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Why is Swift String Manipulation like that

String Manipulation is notoriously tricky in Swift. Some common complaints:

  • Three different datatypes: String, Substring, and Character.
  • Cannot iterate String with Integer. We need to use a separate datatype called String.Index.

So a simple palindrome search goes from this in Python:

if string[i] != string[length - 1 - i]:

To this in Swift:

// ...
if String(string[String.Index(string.startIndex, offsetBy: i)]) != String(string[String.Index(string.endIndex, offsetBy: -i)]) // What is wrong with you?
// ...

See the perplexing syntax? However, there are several reasons why Swift Strings are designed this way.

  • Substrings. Swift Strings are value types copied when assigned or passed to a function. This can be good for stability but bad for efficiency, especially when working with large strings. Therefore we have the substring datatype, which does not create a new instance.

  • String.Index, Character. Swift Strings are Unicode-Correct, which means they can handle complex characters and Emoji; ever experienced where complex Emoji are 3-4 characters combined in length? These include words with "extended grapheme clusters", such as "é", "김" and "🇮🇳". Using Integer, their length will vary, but in String.Index they are all length 1. Apple's Example:

    let cafe = "Cafe\u{301} du 🌍"
    print(cafe) // Prints "Café du 🌍"
    print(cafe.count) // Prints "9"
    print(Array(cafe)) // Prints ["C", "a", "f", "é", " ", "d", "u", " ", "🌍"]

It's always a compromise, one or the other.

The difference between ordinary and extraordinary is that little extra.

– Jimmy Johnson (1943—)