init(_:within:)

Declaration

init?(_ idx: String.Index, within target: String.UTF8View)

Parameters

• idx:

  A position in a String or one of its views.

• target:

  The UTF8View in which to find the new position.

Discussion

The following example finds the position of a space in a string’s utf16 view and then converts that position to an index in the string’s utf8 view.

let cafe = "Café 🍵"

let utf16Index = cafe.utf16.firstIndex(of: 32)!
let utf8Index = String.UTF8View.Index(utf16Index, within: cafe.utf8)!

print(Array(cafe.utf8[..<utf8Index]))
// Prints "[67, 97, 102, 195, 169]"

If the position passed in utf16Index doesn’t have an exact corresponding position in utf8, the result of the initializer is nil. For example, because UTF-8 and UTF-16 represent high Unicode code points differently, an attempt to convert the position of the trailing surrogate of a UTF-16 surrogate pair fails.

The next example attempts to convert the indices of the two UTF-16 code points that represent the teacup emoji ("🍵"). The index of the lead surrogate is successfully converted to a position in utf8, but the index of the trailing surrogate is not.

let emojiHigh = cafe.utf16.index(after: utf16Index)
print(String.UTF8View.Index(emojiHigh, within: cafe.utf8))
// Prints "Optional(String.Index(...))"

let emojiLow = cafe.utf16.index(after: emojiHigh)
print(String.UTF8View.Index(emojiLow, within: cafe.utf8))
// Prints "nil"