How to find max value for Double and Float in Swift

问题

Current learning Swift, there are ways to find max and min value for different kind of Integer like Int.max and Int.min.

Is there a way to find max value for Double and Float? Moreover, which document should I refer for this kind of question? I am currently reading Apple's The Swift Programming Language.

回答1:

While there’s no Double.max, it is defined in the C float.h header, which you can access in Swift via import Darwin.

import Darwin

let fmax = FLT_MAX
let dmax = DBL_MAX

These are roughly 3.4 * 10^38 and 1.79 * 10^308 respectively.

But bear in mind it’s not so simple with floating point numbers (it’s never simple with floating point numbers). When holding numbers this large, you lose precision in a similar way to losing precision with very small numbers, so:

let d = DBL_MAX
let e = d - 1.0
let diff = d - e
diff == 0.0  // true

let maxPlusOne = DBL_MAX + 1
maxPlusOne == d  // true

let inf = DBL_MAX * 2
// perhaps infinity is the “maximum” 
inf == Double.infinity  // true

So before you get into some calculations that might possibly brush up against these limits, you should probably read up on floating point. Here and here are probably a good start.

回答2:

As of Swift 3, you should use:

CGFloat.greatestFiniteMagnitude
Double.greatestFiniteMagnitude
Float.greatestFiniteMagnitude

回答3:

AV's answer is fine, but I find those macros hard to remember and a bit non-obvious, so eventually I made Double.MIN and friends work:

extension Double {
    static var MIN     = -DBL_MAX
    static var MAX_NEG = -DBL_MIN
    static var MIN_POS =  DBL_MIN
    static var MAX     =  DBL_MAX
}

Don't use lowercase min and max -- those symbols are used in Swift 3.