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[SR-10637] Double to String conversion often fails in last digit (e.g. 2/3 != 0.666...7) #53037

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swift-ci opened this issue May 8, 2019 · 2 comments
Closed

[SR-10637] Double to String conversion often fails in last digit (e.g. 2/3 != 0.666...7) #53037

swift-ci opened this issue May 8, 2019 · 2 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella

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@swift-ci
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swift-ci commented May 8, 2019

Previous ID SR-10637
Radar None
Original Reporter ajo (JIRA User)
Type Bug
Status Resolved
Resolution Invalid
Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug
Assignee None
Priority Medium

md5: e0426be603bb5b7bf1fd4b94b68afef8

Issue Description:

There seems to be an bug with last digit rounding of Double-to-String conversion.

#!/usr/bin/swift

import Foundation

typealias FloatType = Double

let twoThirds: FloatType = FloatType(2) / FloatType(3)      // 2/3
let twoThirdsString: String = String(twoThirds)             // 0.66...6 (should be ...7)
let formatString: String = String(format: "%lf", twoThirds) // = 0.666667

print("Type: \(FloatType.self)")
print("init(_: \(FloatType.self): \(twoThirdsString)")  // Double: 0.6666666666666666,  Float: 0.6666667
print("init(format: ...): \(formatString)")             // Double: 0.666667             Float: 0.666667


// Should hold if Double -> String conversion holds
assert(twoThirdsString.last == "7")

The Swift script above works when FloatType = Float, but not when FloatType = Double.
@belkadan
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belkadan commented May 8, 2019

This is just how floating-point works: 2/3 cannot be represented exactly, and the closest value to 2/3 in Double is 0.66666666666666662965923251249478198587894439697265625, with the next value up being 0.66666666666666674068153497501043602824211120605468750.

import Foundation

typealias FloatType = Double

let twoThirds: FloatType = FloatType(2) / FloatType(3)
print(String(format: "%.60lf", twoThirds))
print(String(format: "%.60lf", twoThirds + twoThirds.ulp))

@swift-ci
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Comment by Andreas Jönsson (JIRA)

Yes, that's correct but I don't quite fully agree with the conclusion.

The resulting string (in this case, "0.6666666666666666" with 16 significant decimal digits) is chosen so that it's invertible - i.e. converting a Double back and forth to a String can be done deterministically (without introducing rounding errors).

The part I missed was that this is actually specified in the String API init being used - the init with LosslessStringConvertible that's being called in the sample: https://developer.apple.com/documentation/swift/string/2431675-init

So everything is working as specified.

Thanks.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella
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@belkadan @swift-ci