Scala Doubles, and Precision

Edit: fixed the problem that @ryryguy pointed out. (Thanks!)

If you want it to be fast, Kaito has the right idea. math.pow is slow, though. For any standard use you're better off with a recursive function:

def trunc(x: Double, n: Int) = {
  def p10(n: Int, pow: Long = 10): Long = if (n==0) pow else p10(n-1,pow*10)
  if (n < 0) {
    val m = p10(-n).toDouble
    math.round(x/m) * m
  }
  else {
    val m = p10(n).toDouble
    math.round(x*m) / m
  }
}

This is about 10x faster if you're within the range of Long (i.e 18 digits), so you can round at anywhere between 10^18 and 10^-18.