Swift-Tips之rounded(_:)

let nums = [3.0, 3.001, 3.5, 3.999, -3.0, -3.001, -3.5, -3.999]
let rules = [FloatingPointRoundingRule.awayFromZero, .down, .toNearestOrAwayFromZero, .toNearestOrEven, .towardZero, .up]

for rule in rules {
    print("========\(rule)========")
    for num in nums {
        let value = num.rounded(rule)
        print("Rule: \(rule), Source: \(num), Value: \(value)")
    }
}

/*
========awayFromZero========
Source: 3.0, Value: 3.0
Source: 3.001, Value: 4.0
Source: 3.5, Value: 4.0
Source: 3.999, Value: 4.0
Source: -3.0, Value: -3.0
Source: -3.001, Value: -4.0
Source: -3.5, Value: -4.0
Source: -3.999, Value: -4.0
========down========
Source: 3.0, Value: 3.0
Source: 3.001, Value: 3.0
Source: 3.5, Value: 3.0
Source: 3.999, Value: 3.0
Source: -3.0, Value: -3.0
Source: -3.001, Value: -4.0
Source: -3.5, Value: -4.0
Source: -3.999, Value: -4.0
========toNearestOrAwayFromZero========
Source: 3.0, Value: 3.0
Source: 3.001, Value: 3.0
Source: 3.5, Value: 4.0
Source: 3.999, Value: 4.0
Source: -3.0, Value: -3.0
Source: -3.001, Value: -3.0
Source: -3.5, Value: -4.0
Source: -3.999, Value: -4.0
========toNearestOrEven========
Source: 3.0, Value: 3.0
Source: 3.001, Value: 3.0
Source: 3.5, Value: 4.0
Source: 3.999, Value: 4.0
Source: -3.0, Value: -3.0
Source: -3.001, Value: -3.0
Source: -3.5, Value: -4.0
Source: -3.999, Value: -4.0
========towardZero========
Source: 3.0, Value: 3.0
Source: 3.001, Value: 3.0
Source: 3.5, Value: 3.0
Source: 3.999, Value: 3.0
Source: -3.0, Value: -3.0
Source: -3.001, Value: -3.0
Source: -3.5, Value: -3.0
Source: -3.999, Value: -3.0
========up========
Source: 3.0, Value: 3.0
Source: 3.001, Value: 4.0
Source: 3.5, Value: 4.0
Source: 3.999, Value: 4.0
Source: -3.0, Value: -3.0
Source: -3.001, Value: -3.0
Source: -3.5, Value: -3.0
Source: -3.999, Value: -3.0
 */
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总结

• awayFromZero : 四舍五入到最接近的值，返回值大于或等于源的大小（该规则比较的是绝对值）

let num = 3.5
num.rounded(.awayFromZero) // 4

let num2 = -3.5
num2.rounded(.awayFromZero) //-4
//可以看出 -4是小于-3.5的，所以它的规则说返回值大于或等于源指的是两值的绝对值的比较
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down
toNearestOrAwayFromZero
toNearestOrEven

let num = 3.5
// 这是3与4同等接近，选择偶数4
num.rounded(.toNearestOrEven)  // 4
let num2 = 4.5
// 这是4与5同等接近，选择偶数4
num2.rounded(.toNearestOrEven)  // 4
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• towardZero : 四舍五入到最接近的值，返回值小于于或等于源的大小；通过上面的例子可以看出它是忽略负号的；与 awayFromZero 对应
• up : 四舍五入到最接近的值，返回值大于或等于源的大小；不忽略负号；与 up 对应