为什么DBL_MIN不是负数(在Swift中)？

为什么表达式DBL_MIN< 0返回false？ @H_262_5@为什么与DLB_MIN的比较似乎表明DBL_MIN是正数？

let a = DBL_MIN // 2.225073858507201e-308
let b = DBL_MAX // 1.797693134862316e+308

if a < 0.0 {
    print("1. DBL_MIN is indeed less than zero") // doesn't print (unexpected)
}

if DBL_MIN < 0.0 {
    print("2. DBL_MIN is indeed less than zero") // doesn't print (unexpected)
}

if DBL_MIN > 0.0 {
    print("3. DBL_MIN is larger than zero?") // prints (unexpected)
}

if DBL_MIN > DBL_MAX {
    print("4. DBL_MIN did some strange flip?") // doesn't print (OK)
}

if b > 0.0 {
    print("5. DBL_MAX is indeed larger than zero") // prints  (OK)
}

/* printout: 
3. DBL_MIN is larger than zero?
5. DBL_MAX is indeed larger than zero  */

我怀疑它与浮点精度有关,但我自己无法解释它.

我使用的是Swift 2.1.1和XCode 7.2.

2.225073858507201 * 10 ^ -308