How does String Index work in Swift

Question

I ve been updating some of my old code and answers with Swift 3 but when I got to Swift Strings and Indexing it has been a pain to understand things    Specifically I was trying the following   let str    Hello  playground  let prefixRange   str startIndex   lt str startIndex advancedBy 5     error   where the second line was giving me the following error      advancedBy  is unavailable  To advance an index by n steps call  index   offsetBy    on the CharacterView instance that produced the index    I see that String has the following methods   str index after  String Index  str index before  String Index  str index String Index  offsetBy  String IndexDistance  str index String Index  offsetBy  String IndexDistance  limitedBy  String Index    These were really confusing me at first so I started playing around with them until I understood them  I am adding an answer below to show how they are used

User · Answer

I appreciate this question and all the info with it. I have something in mind that's kind of a question and an answer when it comes to String.Index.

I'm trying to see if there is an O(1) way to access a Substring (or Character) inside a String because string.index(startIndex, offsetBy: 1) is O(n) speed if you look at the definition of index function. Of course we can do something like:

let characterArray = Array(string)

then access any position in the characterArray however SPACE complexity of this is n = length of string, O(n) so it's kind of a waste of space.

I was looking at Swift.String documentation in Xcode and there is a frozen public struct called Index. We can initialize is as:

let index = String.Index(encodedOffset: 0)

Then simply access or print any index in our String object as such:

print(string[index])

Note: be careful not to go out of bounds`

This works and that's great but what is the run-time and space complexity of doing it this way? Is it any better?

User · Answer

All of the following examples use var str    quot Hello  playground quot   startIndex and endIndex  startIndex is the index of the first character endIndex is the index after the last character   Example    character str str startIndex     H str str endIndex       error  after last character     range let range   str startIndex   lt str endIndex str range       quot Hello  playground quot   With Swift 4 s one-sided ranges  the range can be simplified to one of the following forms  let range   str startIndex    let range      lt str endIndex  I will use the full form in the follow examples for the sake of clarity  but for the sake of readability  you will probably want to use the one-sided ranges in your code  after As in  index after  String Index   after refers to the index of the character directly after the given index   Examples    character let index   str index after  str startIndex  str index       quot e quot      range let range   str index after  str startIndex    lt str endIndex str range       quot ello  playground quot   before As in  index before  String Index   before refers to the index of the character directly before the given index   Examples    character let index   str index before  str endIndex  str index      d     range let range   str startIndex   lt str index before  str endIndex  str range      Hello  playgroun  offsetBy As in  index String Index  offsetBy  String IndexDistance   The offsetBy value can be positive or negative and starts from the given index  Although it is of the type String IndexDistance  you can give it an Int   Examples    character let index   str index str startIndex  offsetBy  7  str index      p     range let start   str index str startIndex  offsetBy  7  let end   str index str endIndex  offsetBy  -6  let range   start   lt end str range      play  limitedBy As in  index String Index  offsetBy  String IndexDistance  limitedBy  String Index   The limitedBy is useful for making sure that the offset does not cause the index to go out of bounds  It is a bounding index  Since it is possible for the offset to exceed the limit  this method returns an Optional  It returns nil if the index is out of bounds   Example    character if let index   str index str startIndex  offsetBy  7  limitedBy  str endIndex        str index      p    If the offset had been 77 instead of 7  then the if statement would have been skipped  Why is String Index needed  It would be much easier to use an Int index for Strings  The reason that you have to create a new String Index for every String is that Characters in Swift are not all the same length under the hood  A single Swift Character might be composed of one  two  or even more Unicode code points  Thus each unique String must calculate the indexes of its Characters  It is possibly to hide this complexity behind an Int index extension  but I am reluctant to do so  It is good to be reminded of what is actually happening

User · Answer

func change string  inout String         var character  Character    normal      enum Character           case space         case newLine         case normal            for i in stride from  string count - 1  through  0  by  -1               first get index         let index  String Index          if i    0               index   string index after  string index string startIndex  offsetBy  i - 1             else               index   string startIndex                    if string index        n                 if character     normal                    if character     newLine                       string remove at  index                     else if character     space                       let number   string index after  string index string startIndex  offsetBy  i                       if string number                                   string remove at  number                                            character    newLine                                  else                   character    newLine                          else if string index                         if character     normal                    string remove at  index                  else                   character    space                          else                character    normal                           startIndex     guard string count  gt  0 else   return       if string string startIndex       n     string string startIndex                   string remove at  string startIndex                endIndex - here is a little more complicated      guard string count  gt  0 else   return       let index   string index before  string endIndex      if string index       n     string index                   string remove at  index

User · Answer

Create a UITextView inside of a tableViewController  I used function  textViewDidChange and then checked for return-key-input  then if it detected return-key-input  delete the input of return key and dismiss keyboard   func textViewDidChange   textView  UITextView        tableView beginUpdates       if textView text contains   n            textView text remove at  textView text index before  textView text endIndex           textView resignFirstResponder             tableView endUpdates

[swift] How does String.Index work in Swift

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