Scalacheck: generate list of numbers with total less than a certain value

Question

I need to generate a random list of numbers of a given size whose total is less than a given fixed threshold using ScalaCheck. In other words, do something like:

val threshold = 3000    
val listGenerator = Gen.listOfN(2000, Gen.choose(1,2000)) suchThat { _.sum < theshold }

While the above expresses exactly what I want to achieve, it doesn't work as the suchThat clause ends up discarding the vast majority of generated values.

Are you sure you need to do this? Every constraint you put on a ScalaCheck generator means more potential inputs that aren't being checked. — Travis Brown, Feb 20 '16 at 23:14

score 0 · Answer 1 · answered Feb 20 '16 at 22:57

Well, given your maximum is 3k and you want a list of 2000 with the minimum value of 1, I'd expect a list of mostly 1s and 2s. One solution is below (With current numbers will only generate a list of 1s),

val threshold = 3000

val max = 2000
val max = threshold / n

val listGenerator = Gen.listOfN(max, Gen.choose(1,max))

The math would change a bit if you allow 0s.

score 0 · Answer 2 · answered Jul 01 '22 at 22:33

Assuming the threshold is T, and the length of the list is N, You can generate this list by picking N + 1 distinct sorted random numbers within 0 until T, and then compute the differences between adjacent numbers.

For example, let T = 10 and N = 3, generate 4 distinct sorted numbers first:

1, 4, 6, 9

By computing the differences of adjacent numbers, you get:

3, 2, 3

whose sum is 8, less than 10.

Following this, you can have the following generator:

  def gen(T: Int, N: Int): Gen[collection.Seq[Int]] =
    Gen.pick(N + 1, 0 until T).map(_.sortBy(-_)).map { sorted =>
      (sorted.init, sorted.tail).zipped.map(_ - _)
    }

Scalacheck: generate list of numbers with total less than a certain value

2 Answers2