nan

A NaN compares not equal, not greater than, and not less than every value, including itself. Passing a NaN to an operation generally results in NaN.

let x = 1.21
// x > Double.nan == false
// x < Double.nan == false
// x == Double.nan == false

Because a NaN always compares not equal to itself, to test whether a floating-point value is NaN, use its isNaN property instead of the equal-to operator (==). In the following example, y is NaN.

let y = x + Double.nan
print(y == Double.nan)
// Prints "false"
print(y.isNaN)
// Prints "true"