[swift-users] function advancedBy() bugs

[zh ao]

The second half on mutating is wrong. Please ignore that.

Zhaoxin

On Tue, May 17, 2016 at 8:47 PM, zh ao <owenzx at gmail.com> wrote:

> Xcode Version 7.3.1 (7D1014), swift 2.2 along with Xcode.
>
> Zhaoxin
>
> On Tue, May 17, 2016 at 8:46 PM, zh ao <owenzx at gmail.com> wrote:
>
>> It seams that I encountered bugs in function advancedBy().
>>
>> //: Playground - noun: a place where people can play
>>
>>
>> import Foundation
>>
>>
>> do {
>>
>>     var str = "abcdefg"
>>
>>     var range = str.startIndex..<str.endIndex // 0..<7
>>
>>     str += "hijklmn"
>>
>>     range.endIndex = str.endIndex // 0..<14
>>
>>     let index = range.startIndex.advancedBy(10)
>>
>>     //fatal error: cannot increment endIndex
>>
>> }
>>
>>
>> do {
>>
>>     var str = "abcdefg"
>>
>>     str += "hijklmn"
>>
>>
>>
>>     var range = str.startIndex..<str.endIndex // 0..<14
>>
>>     let index = range.startIndex.advancedBy(10)
>>
>>     range // 0..<14
>>
>> }
>>
>>
>> do {
>>
>>     var range = 0..<7
>>
>>     let index = range.startIndex.advancedBy(10)
>>
>>     range // 0..<7
>>
>> }
>>
>>
>> There are three do blocks. All codes are similar.
>>
>>
>> First block and second block is the almost same. But I encounter an error
>> in the first block, saying "fatal error: cannot increment endIndex". It
>> shouldn't appear as the endIndex of range is 14 instead 7.
>>
>>
>> Second block and third block both used function advancedBy, however, in
>> the second block, the method is from BidirectionalIndexType,
>>
>>
>> extension BidirectionalIndexType {
>>
>>     @warn_unused_result
>>
>>     public func advancedBy(n: Self.Distance) -> Self
>>
>>     @warn_unused_result
>>
>>     public func advancedBy(n: Self.Distance, limit: Self) -> Self
>>
>> }
>>
>>
>> ​in the ​third block, it is RandomAccessIndexType,
>>
>>
>> extension Int : RandomAccessIndexType {
>>
>>     /// Returns the next consecutive value after `self`.
>>
>>     ///
>>
>>     /// - Requires: The next value is representable.
>>
>>     public func successor() -> Int
>>
>>     /// Returns the previous consecutive value before `self`.
>>
>>     ///
>>
>>     /// - Requires: The previous value is representable.
>>
>>     public func predecessor() -> Int
>>
>>     public func distanceTo(other: Int) -> Distance
>>
>>     public func advancedBy(n: Distance) -> Int
>>
>> }
>>
>>
>> ​So the range in the third block after advancedBy is unchanged. It makes
>> sense here. But I think people may feel strange as one is a mutating
>> function and the other is not.
>>
>> ​Zh​aoxin
>>
>>
>
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