# init(_:within:) Creates an index in the given string that corresponds exactly to the specified position. ``` init?(_ sourcePosition: String.Index, within target: S) where S : StringProtocol ``` ## Parameters `sourcePosition` A position in a view of the `target` parameter. `sourcePosition` must be a valid index of at least one of the views of `target`. `target` The string referenced by the resulting index. ## Discussion If the index passed as `sourcePosition` represents the start of an extended grapheme cluster—the element type of a string—then the initializer succeeds. The following example converts the position of the Unicode scalar `"e"` into its corresponding position in the string. The character at that position is the composed `"é"` character. ``` let cafe = "Cafe\u{0301}" print(cafe) // Prints "Café" let scalarsIndex = cafe.unicodeScalars.firstIndex(of: "e")! let stringIndex = String.Index(scalarsIndex, within: cafe)! print(cafe[...stringIndex]) // Prints "Café" ``` If the index passed as `sourcePosition` doesn’t have an exact corresponding position in `target`, the result of the initializer is `nil`. For example, an attempt to convert the position of the combining acute accent (`"\u{0301}"`) fails. Combining Unicode scalars do not have their own position in a string. ``` let nextScalarsIndex = cafe.unicodeScalars.index(after: scalarsIndex) let nextStringIndex = String.Index(nextScalarsIndex, within: cafe) print(nextStringIndex) // Prints "nil" ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)