Ubuntu 22.04下NumberFormatter格式化Float小数位数异常求助

需要将数值始终保留逗号后2位小数，但部分Float值在Linux（Ubuntu 22.04）环境下无法通过NumberFormatter正确格式化，macOS 14.7下运行正常。

代码示例 #

import Foundation
let floatNumber: Float = 17889.055
let roundedNumber: Float = (floatNumber * 100).rounded() / 100
let formatter: NumberFormatter = NumberFormatter()
formatter.locale = Locale(identifier: "de_DE")
formatter.numberStyle = .decimal
formatter.maximumFractionDigits = 2
formatter.minimumFractionDigits = 2
formatter.groupingSeparator = "."
formatter.decimalSeparator = ","

print(formatter.string(from: NSNumber(value: floatNumber)) ?? String(format: "%.2f", floatNumber)) 
print(formatter.string(from: NSNumber(value: roundedNumber)) ?? String(format: "%.2f", roundedNumber)) 

实际输出（Linux） #

swift test.swift 
17.889,055
17.889,061

预期输出 #

swift test.swift 
17.889,06
17.889,06

macOS下输出 #

17.889,05
17.889,06

解决办法 #

方案1：改用Double类型 #

Float是单精度浮点型，无法精确存储17889.055这类十进制小数，实际存储值存在微小误差。Linux下Foundation对该误差的处理逻辑和macOS不同，导致格式化结果异常。改用双精度的Double能大幅降低精度偏差：

import Foundation
let doubleNumber: Double = 17889.055
let roundedDouble: Double = (doubleNumber * 100).rounded() / 100
let formatter: NumberFormatter = NumberFormatter()
formatter.locale = Locale(identifier: "de_DE")
formatter.numberStyle = .decimal
formatter.maximumFractionDigits = 2
formatter.minimumFractionDigits = 2
formatter.groupingSeparator = "."
formatter.decimalSeparator = ","

print(formatter.string(from: NSNumber(value: doubleNumber)) ?? String(format: "%.2f", doubleNumber)) 
print(formatter.string(from: NSNumber(value: roundedDouble)) ?? String(format: "%.2f", roundedDouble)) 

方案2：使用Decimal类型精确处理 #

Decimal专为十进制精确运算设计，完全避免浮点精度丢失问题，是处理货币、精确小数场景的最优选择：

import Foundation
let decimalNumber = Decimal(string: "17889.055")!
let roundedDecimal = decimalNumber.rounding(accordingTo: .init(scale: 2, roundingMode: .plain))
let formatter: NumberFormatter = NumberFormatter()
formatter.locale = Locale(identifier: "de_DE")
formatter.numberStyle = .decimal
formatter.maximumFractionDigits = 2
formatter.minimumFractionDigits = 2
formatter.groupingSeparator = "."
formatter.decimalSeparator = ","

print(formatter.string(from: roundedDecimal as NSNumber) ?? "\(roundedDecimal)")

原因总结 #

• Float的单精度存储特性导致无法精确表示部分十进制小数，存储值存在误差；
• Linux与macOS的Foundation框架对浮点误差的格式化处理逻辑存在差异，引发输出异常；
• Double提升精度减少误差，Decimal则从根源上解决精度问题。

内容的提问来源于stack exchange，提问作者lain