In Scala, why is NaN not being picked up by pattern matching?

scala pattern-matching ieee-754 nan
deltanovember picture deltanovember · Aug 2, 2011 · Viewed 12.1k times · Source

My method is as follows

  def myMethod(myDouble: Double): Double = myDouble match {
    case Double.NaN => ...
    case _ => ...
  }

The IntelliJ debugger is showing NaN but this is not being picked up in my pattern matching. Are there possible cases I am omitting

Answer

Tomasz Nurkiewicz picture Tomasz Nurkiewicz · Aug 2, 2011

It is a general rule how 64-bit floating point numbers are compared according to IEEE 754 (not Scala or even Java related, see NaN):

double n1 = Double.NaN;
double n2 = Double.NaN;
System.out.println(n1 == n2);     //false

The idea is that NaN is a marker value for unknown or indeterminate. Comparing two unknown values should always yields false as they are well... unknown.

If you want to use pattern matching with NaN, try this:

myDouble match {
    case x if x.isNaN => ...
    case _ => ...
}

But I think pattern matching will use strict double comparison so be careful with this construct.

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