Scala for-comprehension if filtering too much?

Question

I have the following program: (Scala 2.9.2, Java6)

object Forcomp {
  def main(args: Array[String]): Unit = {
    val xs = List(-1, 0, 1)
    val xss = for (a <- xs; b <- xs if a != 0 && b != 0) yield (a,b)
    println(xss)
  }
}

It produces this output: List((-1,-1), (-1,1), (1,-1), (1,1)) I would have expected it to only filter out values where a and b are both 0 – not all values where either a or b are 0.

I can get the behaviour I want by changing the if-clause to this: if (a,b) != (0,0) – however, should I really have to? Is this a bug or is this intentional behaviour? I, for one, was surprised by this.

Answer 1

The truth table for the filter you have is this:

 a==0  |  b==0 | (a!=0 && b!=0)
--------------------------------
  0    |   0   |      0
  0    |   1   |      0
  1    |   0   |      0
  1    |   1   |      1

whereas the behaviour you say you want is:

 a==0  |  b==0 | !(a==0 && b==0)
--------------------------------
  0    |   0   |      0
  0    |   1   |      1
  1    |   0   |      1
  1    |   1   |      1