【Swift】少数点付きの四則演算

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NSDecimalNumber を使うといいらしい。

        let str1 = "5.5"
let double1 = 0.5
let decimal1 = NSDecimalNumber(string: str1)
let decimal2 = NSDecimalNumber(value: double1)

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        let decimal3 = decimal1.adding(decimal2)
or
let decimal3 = decimal1.subtracting(decimal2)
or
let decimal3 = decimal1.multiplying(by: decimal2)
or
let decimal3 = decimal1.dividing(by: decimal2)

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        let double2 = decimal3.doubleValue