PROSAGA码农传奇-swift/objective-C-获取长数字中的 x 个初始数字

@matt 建议的是获取字符串的前四位数字和指数。试试这个：

let string = "9.272685136e-9"
    if let lowerBound = string.range(of: "e-")?.lowerBound,
        case let mantissa = string[..<lowerBound].prefix(5),
        case let exponent = string[lowerBound...],
        case let finalValue = [mantissa,exponent].joined(),
        let decimal = Decimal(string: finalValue) {
        print(decimal)
    }
编辑/更新：

如果您的值没有科学记数法，您可以简单地使用数字格式化程序来转换它：

extension Formatter {
        static let scientific: NumberFormatter = {
            let formatter = NumberFormatter()
            formatter.numberStyle = .scientific
            return formatter
        }()
    }
    extension Decimal {
        var scientific: String { Formatter.scientific.string(for: self) ?? "" }
    }
    let decimal = Decimal(string: "0.0000000092726851369802")!
    let scientific = decimal.scientific  // "9.2726851369802E-9"
    if let lowerBound = scientific.range(of: "e-", options: .caseInsensitive)?.lowerBound,
        case let mantissa = scientific[..<lowerBound].prefix(5),
        case let exponent = scientific[lowerBound...],
        case let finalValue = [mantissa,exponent].joined(),
        let decimal = Decimal(string: finalValue) {
        print(decimal)  // "0.000000009272\n"
    }
    @available（macOS 12.0、iOS 15.0、tvOS 15.0、watchOS 8.0、*）

您可以使用格式化方法并将数字精度有效数字设置为四位：

let decimal = Decimal(string: "9.272685136e-9")!
    decimal.formatted(.number.precision(.significantDigits(4)))  // "0.000000009273"