Swift Strings

A String in Swift is a sequence of characters. Swift strings are powerful, Unicode-correct, and packed with built-in tools for searching, modifying, and formatting text. You use strings constantly — for user names, messages, file paths, URLs, and more.

Creating Strings

let greeting = "Hello, Swift!"
let empty = ""
let alsoEmpty = String()

var multiline = """
Line one
Line two
Line three
"""
print(multiline)

Triple-quoted strings (""") preserve line breaks and are perfect for paragraphs, JSON templates, or HTML snippets.

String Interpolation

Embed any value directly inside a string using \(). Swift converts the value to text automatically.

let name = "Alice"
let age = 30
let score = 98.5

print("Name: \(name), Age: \(age), Score: \(score)")
// Output: Name: Alice, Age: 30, Score: 98.5

print("Next year she will be \(age + 1).")
// Output: Next year she will be 31.

String Properties

let word = "Swift"

print(word.count)       // Output: 5
print(word.isEmpty)     // Output: false
print(word.uppercased()) // Output: SWIFT
print(word.lowercased()) // Output: swift

Diagram: String as a Character Sequence

"Swift"
  |
  S  w  i  f  t
  ↑              ↑
startIndex    endIndex (past the last character)

word.first → "S"
word.last  → "t"
word.count → 5

Checking String Content

hasPrefix and hasSuffix

let filename = "report_2024.pdf"

print(filename.hasPrefix("report"))   // Output: true
print(filename.hasSuffix(".pdf"))     // Output: true
print(filename.hasSuffix(".docx"))    // Output: false

contains

let sentence = "Swift is fast and safe."

print(sentence.contains("fast"))    // Output: true
print(sentence.contains("slow"))    // Output: false

Modifying Strings

Concatenation

var message = "Hello"
message += ", World"
message.append("!")
print(message)   // Output: Hello, World!

Replacing Substrings

var text = "I love cats. Cats are great."
let updated = text.replacingOccurrences(of: "cats", with: "dogs",
                                        options: .caseInsensitive)
print(updated)   // Output: I love dogs. dogs are great.

Trimming Whitespace

let padded = "   hello   "
let trimmed = padded.trimmingCharacters(in: .whitespaces)
print(trimmed)   // Output: "hello"

Splitting and Joining

Split a String Into Parts

let csv = "Alice,Bob,Carol,Dave"
let names = csv.split(separator: ",")
print(names)
// Output: ["Alice", "Bob", "Carol", "Dave"]

for name in names {
    print(name)
}

Join an Array Into a String

let words = ["Swift", "is", "awesome"]
let joined = words.joined(separator: " ")
print(joined)   // Output: Swift is awesome

let csv2 = words.joined(separator: ",")
print(csv2)     // Output: Swift,is,awesome

String Comparison

let a = "apple"
let b = "Apple"

print(a == b)                                        // false (case-sensitive)
print(a.lowercased() == b.lowercased())              // true
print(a.caseInsensitiveCompare(b) == .orderedSame)   // true

Substrings

Swift extracts substrings using string indices. A Substring shares memory with the original string, making it very efficient.

let email = "alice@example.com"

if let atSign = email.firstIndex(of: "@") {
    let username = email[..<atSign]
    print(username)   // Output: alice

    let domain = email[email.index(after: atSign)...]
    print(domain)     // Output: example.com
}

Convert Substring to String

let sub = email.prefix(5)
let str = String(sub)
print(str)   // Output: alice

String Formatting Numbers

let price = 9.5
let formatted = String(format: "Price: $%.2f", price)
print(formatted)   // Output: Price: $9.50

let big = 1_234_567
let readable = String(format: "%d items", big)
print(readable)    // Output: 1234567 items

Converting Between Strings and Other Types

// String to Int
let numStr = "42"
if let number = Int(numStr) {
    print(number + 8)   // Output: 50
}

// String to Double
if let pi = Double("3.14159") {
    print(pi * 2)       // Output: 6.28318
}

// Int to String
let count = 100
let label = "Score: \(count)"
print(label)            // Output: Score: 100

Iterating Over Characters

let word2 = "Swift"

for char in word2 {
    print(char)
}
// Output: S w i f t (each on its own line)

let vowels: Set<Character> = ["a", "e", "i", "o", "u"]
let vowelCount = word2.lowercased().filter { vowels.contains($0) }.count
print("Vowels in Swift: \(vowelCount)")   // Output: 1

Diagram: Common String Operations

Original: "  Hello, Swift!  "
          |
          trimmed → "Hello, Swift!"
          |
          uppercased → "HELLO, SWIFT!"
          |
          split(separator: ",") → ["Hello", " Swift!"]
          |
          replacingOccurrences("Swift", "World") → "Hello, World!"

Raw Strings

Prefix and suffix a string with # to treat backslashes and quotes as literal characters. Useful for regular expressions and file paths.

let path = #"C:\Users\Alice\Documents"#
print(path)   // Output: C:\Users\Alice\Documents

let regex = #"\d{3}-\d{4}"#
print(regex)   // Output: \d{3}-\d{4}

Summary

Swift strings are Unicode-safe sequences with a rich set of methods. Use interpolation (\()) to embed values, split and joined to work with parts, and prefix/suffix/firstIndex to extract substrings. Convert between strings and numbers with Int(), Double(), and String(format:). Strings are immutable when declared with let and fully mutable with var.