Unexpected results when using String.CharacterView.Index


(Zhao Xin) #1

When using subscript of `String.CharacterView`, I got an unexpected error.

fatal error: Can't form a Character from an empty String

func test() {

    let s = "Original Script:"

    let cs = s.characters

// let startIndex = cs.startIndex

    let nextIndex = "Original ?".characters.endIndex

    let nextCharacter = cs[nextIndex]// above error

}

test()

​However, if I chose​ another way to get the nextIndex. It works.

func test() {

    let s = "Original Script:"

    let cs = s.characters

    let startIndex = cs.startIndex

// let nextIndex = "Original ?".characters.endIndex

    let nextIndex01 = cs.index(startIndex, offsetBy: "Original ?".characters
.count)

    let nextCharacter = cs[nextIndex01]

}

test()

Further more, I compared the two `nextIndex`. They were equal.

func test() {

    let s = "Original Script:"

    let cs = s.characters

    let startIndex = cs.startIndex

    let nextIndex = "Original ?".characters.endIndex

    let nextIndex01 = cs.index(startIndex, offsetBy: "Original ?".characters
.count)

    let nextCharacter = cs[nextIndex01]

    print(nextIndex01 == nextIndex) // true

}

test()

So I wonder, is there a bug here?

Xcode 8.2.1 (8C1002), Swift 3.0.2

Zhaoxin


(Ole Begemann) #2

When using subscript of `String.CharacterView`, I got an unexpected error.

    fatal error: Can't form a Character from an empty String

func test() {
    let s = "Original Script:"
    let cs = s.characters
// let startIndex = cs.startIndex
    let nextIndex = "Original ?".characters.endIndex
    let nextCharacter = cs[nextIndex]// above error
}

test()

First of all, it's not guaranteed that an index derived from one string can be used to subscript another string. Don't rely on that.

endIndex is also different, and this is why you're seeing a crash here. Let's inspect nextIndex with dump(nextIndex):

▿ Swift.String.CharacterView.Index
   ▿ _base: Swift.String.UnicodeScalarView.Index
     - _position: 10
   - _countUTF16: 0

You see that _countUTF16 is 0, i.e. internally, String.CharacterView assigns its endIndex a length of 0 (in terms of UTF-16 code units). This is why it traps when you use the index for subscripting. The endIndex is not a valid index for subscripting, not for the string it was derived from and not for any other string.

​However, if I chose​ another way to get the nextIndex. It works.

functest() {
    let s = "Original Script:"
    let cs = s.characters
    let startIndex = cs.startIndex
// let nextIndex = "Original ?".characters.endIndex
    let nextIndex01 = cs.index(startIndex, offsetBy: "Original
?".characters.count)
    let nextCharacter = cs[nextIndex01]
}

test()

Here, dump(nextIndex01) prints this:

▿ Swift.String.CharacterView.Index
   ▿ _base: Swift.String.UnicodeScalarView.Index
     - _position: 10
   - _countUTF16: 1

Notice that _countUTF16 is 1, so it looks like a valid index from the perspective of cs. But again, don't rely on this! The results of subscripting a collection with an index derived from another collection are undefined unless the collection explicitly documents otherwise.

Further more, I compared the two `nextIndex`. They were equal.

functest() {
    let s = "Original Script:"
    let cs = s.characters
    let startIndex = cs.startIndex
    let nextIndex = "Original ?".characters.endIndex
    let nextIndex01 = cs.index(startIndex, offsetBy: "Original
?".characters.count)
    let nextCharacter = cs[nextIndex01]
    print(nextIndex01 == nextIndex) // true
}

test()

It looks like String.Index only takes the position into account to determine equality, not its _countUTF16. This makes sense for the way endIndex and index(_:offsetBy:) are implemented. After all, nextIndex and nextIndex01 _should be equal_. It would certainly be possible to implement it differently (where endIndex and index(_:offsetBy:) returned identical indices, including _countUTF16:) and I don't know why the stdlib team chose to do it this way (maybe performance?).

In any case, much of this implementation may change with the work going into strings for Swift 4.

···

On 09/03/2017 08:27, Zhao Xin via swift-users wrote:


(Zhao Xin) #3

Thanks a lot, Ole. I understand now.

Zhaoxin

···

On Thu, Mar 9, 2017 at 7:54 PM, Ole Begemann <ole@oleb.net> wrote:

On 09/03/2017 08:27, Zhao Xin via swift-users wrote:

When using subscript of `String.CharacterView`, I got an unexpected error.

    fatal error: Can't form a Character from an empty String

func test() {
    let s = "Original Script:"
    let cs = s.characters
// let startIndex = cs.startIndex
    let nextIndex = "Original ?".characters.endIndex
    let nextCharacter = cs[nextIndex]// above error
}

test()

First of all, it's not guaranteed that an index derived from one string
can be used to subscript another string. Don't rely on that.

endIndex is also different, and this is why you're seeing a crash here.
Let's inspect nextIndex with dump(nextIndex):

▿ Swift.String.CharacterView.Index
  ▿ _base: Swift.String.UnicodeScalarView.Index
    - _position: 10
  - _countUTF16: 0

You see that _countUTF16 is 0, i.e. internally, String.CharacterView
assigns its endIndex a length of 0 (in terms of UTF-16 code units). This is
why it traps when you use the index for subscripting. The endIndex is not a
valid index for subscripting, not for the string it was derived from and
not for any other string.

​However, if I chose​ another way to get the nextIndex. It works.

functest() {
    let s = "Original Script:"
    let cs = s.characters
    let startIndex = cs.startIndex
// let nextIndex = "Original ?".characters.endIndex
    let nextIndex01 = cs.index(startIndex, offsetBy: "Original
?".characters.count)
    let nextCharacter = cs[nextIndex01]
}

test()

Here, dump(nextIndex01) prints this:

▿ Swift.String.CharacterView.Index
  ▿ _base: Swift.String.UnicodeScalarView.Index
    - _position: 10
  - _countUTF16: 1

Notice that _countUTF16 is 1, so it looks like a valid index from the
perspective of cs. But again, don't rely on this! The results of
subscripting a collection with an index derived from another collection are
undefined unless the collection explicitly documents otherwise.

Further more, I compared the two `nextIndex`. They were equal.

functest() {
    let s = "Original Script:"
    let cs = s.characters
    let startIndex = cs.startIndex
    let nextIndex = "Original ?".characters.endIndex
    let nextIndex01 = cs.index(startIndex, offsetBy: "Original
?".characters.count)
    let nextCharacter = cs[nextIndex01]
    print(nextIndex01 == nextIndex) // true
}

test()

It looks like String.Index only takes the position into account to
determine equality, not its _countUTF16. This makes sense for the way
endIndex and index(_:offsetBy:) are implemented. After all, nextIndex and
nextIndex01 _should be equal_. It would certainly be possible to implement
it differently (where endIndex and index(_:offsetBy:) returned identical
indices, including _countUTF16:) and I don't know why the stdlib team chose
to do it this way (maybe performance?).

In any case, much of this implementation may change with the work going
into strings for Swift 4.