Swift的小数精度问题 [英] Swift's Decimal precision issue

问题描述

根据文档此处，Swift 3/4十进制类型是一种表示形式在以10为基数桥接到NSDecimalNumber.但是，我遇到了使用NSDecimalNumber时无法重现的精度问题.

According to the docs here, Swift 3/4 Decimal type is a representation in base 10 bridged to NSDecimalNumber. However I'm having precision issues that do not reproduce when using NSDecimalNumber.

let dec24 = Decimal(integerLiteral: 24)
let dec1 = Decimal(integerLiteral: 1)
let decResult = dec1/dec24*dec24 
// prints 0.99999999999999999999999999999999999984

let dn24 = NSDecimalNumber(value: 24)
let dn1 = NSDecimalNumber(value: 1)
let dnResult = dn1.dividing(by: dn24).multiplying(by: dn24)
// prints 1

小数"结构不准确，还是我误会了什么?

Shouldn't the Decimal struct be accurate, or am I misunderstanding something?

推荐答案

NSDecimalNumber(及其覆盖类型Decimal)可以代表

...可以表示为mantissa x 10^exponent的任何数字，其中mantissa是长度不超过38位的十进制整数，而exponent是从–128到127的整数.

... any number that can be expressed as mantissa x 10^exponent where mantissa is a decimal integer up to 38 digits long, and exponent is an integer from –128 through 127.

因此，十进制分数(最多38个十进制数字)可以表示 准确，但不是任意数字.特别是1/24 = 0.416666666... 具有无限多个小数位数(重复小数)，并且不能为 完全表示为Decimal.

So decimal fractions (with up to 38 decimal digits) can be represented exactly, but not arbitrary numbers. In particular 1/24 = 0.416666666... has infinitely many decimal digits (a repeating decimal) and cannot be represented exactly as a Decimal.

Decimal和NSDecimalNumber之间也没有精度差异.如果我们打印出差异，这将变得显而易见 在实际结果和理论结果"之间:

Also there is no precision difference between Decimal and NSDecimalNumber. That becomes apparent if we print the difference between the actual result and the "theoretical result":

let dec24 = Decimal(integerLiteral: 24)
let dec1 = Decimal(integerLiteral: 1)
let decResult = dec1/dec24*dec24

print(decResult - dec1)
// -0.00000000000000000000000000000000000016


let dn24 = NSDecimalNumber(value: 24)
let dn1 = NSDecimalNumber(value: 1)
let dnResult = dn1.dividing(by: dn24).multiplying(by: dn24)

print(dnResult.subtracting(dn1))
// -0.00000000000000000000000000000000000016

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