j <- Gen.choose(i+1,1000)
      if (i < j)
    } yield (i,j)

    val kk = forAll (genUnequalIntPairs)  {
      case (x,y) => println("x =" + x + ", y =" + y)
        x == y
    }

kk.check

因为，我明确提到所选值的最小值为非零，我不应该在属性中看到任何零，对吗？至少，这是我的理解。但这就是我所看到的:

x =134, y =547
x =0, y =547
x =0, y =0
x =0, y =274
x =0, y =0
x =0, y =137
x =0, y =0
x =0, y =69
x =0, y =0
x =0, y =35
x =0, y =0
x =0, y =18
x =0, y =0
x =0, y =9
x =0, y =0
x =0, y =5
x =0, y =0
x =0, y =3
x =0, y =0
x =0, y =2
x =0, y =0
x =0, y =1
x =0, y =0
! Falsified after 0 passed tests.
> ARG_0: (0,1)
> ARG_0_ORIGINAL: (134,547)
res0: Unit = ()

这些零是从哪里来的？我缺少什么？也许，一些显而易见的事情，但我的眼睛没有捕捉到它们。

我正在使用 Scalcheck 版本 1.12.1

Seq("org.scalacheck" %% "scalacheck" % "1.12.1")

任何帮助，不胜感激。

更新 I:正如@samar 所建议的(我还在 gitbook 中找到了对相同内容的引用)，我尝试使用此类过滤生成器。但运气不佳(下面的代码):

implicit lazy val genUnequalIntPairs = for {
  i <- Gen.choose(1,1000).suchThat(_ > 0)
  j <- Gen.choose(i+1,1000).suchThat(_ > 0)
  if (i < j)
} yield (i,j)

val kk = forAll (genUnequalIntPairs)  {
  case (x,y) => println("x =" + x + ", y =" + y)
    x == y
}

kk.check

我运行这段代码大约 6-7 次。这是一种随机输出。

x =536, y =730
x =0, y =730
x =0, y =0
x =0, y =365
x =0, y =0
x =0, y =183
x =0, y =0
x =0, y =92
x =0, y =0
x =0, y =46
x =0, y =0
x =0, y =23
x =0, y =0
x =0, y =12
x =0, y =0
x =0, y =6
x =0, y =0
x =0, y =3
x =0, y =0
x =0, y =2
x =0, y =0
x =0, y =1
x =0, y =0
! Falsified after 0 passed tests.
> ARG_0: (0,1)
> ARG_0_ORIGINAL: (536,730)
res0: Unit = ()

更新二:为了确认，我遵循了 Eric 的建议并做到了这一点:

implicit lazy val genUnequalIntPairs = for {
  i <- Gen.choose(1,1000)
  j <- Gen.choose(i+1,1000)
  if (i != 0 && j != 0)
} yield {
  (i,j)
}

val kk = forAllNoShrink(genUnequalIntPairs)  {
  case (x,y) => println("x =" + x + ", y =" + y)
    x < y
}

kk.check

这有效:

x =805, y =1000
x =742, y =926
x =772, y =919
x =219, y =686
x =999, y =1000
x =752, y =792
x =163, y =585
x =721, y =735
x =251, y =866
x =383, y =887
x =797, y =939
x =925, y =947
x =291, y =951
x =502, y =619
x =422, y =756
x =159, y =886
x =49, y =955
x =624, y =819
x =922, y =979
x =881, y =894
x =936, y =952
x =908, y =975
x =802, y =976
x =918, y =946
x =564, y =823
x =544, y =751
x =916, y =938
x =57, y =762
x =614, y =963
x =497, y =986
x =102, y =332
x =264, y =643
x =611, y =720
x =542, y =797
x =704, y =784
Output exceeds cutoff limit.

更新三:
FWIW，我发现通过迁移到 Scalacheck 版本 1.13.1(最初是 1.12.x)，我得到以下行为:

implicit lazy val genUnequalIntPairs = for {
  i <- Gen.choose(1,1000)
  j <- Gen.choose(1,1000)
  if (i < j)
} yield {
  (i,j)
}

val kk = forAll(genUnequalIntPairs)  {
  case (x,y) => println("x =" + x + ", y =" + y)
    x < y
}

kk.check

产生

x =56, y =752
x =395, y =532
x =468, y =828
x =326, y =749
x =203, y =973
x =294, y =393
x =589, y =975
x =44, y =75
x =406, y =533
x =33, y =381
x =405, y =767
x =13, y =883
x =201, y =341
x =593, y =991
x =636, y =913
x =508, y =782
x =333, y =861
x =306, y =863
x =428, y =537
x =373, y =775
x =74, y =462
x =196, y =299
x =245, y =535
x =312, y =993
x =940, y =989
x =12, y =708

但是通过将条件从小于等于相等，会发生这种情况:

implicit lazy val genUnequalIntPairs = for {
  i <- Gen.choose(1,1000)
  j <- Gen.choose(1,1000)
  if (i < j)
} yield {
  (i,j)
}

val kk = forAll(genUnequalIntPairs)  {
  case (x,y) => println("x =" + x + ", y =" + y)
    x == y
}

kk.check

产生

x =370, y =585
x =0, y =585
x =0, y =0
x =0, y =293
x =0, y =0
x =0, y =147
x =0, y =0
x =0, y =74
x =0, y =0
x =0, y =37
x =0, y =0
x =0, y =19
x =0, y =0
x =0, y =10
x =0, y =0
x =0, y =5
x =0, y =0
x =0, y =3

显然，所应用的条件覆盖了我留给 Generator 的显式指令。这不是有点 不直观 ?其他人怎么看？

Answer 1

它可能来自属性的收缩。试试 forAllNoShrink反而。