Which is the Swift equivalent of isnan()?

It's defined in the FloatingPointNumber protocol, which both the Float and Double types conform to. Usage is as follows:

let d = 3.0let isNan = d.isNaN // Falselet d = Double.NaNlet isNan = d.isNaN // True

If you're looking for a way to make this check yourself, you can. IEEE defines that NaN != NaN, meaning you can't directly compare NaN to a number to determine its is-a-number-ness. However, you can check that maybeNaN != maybeNaN. If this condition evaluates as true, you're dealing with NaN.

Although you should prefer using aVariable.isNaN to determine if a value is NaN.

As a bit of a side note, if you're less sure about the classification of the value you're working with, you can switch over value of your FloatingPointNumber conforming type's floatingPointClass property.

let noClueWhatThisIs: Double = // ...switch noClueWhatThisIs.floatingPointClass {case .SignalingNaN:    print(FloatingPointClassification.SignalingNaN)case .QuietNaN:    print(FloatingPointClassification.QuietNaN)case .NegativeInfinity:    print(FloatingPointClassification.NegativeInfinity)case .NegativeNormal:    print(FloatingPointClassification.NegativeNormal)case .NegativeSubnormal:    print(FloatingPointClassification.NegativeSubnormal)case .NegativeZero:    print(FloatingPointClassification.NegativeZero)case .PositiveZero:    print(FloatingPointClassification.PositiveZero)case .PositiveSubnormal:    print(FloatingPointClassification.PositiveSubnormal)case .PositiveNormal:    print(FloatingPointClassification.PositiveNormal)case .PositiveInfinity:    print(FloatingPointClassification.PositiveInfinity)}

Its values are declared in the FloatingPointClassification enum.

The accepted answer works but when I first saw it I wasn't exactly clear because of the example and i didn't know that NaN is an acronym for "not a number".

Here's an example from Apple for anyone who isn't clear:

let x = 0.0let y = x * .infinity // y is a NaN because .infinity is not a numberif y.isNan {    print("this is NaN") // this will print} else {    print("this isn't Nan")}