Nirmalya Nirmalya - 3 months ago 6
Scala Question

ScalaCheck: Gen.choose outputting values beyond a specified range

I am using Scalacheck for finding defects, as a part of an assignment. Unbelievably perhaps, I am stuck as it is generating a pair of non-zero integers.

From my IntelliJ worksheet, ad verbatim:

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


Because, I am explicitly mentioning the minimum of the chosen values to be non-zero, I should not see any zero inside the property, right? At least, that is my understanding. But this is what I see:

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 = ()


From where are these zeroes coming? What I am missing? Perhaps, something obvious, but my eyes don't catch them.

I am using Scalcheck version 1.12.1

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


Any help, appreciated.

Update I: as suggested by @samar (I also found a reference to the same in gitbook), I tried to filter the generator with a suchThat. But not much luck (code below):

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


I ran this code for about 6-7 times. This is one random output.

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 = ()


Update II: Just to confirm, I have followed Eric's suggestions and have done this:

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


This works:

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.


Update III:
FWIW, I have found that by moving to Scalacheck version 1.13.1 (it was 1.12.x originally), I get the following behaviour:

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


produces

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


but by changing the condition to equality from less-than, this happens:

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


produces

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


Clearly, the condition being applied overrides the explicit instructions that I leave with the Generator. Isn't this somewhat non-intuitive? What do others think?

Answer

It probably comes from the shrinking of properties. Try with forAllNoShrink instead.