在swift中将Double转换为Scientific Notation

Question

我试图将给定的双精度转换为科学记数法,并遇到一些问题.我似乎无法找到关于如何做到这一点的大量文档.目前我正在使用:

 var val = 500
 var numberFormatter = NSNumberFormatter()
 numberFormatter.numberStyle = NSNumberFormatterStyle.ScientificStyle
 let number = numberFormatter.numberFromString("\(val)")
 println(number as Double?) 
 // Prints optional(500) instead of optional(5e+2)

我究竟做错了什么？

Answer 1

你可以设置NumberFormatter属性positiveFormat和exponent Symbol来格式化你想要的字符串,如下所示:

let val = 500
let formatter = NumberFormatter()
formatter.numberStyle = .scientific
formatter.positiveFormat = "0.###E+0"
formatter.exponentSymbol = "e"
if let scientificFormatted = formatter.string(for: val) {
    print(scientificFormatted)  // "5e+2"
}

更新:Xcode 9•Swift 4

您还可以创建扩展以从数字类型获取科学格式化描述,如下所示:

extension Formatter {
    static let scientific: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.numberStyle = .scientific
        formatter.positiveFormat = "0.###E+0"
        formatter.exponentSymbol = "e"
        return formatter
    }()
}

extension Numeric {
    var scientificFormatted: String {
        return Formatter.scientific.string(for: self) ?? ""
    }
}

print(500.scientificFormatted)   // "5e+2"