Why is UInt64 max equal -1 in Swift?

Question

Using Swift, wether in terminal or playground, when I enter the following on a new line:

UInt64.max

I get a result value of -1.

Is this a bug?

Edit: Examples

$ xcrun swift
Welcome to Swift!  Type :help for assistance.
  1> UInt64.max / 2
$R1: UInt64 = 9223372036854775807
  2> UInt64.max / 2 + 1
$R2: UInt64 = -9223372036854775808
  3> UInt64.max
$R3: UInt64 = -1
  4> println(UInt64.max / 2)
9223372036854775807
  5> println(UInt64.max / 2 + 1)
9223372036854775808
  6> println(UInt64.max)
18446744073709551615

I meant just typing it in a playground line or Swift terminal - updated question. — Khaled Barazi, Jun 14 '14 at 10:24
I think it's a bug in the playground implementation: they re-interpret it as something else (either a signed int64 or even int32). What do you get for `UInt64.max/4`? — Sergey Kalinichenko, Jun 14 '14 at 10:28
obviously it is caused by integer overflow and most likely a bug so you should file a bug report : https://bugreport.apple.com — Bryan Chen, Jun 14 '14 at 10:29
@Spectravideo328 So they print it as `Int64` then. It's a bug. — Sergey Kalinichenko, Jun 14 '14 at 11:05
`println(UInt64.max)` that does not give `-1` at all. how do you get that value? — holex, Jun 14 '14 at 12:03
@holex looks like it is a playground and lldb bug only (I hope). So if you just type UInt64.max in a line in playground or Swift terminal or lldb at a break, you get the -1. — Khaled Barazi, Jun 14 '14 at 14:04
@Spectravideo328, I've not been able to try any playground stuff, regarding the Xcode6 directly crashes on my computer, if I open any playgound. — holex, Jun 14 '14 at 17:50

mono · Answer 1 · 2014-06-14T12:55:42.950

6

I think it is a bug of terminal and playground.

But, the output of the println is correct(18446744073709551615) in both terminal and playground.

edited Jun 14 '14 at 12:55

answered Jun 14 '14 at 10:31

mono

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score 0 · Answer 2 · answered Jun 21 '14 at 16:35

0

Looks like the same bug I've run into with any unsigned type.

echo "UInt8(255)" | xcrun swift

will also print -1

answered Jun 21 '14 at 16:35

Brian

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This appears to be fixed in Swift 2.1.1 : `echo "UInt8(255)" | xcrun swift` now outputs `$R0: UInt8 = 255` – Adrian Sluyters Feb 10 '16 at 03:32
or optionally: `echo "UInt8.max" | xcrun` swift now outputs $R0: UInt8 = 255 – Adrian Sluyters Feb 10 '16 at 03:34

score -1 · Answer 3 · answered Apr 04 '16 at 04:16

-1

This is still a bug, even with the REPL in Swift 2.2. See https://bugs.swift.org/browse/SR-1007 or https://bugs.swift.org/browse/SR-1138.

Fixed in Playgrounds with XCode 7.3 though.

answered Apr 04 '16 at 04:16

jd20

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score -2 · Answer 4 · answered Aug 31 '15 at 07:15

-2

It's because your processor is of 64 bit and UInt is 64 bit * 2. The type Int can't run this number, it's out of range.

answered Aug 31 '15 at 07:15

Segev

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That is not true, UInt = UInt64, which is exactly 64 bits. – kareman Aug 21 '20 at 12:41

Why is UInt64 max equal -1 in Swift?

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