How does String substring 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 with substrings things got confusing    Specifically I was trying the following   let str    Hello  playground  let prefixRange   str startIndex   lt str startIndex advancedBy 5  let prefix   str substringWithRange prefixRange    where the second line was giving me the following error     Value of type  String  has no member  substringWithRange    I see that String does have the following methods now   str substring to  String Index  str substring from  String Index  str substring with  Range lt String Index gt     These were really confusing me at first so I started playing around index and range  This is a followup question and answer for substring  I am adding an answer below to show how they are used

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Here s a function that returns substring of a given substring when start and end indices are provided  For complete reference you can visit the links given below   func substring string  String  fromIndex  Int  toIndex  Int  - gt  String        if fromIndex  lt  toIndex  amp  amp  toIndex  lt  string count   use string characters count for swift3            let startIndex   string index string startIndex  offsetBy  fromIndex          let endIndex   string index string startIndex  offsetBy  toIndex          return String string startIndex   lt endIndex        else          return nil           Here s a link to the blog post that I have created to deal with string manipulation in swift   String manipulation in swift  Covers swift 4 as well   Or you can see this gist on github

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I m really frustrated at Swift s String access model  everything has to be an Index  All I want is to access the i-th character of the string using Int  not the clumsy index and advancing  which happens to change with every major release   So I made an extension to String   extension String       func index from  Int  - gt  Index           return self index startIndex  offsetBy  from             func substring from  Int  - gt  String           let fromIndex   index from  from          return String self fromIndex                 func substring to  Int  - gt  String           let toIndex   index from  to          return String self    lt toIndex              func substring with r  Range lt Int gt   - gt  String           let startIndex   index from  r lowerBound          let endIndex   index from  r upperBound          return String self startIndex   lt endIndex            let str    Hello  playground  print str substring from  7              playground print str substring to  5                Hello print str substring with  7   lt 11         play

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I m quite mechanical thinking  Here are the basics     Swift 4 Swift 5    let t    abracadabra     let start1   t index t startIndex  offsetBy 0    let   end1   t index t endIndex  offsetBy -5    let start2   t index t endIndex  offsetBy -5    let   end2   t index t endIndex  offsetBy 0     let t2   t start1    lt  end1    let t3   t start2    lt  end2                       or a shorter form     let t4   t    lt end1    let t5   t start2        print    t2    t3    t      print    t4    t5    t          result       abraca dabra abracadabra   The result is a substring  meaning that it is a part of the original string  To get a full blown separate string just use e g       String t3      String t4    This is what I use       let mid   t index t endIndex  offsetBy -5      let firstHalf   t    lt mid      let secondHalf   t mid

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Came across this fairly short and simple way of achieving this  var str    quot Hello  World quot  let arrStr   Array str  print arrStr 0   lt 5       quot H quot    quot e quot    quot l quot    quot l quot    quot o quot   print arrStr 7   lt 12       quot W quot    quot o quot    quot r quot    quot l quot    quot d quot   print String arrStr 0   lt 5      Hello print String arrStr 7   lt 12      World

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I am new in Swift 3  but looking the String  index  syntax for analogy I think that index is like a  pointer  constrained to string and Int can help as an independent object  Using the base   offset syntax   then  we can get the i-th character from string with the code bellow    let s    abcdefghi  let i   2 print  s s index s startIndex  offsetBy i       print c   For a range of characters   indexes  from string using String  range  syntax we can get from i-th to f-th characters with the code bellow   let f   6 print  s s index s startIndex  offsetBy i     lt s index s startIndex  offsetBy f 1       print cdefg   For a substring  range  from a string using String substring  range  we can get the substring using the code bellow   print  s substring  with s index s startIndex  offsetBy i     lt s index s startIndex  offsetBy f 1         print cdefg   Notes     The i-th and f-th begin with 0  To f-th  I use offsetBY  f   1  because the range of subscription use    lt   half-open operator   not include the f-th position  Of course must include validate errors like invalid index

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The specificity of String has mostly been addressed in other answers  To paraphrase  String has a specific Index which is not of type Int because string elements do not have the same size in the general case  Hence  String does not conform to RandomAccessCollection and accessing a specific index implies the traversal of the collection  which is not an O 1  operation  Many answers have proposed workarounds for using ranges  but they can lead to inefficient code as they use String methods  index from    index  offsetBy         that are not O 1   To access string elements like in an array you should use an Array  let array   Array  quot Hello  world  quot   let letter   array 5   This is a trade-off  the array creation is an O n  operation but array accesses are then O 1   You can convert back to a String when you want with String array

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I had the same initial reaction   I too was frustrated at how syntax and objects change so drastically in every major release   However  I realized from experience how I always eventually suffer the consequences of trying to fight  change  like dealing with multi-byte characters which is inevitable if you re looking at a global audience   So I decided to recognize and respect the efforts exerted by Apple engineers and do my part by understanding their mindset when they came up with this  horrific  approach   Instead of creating extensions which is just a workaround to make your life easier  I m not saying they re wrong or expensive   why not figure out how Strings are now designed to work   For instance  I had this code which was working on Swift 2 2   let rString   cString substringToIndex 2  let gString    cString substringFromIndex 2  as NSString  substringToIndex 2  let bString    cString substringFromIndex 4  as NSString  substringToIndex 2    and after giving up trying to get the same approach working e g  using Substrings  I finally understood the concept of treating Strings as a bidirectional collection for which I ended up with this version of the same code   let rString   String cString characters prefix 2   cString   String cString characters dropFirst 2   let gString   String cString characters prefix 2   cString   String cString characters dropFirst 2   let bString   String cString characters prefix 2     I hope this contributes

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Swift 4  amp  5  extension String     subscript   i  Int  - gt  String       let idx1   index startIndex  offsetBy  i      let idx2   index idx1  offsetBy  1      return String self idx1   lt idx2          subscript  r  Range lt Int gt   - gt  String       let start   index startIndex  offsetBy  r lowerBound      let end   index startIndex  offsetBy  r upperBound      return String self start    lt  end          subscript  r  CountableClosedRange lt Int gt   - gt  String       let startIndex    self index self startIndex  offsetBy  r lowerBound      let endIndex   self index startIndex  offsetBy  r upperBound - r lowerBound      return String self startIndex   endIndex          How to use it    quot abcde quot  0  -- gt   quot a quot   quot abcde quot  0   2  -- gt   quot abc quot   quot abcde quot  2   lt 4  -- gt   quot cd quot

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I created a simple extension for this  Swift 3   extension String       func substring location  Int  length  Int  - gt  String            guard characters count  gt   location   length else   return nil           let start   index startIndex  offsetBy  location          let end   index startIndex  offsetBy  location   length          return substring with  start   lt end

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Swift 4 In swift 4 String conforms to Collection  Instead of substring  we should now use a subscript  So if you want to cut out only the word  quot play quot  from  quot Hello  playground quot   you could do it like this  var str    quot Hello  playground quot  let start   str index str startIndex  offsetBy  7  let end   str index str endIndex  offsetBy  -6  let result   str start   lt end     The result is of type Substring  It is interesting to know  that doing so will give you a Substring instead of a String  This is fast and efficient as Substring shares its storage with the original String  However sharing memory this way can also easily lead to memory leaks  This is why you should copy the result into a new String  once you want to clean up the original String  You can do this using the normal constructor  let newString   String result   You can find more information on the new Substring class in the  Apple documentation  1 So  if you for example get a Range as the result of an NSRegularExpression  you could use the following extension  extension String        subscript   range  NSRange  - gt  String           let start   self index self startIndex  offsetBy  range lowerBound          let end   self index self startIndex  offsetBy  range upperBound          let subString   self start   lt end          return String subString

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Swift 4  extension String       subscript   i  Int  - gt  String           let idx1   index startIndex  offsetBy  i          let idx2   index idx1  offsetBy  1          return String self idx1   lt idx2            let s    hello   s 0        h s 1        e s 2        l s 3        l s 4        o

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Same frustration  this should not be that hard     I compiled this example of getting positions for substring s  from larger text         Play with finding substrings returning an array of the non-unique words and positions in text        import UIKit  let Bigstring    Why is it so hard to find substrings in Swift3  let searchStrs   Array lt String gt       Why    substrings    Swift3    FindSubString inputStr  Bigstring  subStrings  searchStrs    func FindSubString inputStr   String  subStrings  Array lt String gt    - gt     Array lt  String  Int  Int  gt        var resultArray   Array lt  String  Int  Int  gt           for i  Int in 0    subStrings  count  -1           if inputStr contains  subStrings  i                   let range  Range lt String Index gt    inputStr range of  subStrings  i                let lPos   inputStr distance from  inputStr startIndex  to  range lowerBound              let uPos   inputStr distance from  inputStr startIndex  to  range upperBound              let element     subStrings  i    as String  lPos  uPos              resultArray append element                      for words in resultArray           print words            return resultArray     returns   Why   0  3    substrings   26  36    Swift3   40  46

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Swift 4   extension String       func take   n  Int  - gt  String           guard n  gt   0 else               fatalError  n should never negative                     let index   self index self startIndex  offsetBy  min n  self count           return String self    lt index             Returns a subsequence of the first n characters  or the entire string if the string is shorter   inspired by  https   kotlinlang org api latest jvm stdlib kotlin text take html   Example   let text    Hello  World   let substring   text take 5    Hello

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Swift 4   Substring   https   developer apple com documentation swift substring    let greeting    Hi there  It s nice to meet you    let endOfSentence   greeting index of        let firstSentence   greeting    endOfSentence     firstSentence     Hi there     Example of extension String   private typealias HowDoYouLikeThatElonMusk   String private extension HowDoYouLikeThatElonMusk        subscript   from  Character     to  Character     include  Bool  - gt  String            if let  from  Character   from  let  to  Character   to               let dynamicSourceForEnd  String     from     to   String self reversed      self              guard let startOfSentence  String Index   self index of   from                   let endOfSentence  String Index   dynamicSourceForEnd index of   to  else                   return nil                            let result  String   String self startOfSentence   endOfSentence               if include    false                   guard result count  gt  2 else                           return nil                                   return String result result index result startIndex  offsetBy  1    lt result index result endIndex  offsetBy  -1                              return result           else if let  from  Character   from               guard let startOfSentence  String Index   self index of   from  else                   return nil                           let result  String   String self startOfSentence                  if include    false                   guard result count  gt  1 else                       return nil                                   return String result result index result startIndex  offsetBy  1                                 return result           else if let  to  Character   to               guard let endOfSentence  String Index   self index of   to  else                       return nil                           let result  String   String self    endOfSentence               if include    false                   guard result count  gt  1 else                       return nil                                   return String result    lt result index result endIndex  offsetBy  -1                              return result                   return nil           example of using the extension String   let source                                       gt  gt  gt 01234  56789 lt  lt  lt      include   true var from            source  3   nil  true             34  56789 lt  lt  lt   var to              source nil   6   true        gt  gt  gt 01234  56  var fromTo          source  3    6   true             34  56  let notFound        source  a   nil  true      nil    include   false from                source  3   nil  false             4  56789 lt  lt  lt   to                  source nil   6   false       gt  gt  gt 01234  5  fromTo              source  3    6   false             4  5  let outOfBounds     source           false     nil  let str    Hello  playground  let hello   str nil       false      Hello

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Building on the above I needed to split a string at a non-printing character dropping the non-printing character  I developed two methods   var str    abc u 1A 12345sdf  let range1  Range lt String Index gt    str range of    u 1A     let index1  Int   str distance from  str startIndex  to  range1 lowerBound  let start   str index str startIndex  offsetBy  index1  let end   str index str endIndex  offsetBy  -0  let result   str start   lt end     The result is of type Substring let firstStr   str str startIndex   lt range1 lowerBound    which I put together using some of the answers above   Because a String is a collection I then did the following   var fString   String   for  n c  in str enumerated      if c      u 1A         print fString       let lString   str dropFirst n   1      print lString      break       fString    String c       Which for me was more intuitive  Which one is best  I have no way of telling They both work with Swift 5

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Swift 5 Extension   extension String       subscript   range  CountableRange lt Int gt   - gt  String           let start   index startIndex  offsetBy  max 0  range lowerBound           let end   index start  offsetBy  min self count - range lowerBound                                                range upperBound - range lowerBound           return String self start   lt end              subscript   range  CountablePartialRangeFrom lt Int gt   - gt  String           let start   index startIndex  offsetBy  max 0  range lowerBound            return String self start                Usage    let s    hello  s 0   lt 3      hel  s 3          lo    Or unicode   let s      s 0   lt 1

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Heres a more generic implementation   This technique still uses index to keep with Swift s standards  and imply a full Character   extension String       func subString  lt R gt     range  R  - gt  String  where R   RangeExpression  String Index    R Bound               return String self range              func index at  Int  - gt  Index               return self index self startIndex  offsetBy  at            To sub string from the 3rd character   let item    Fred looks funny  item subString item index at  2          ed looks funny    I ve used camel subString to indicate it returns a String and not a Substring

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Swift 5  let desiredIndex  Int   7   let substring   str String Index encodedOffset  desiredIndex       This substring variable will give you the result    Simply here Int is converted to Index and then you can split the strings  Unless you will get errors

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All of the following examples use   var str    Hello  playground    Swift 4  Strings got a pretty big overhaul in Swift 4  When you get some substring from a String now  you get a Substring type back rather than a String  Why is this  Strings are value types in Swift  That means if you use one String to make a new one  then it has to be copied over  This is good for stability  no one else is going to change it without your knowledge  but bad for efficiency    A Substring  on the other hand  is a reference back to the original String from which it came  Here is an image from the documentation illustrating that     No copying is needed so it is much more efficient to use  However  imagine you got a ten character Substring from a million character String  Because the Substring is referencing the String  the system would have to hold on to the entire String for as long as the Substring is around  Thus  whenever you are done manipulating your Substring  convert it to a String   let myString   String mySubstring    This will copy just the substring over and the memory holding old String can be reclaimed  Substrings  as a type  are meant to be short lived   Another big improvement in Swift 4 is that Strings are Collections  again   That means that whatever you can do to a Collection  you can do to a String  use subscripts  iterate over the characters  filter  etc     The following examples show how to get a substring in Swift   Getting substrings  You can get a substring from a string by using subscripts or a number of other methods  for example  prefix  suffix  split   You still need to use String Index and not an Int index for the range  though   See my other answer if you need help with that    Beginning of a string  You can use a subscript  note the Swift 4 one-sided range    let index   str index str startIndex  offsetBy  5  let mySubstring   str    lt index     Hello   or prefix   let index   str index str startIndex  offsetBy  5  let mySubstring   str prefix upTo  index     Hello   or even easier   let mySubstring   str prefix 5     Hello   End of a string  Using subscripts   let index   str index str endIndex  offsetBy  -10  let mySubstring   str index        playground   or suffix   let index   str index str endIndex  offsetBy  -10  let mySubstring   str suffix from  index     playground   or even easier   let mySubstring   str suffix 10     playground   Note that when using the suffix from  index  I had to count back from the end by using -10  That is not necessary when just using suffix x   which just takes the last x characters of a String   Range in a string  Again we simply use subscripts here   let start   str index str startIndex  offsetBy  7  let end   str index str endIndex  offsetBy  -6  let range   start   lt end  let mySubstring   str range      play   Converting Substring to String  Don t forget  when you are ready to save your substring  you should convert it to a String so that the old string s memory can be cleaned up   let myString   String mySubstring    Using an Int index extension   I m hesitant to use an Int based index extension after reading the article Strings in Swift 3 by Airspeed Velocity and Ole Begemann  Although in Swift 4  Strings are collections  the Swift team purposely hasn t used Int indexes  It is still String Index  This has to do with Swift Characters being composed of varying numbers of Unicode codepoints  The actual index has to be uniquely calculated for every string    I have to say  I hope the Swift team finds a way to abstract away String Index in the future  But until them I am choosing to use their API  It helps me to remember that String manipulations are not just simple Int index lookups

[swift] How does String substring work in Swift

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