@wonder posted https://www.b4x.com/android/forum/threads/return-nan-and-infinity.106046/post-663817

This is an addendum to his posting to show how these two objects are both numbers and not numbers, and how to test for that.

This is an addendum to his posting to show how these two objects are both numbers and not numbers, and how to test for that.

B4X:

```
Private Sub specialNumbers
'Infinity behaves in the expected way as as an infinitely large number
Dim infinity As Double = 1 / 0
Log(infinity = -Logarithm(0, cE)) 'True
Log((infinity = infinity)) 'True
Log((infinity = -infinity)) 'False
Log(TAB)
Log((infinity + 5)) 'Infinity
Log((infinity - 5)) 'Infinity
Log((infinity * 5)) 'Infinity
Log((infinity / 5)) 'Infinity
Log((infinity / 0)) 'Infinity
Log(infinity * infinity) 'Infinity
Log(Sqrt(infinity)) 'Infinity
Log(Power(cE, infinity)) 'Infinity
'Numberformat shows a nice infinity symbol
Log(NumberFormat2(infinity, 1, 0, 0, False)) '∞
Log((1 / infinity)) '0
Log((100 / infinity)) '0
Log((1000 / infinity)) '0
Log((1000 / -infinity)) '-0
Log(TAB)
'However, like the Sqrt(-1), the following are undefined and therefore NaN
Log((infinity / infinity)) 'NaN
Log((infinity / -infinity)) 'NaN
'The simplest NaN is 0 / 0
Dim NaN As Double = 0 / 0
Log((NaN = Sqrt(-1000))) 'False - this is unexpected, but understandable given that the comparison of NaN with anything return False
Log((NaN = NaN)) 'False
Log((NaN = -NaN)) 'False
Log((NaN = "NaN")) 'False - this is unexpected, but understandable given the order of the string conversion
Log(("NaN" = NaN)) 'True
'IsNumber only checks if string can be converted to number, so it's argument is first converted to a string "NaN"
' which is (paradoxically) interpreted as a valid a number
Log(IsNumber(Sqrt(-1))) 'True
Log(IsNumber(0 / 0)) 'True
Log(IsNumber(0)) 'True
Log(IsNumber(infinity)) 'True
'NaN is the only "number" that is not equal to itself - this can be used to test for NaN - see
'Private Sub IsNaN(z As Double) As Boolean
' Return z <> z
'End Sub
Log(IsNaN(Sqrt(-1))) 'True
Log(IsNaN(0 / 0)) 'True
Log(IsNaN(0)) 'False
Log(IsNaN(infinity)) 'False
Log(IsNaN(NaN)) 'True
'all computations with NaN result in NaN [NaN could be used as "missing" value in Statistics]
Log((NaN + 5)) 'NaN
Log((NaN - 5)) 'NaN
Log((NaN * 5)) 'NaN
Log((NaN / 5)) 'NaN
Log((NaN / 0)) 'NaN
Log(NaN * NaN) 'NaN
Log(Sqrt(NaN)) 'NaN
Log(Power(cE, NaN)) 'NaN
Log((1 / NaN)) 'NaN
Log((100 / NaN)) 'NaN
Log((1000 / NaN)) 'NaN
Log((NaN / NaN)) 'NaN
End Sub
Private Sub IsNaN(z As Double) As Boolean
Return z <> z
End Sub
Private Sub NotNaN(z As Double) As Boolean
Return z = z
End Sub
```

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