1.9 · Strings and Unicode

A Swift String is a sequence of grapheme clusters, not characters or bytes. Every surprising count, index, and comparison behaviour follows from that.

Mental model. Three layers: bytes (UTF-8) → scalars (code points) → Characters (what a human sees). Swift’s default view is the top layer, and it is the most expensive to index.

What a character is

Graphemes, scalars and bytes.

1. Why is String not a RandomAccessCollection of Character? Why is string[i + 1] not O(1)? 1.9.1 · L1
   A Character is an extended grapheme cluster: a user-perceived character that may be composed of many Unicode scalars — flags (two regional indicators), emoji…
2. Character vs Unicode.Scalar vs UTF-8 code unit? 1.9.2 · L2
   Unicode scalar — one code point (roughly, one “letter” in the Unicode table). Character — a grapheme cluster: one or more scalars that a human reads as one…

Comparing strings

Equality means different things for UI and IDs.

1. How do you compare strings for UI versus for identifiers? 1.9.3 · L3
   UI / search: use localized, human-facing comparison — localizedStandardCompare, localizedLowercase, formatted(), and .diacriticInsensitive search options. This…

More in Swift language

• 1.1 · Value vs reference types
• 1.2 · Optionals, pattern matching, control flow
• 1.3 · Protocols, extensions, associated types
• 1.4 · Generics, <code>some</code>, <code>any</code>
• 1.5 · Closures, escaping, capture lists
• 1.6 · Properties, observers, wrappers
• 1.7 · Access control
• 1.8 · Error handling, <code>defer</code>, ownership
• 1.10 · Codable, Sendable, macros, ABI
• 1.11 · Static vs dynamic dispatch