TopolearnText is everywhere in apps, and Swift's Unicode model explains why you can't index a string by an Int.
A String literal goes in double quotes; insert values with interpolation \(expression). Multi-line strings use triple quotes, and their indentation is stripped relative to the closing """.
Strings are value types: assigning or passing one copies it (cheaply, thanks to copy-on-write), so a function can never change your string behind your back.
A String is a collection of Characters, and each Character is an extended grapheme cluster — what a user sees as one symbol. "é" might be one or two Unicode scalars, and a flag emoji is two, but each counts as one Character. That is why count walks the string, and why you cannot write s[3]: indexing uses String.Index, obtained from s.startIndex, s.index(_:offsetBy:) or methods like firstIndex(of:).
Useful API:
hasPrefix,hasSuffix,contains,lowercased(),split(separator:).s.isEmptyis faster and clearer thans.count == 0.- Slicing yields a
Substring, which shares storage with the original; convert withString(sub)before storing it long-term.
For heavy byte-level work, use the utf8 view, which is fast and indexable in constant time per step.
Example
let user = "Ana"
let items = 3
print("Hi \(user), you have \(items * 2) points") // Hi Ana, you have 6 points
let word = "café"
print(word.count) // 4 characters
print(word.utf8.count) // 5 bytes
let i = word.index(word.startIndex, offsetBy: 3)
print(word[i]) // é
let csv = "a,b,c"
let parts = csv.split(separator: ",") // [Substring]
print(parts.map { String($0).uppercased() }) // ["A", "B", "C"]
let poem = """
Roses are red
Swift is fast
"""
print(poem.hasPrefix("Roses")) // trueRemember
- Interpolate with
\(value) Character= what a user sees;countis O(n)- Index with
String.Index, not Int; convertSubstringtoStringto keep it
Watch out for
- Keeping a small
Substringof a huge string alive, which keeps the whole buffer in memory.