1var fruits = ["Apple", "Banana", "Cherry", "Date"]
2
3// Replace element at index 1
4fruits[1] = "Blueberry"
5print(fruits)  // ["Apple", "Blueberry", "Cherry", "Date"]
6
7// Replace the first element
8fruits[0] = "Avocado"
9print(fruits)  // ["Avocado", "Blueberry", "Cherry", "Date"]
10
11// Replace the last element
12fruits[fruits.count - 1] = "Elderberry"
13print(fruits)  // ["Avocado", "Blueberry", "Cherry", "Elderberry"]

1var items = ["A", "B", "C"]
2
3// CRASH: index out of range
4// items[5] = "X"  // Fatal error: Index out of range
5
6// Safe replacement with bounds checking
7func safeReplace(_ array: inout [String], at index: Int, with value: String) {
8    guard index >= 0, index < array.count else {
9        print("Index \(index) out of bounds (0..<\(array.count))")
10        return
11    }
12    array[index] = value
13}
14
15safeReplace(&items, at: 1, with: "Z")
16print(items)  // ["A", "Z", "C"]
17
18safeReplace(&items, at: 10, with: "X")
19// Prints: Index 10 out of bounds (0..<3)

1extension Array {
2    subscript(safe index: Int) -> Element? {
3        get {
4            return indices.contains(index) ? self[index] : nil
5        }
6        set {
7            guard let newValue = newValue, indices.contains(index) else { return }
8            self[index] = newValue
9        }
10    }
11}
12
13var names = ["Alice", "Bob", "Charlie"]
14
15// Safe read
16print(names[safe: 1])  // Optional("Bob")
17print(names[safe: 5])  // nil
18
19// Safe write
20names[safe: 1] = "Barbara"
21print(names)  // ["Alice", "Barbara", "Charlie"]
22
23names[safe: 10] = "Nobody"  // No crash, silently ignored

1var colors = ["Red", "Green", "Blue", "Yellow", "Purple"]
2
3// Replace indices 1 through 3 with new elements
4colors[1...3] = ["Cyan", "Magenta"]
5print(colors)  // ["Red", "Cyan", "Magenta", "Purple"]
6
7// The replacement can have a different count than the range
8colors[0..<2] = ["Black", "White", "Gray"]
9print(colors)  // ["Black", "White", "Gray", "Magenta", "Purple"]

1var letters = ["A", "B", "C", "D", "E"]
2
3// Replace range with new elements
4letters.replaceSubrange(1...3, with: ["X", "Y"])
5print(letters)  // ["A", "X", "Y", "E"]
6
7// Insert elements (replace empty range)
8letters.replaceSubrange(2..<2, with: ["Z1", "Z2"])
9print(letters)  // ["A", "X", "Z1", "Z2", "Y", "E"]
10
11// Remove elements (replace with empty array)
12letters.replaceSubrange(2...3, with: [])
13print(letters)  // ["A", "X", "Y", "E"]

1var arr = ["First", "Second", "Third", "Fourth"]
2
3// Swap two elements
4arr.swapAt(0, 3)
5print(arr)  // ["Fourth", "Second", "Third", "First"]
6
7// Reverse the entire array
8arr.reverse()
9print(arr)  // ["First", "Third", "Second", "Fourth"]

1var scores = ["Pass", "Fail", "Pass", "Fail", "Pass"]
2
3// Replace all "Fail" with "Retry"
4for i in scores.indices where scores[i] == "Fail" {
5    scores[i] = "Retry"
6}
7print(scores)  // ["Pass", "Retry", "Pass", "Retry", "Pass"]
8
9// Using map for a new array (non-mutating)
10let updated = scores.map { $0 == "Retry" ? "Pending" : $0 }
11print(updated)  // ["Pass", "Pending", "Pass", "Pending", "Pass"]

1var cities = ["London", "Paris", "Berlin", "Paris", "Tokyo"]
2
3// Replace first occurrence
4if let index = cities.firstIndex(of: "Paris") {
5    cities[index] = "Lyon"
6}
7print(cities)  // ["London", "Lyon", "Berlin", "Paris", "Tokyo"]
8
9// Replace all occurrences
10while let index = cities.firstIndex(of: "Paris") {
11    cities[index] = "Marseille"
12}
13print(cities)  // ["London", "Lyon", "Berlin", "Marseille", "Tokyo"]