Get nth character of a string in Swift programming language

Attention: Please see Leo Dabus' answer for a proper implementation for Swift 4 and Swift 5.

Swift 4 or later

The Substring type was introduced in Swift 4 to make substrings faster and more efficient by sharing storage with the original string, so that's what the subscript functions should return.

Try it out here

extension StringProtocol {
    subscript(offset: Int) -> Character { self[index(startIndex, offsetBy: offset)] }
    subscript(range: Range<Int>) -> SubSequence {
        let startIndex = index(self.startIndex, offsetBy: range.lowerBound)
        return self[startIndex..<index(startIndex, offsetBy: range.count)]
    }
    subscript(range: ClosedRange<Int>) -> SubSequence {
        let startIndex = index(self.startIndex, offsetBy: range.lowerBound)
        return self[startIndex..<index(startIndex, offsetBy: range.count)]
    }
    subscript(range: PartialRangeFrom<Int>) -> SubSequence { self[index(startIndex, offsetBy: range.lowerBound)...] }
    subscript(range: PartialRangeThrough<Int>) -> SubSequence { self[...index(startIndex, offsetBy: range.upperBound)] }
    subscript(range: PartialRangeUpTo<Int>) -> SubSequence { self[..<index(startIndex, offsetBy: range.upperBound)] }
}

To convert the Substring into a String, you can simply do String(string[0..2]), but you should only do that if you plan to keep the substring around. Otherwise, it's more efficient to keep it a Substring.

It would be great if someone could figure out a good way to merge these two extensions into one. I tried extending StringProtocol without success, because the index method does not exist there. Note: This answer has been already edited, it is properly implemented and now works for substrings as well. Just make sure to use a valid range to avoid crashing when subscripting your StringProtocol type. For subscripting with a range that won't crash with out of range values you can use this implementation

Why is this not built-in?

The error message says "see the documentation comment for discussion". Apple provides the following explanation in the file UnavailableStringAPIs.swift:

Subscripting strings with integers is not available.

The concept of "the ith character in a string" has different interpretations in different libraries and system components. The correct interpretation should be selected according to the use case and the APIs involved, so String cannot be subscripted with an integer.

Swift provides several different ways to access the character data stored inside strings.

• String.utf8 is a collection of UTF-8 code units in the string. Use this API when converting the string to UTF-8. Most POSIX APIs process strings in terms of UTF-8 code units.

• String.utf16 is a collection of UTF-16 code units in string. Most Cocoa and Cocoa touch APIs process strings in terms of UTF-16 code units. For example, instances of NSRange used with NSAttributedString and NSRegularExpression store substring offsets and lengths in terms of UTF-16 code units.

• String.unicodeScalars is a collection of Unicode scalars. Use this API when you are performing low-level manipulation of character data.

• String.characters is a collection of extended grapheme clusters, which are an approximation of user-perceived characters.

Note that when processing strings that contain human-readable text, character-by-character processing should be avoided to the largest extent possible. Use high-level locale-sensitive Unicode algorithms instead, for example, String.localizedStandardCompare(), String.localizedLowercaseString, String.localizedStandardRangeOfString() etc.

Swift 5.2

let str = "abcdef"
str[1 ..< 3] // returns "bc"
str[5] // returns "f"
str[80] // returns ""
str.substring(fromIndex: 3) // returns "def"
str.substring(toIndex: str.length - 2) // returns "abcd"

You will need to add this String extension to your project (it's fully tested):

extension String {

    var length: Int {
        return count
    }

    subscript (i: Int) -> String {
        return self[i ..< i + 1]
    }

    func substring(fromIndex: Int) -> String {
        return self[min(fromIndex, length) ..< length]
    }

    func substring(toIndex: Int) -> String {
        return self[0 ..< max(0, toIndex)]
    }

    subscript (r: Range<Int>) -> String {
        let range = Range(uncheckedBounds: (lower: max(0, min(length, r.lowerBound)),
                                            upper: min(length, max(0, r.upperBound))))
        let start = index(startIndex, offsetBy: range.lowerBound)
        let end = index(start, offsetBy: range.upperBound - range.lowerBound)
        return String(self[start ..< end])
    }
}

Even though Swift always had out of the box solution to this problem (without String extension, which I provided below), I still would strongly recommend using the extension. Why? Because it saved me tens of hours of painful migration from early versions of Swift, where String's syntax was changing almost every release, but all I needed to do was to update the extension's implementation as opposed to refactoring the entire project. Make your choice.

let str = "Hello, world!"
let index = str.index(str.startIndex, offsetBy: 4)
str[index] // returns Character 'o'

let endIndex = str.index(str.endIndex, offsetBy:-2)
str[index ..< endIndex] // returns String "o, worl"

String(str.suffix(from: index)) // returns String "o, world!"
String(str.prefix(upTo: index)) // returns String "Hell"

I just came up with this neat workaround

var firstChar = Array(string)[0]