Generic subscripts in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of generic subscript accessing dictionary

What is the output of this Swift code using a generic subscript to access dictionary values?

Swift

struct Container {
    private var data = ["a": 1, "b": 2, "c": 3]
    subscript<T>(key: T) -> Int? where T == String {
        return data[key]
    }
}

let container = Container()
print(container["b"] ?? 0)

A nil

B Optional(2)

C Compile-time error

D 2

💡 Hint

The subscript returns an Int? but the print unwraps it with ?? operator.

The subscript returns an optional Int. Using ?? 0 unwraps it or returns 0. Since key "b" exists with value 2, it prints 2.

❓ Predict Output

intermediate

Generic subscript with multiple constraints output

What will this Swift code print when using a generic subscript constrained to Numeric & Comparable types?

Swift

struct Numbers {
    private var values = [10, 20, 30, 40]
    subscript<T: Numeric & Comparable>(index: T) -> T? {
        guard let i = index as? Int, i < values.count else { return nil }
        return values[i] as? T
    }
}

let nums = Numbers()
print(nums[2] ?? -1)

A 30

B -1

C nil

D Runtime error

💡 Hint

The subscript converts generic index to Int and returns the value if valid.

Index 2 is valid, so values[2] = 30 is returned and printed.

🔧 Debug

advanced

Identify the error in generic subscript declaration

This Swift struct uses a generic subscript but causes a compile error. What is the error?

Swift

struct Box {
    var items = ["apple", "banana"]
    subscript<T>(index: T) -> String {
        return items[index]
    }
}

A Missing return type in subscript

B Array items is not initialized

C Cannot subscript array with generic type T without constraint

D Subscript must be static

💡 Hint

Array subscripts require Int index, generic T is unconstrained.

The subscript uses generic T as index but array requires Int index. Without constraint or cast, this causes a compile error.

📝 Syntax

advanced

Which generic subscript syntax is correct?

Which option shows the correct syntax for a generic subscript in Swift that returns the count of elements for a generic collection?

A subscript<T: Collection>(item: T) -> Int { return item.count }

B subscript<T>(item: T) -> Int where T: Collection { return item.count }

C subscript<T>(item: T: Collection) -> Int { return item.count }

D subscript<T>(item: T) -> Int where T Collection { return item.count }

💡 Hint

Generic constraints use 'where' clause after return type.

Option B correctly uses 'where T: Collection' after the return type. Option B places constraint incorrectly. Options C and D have syntax errors.

🚀 Application

expert

Using generic subscripts to access heterogeneous dictionary values

Given this Swift struct with a generic subscript, what will be the output of the print statements?

Swift

struct AnyDict {
    private var dict: [String: Any] = ["int": 42, "str": "hello", "dbl": 3.14]
    subscript<T>(key: String) -> T? {
        return dict[key] as? T
    }
}

let anyDict = AnyDict()
print(anyDict["int"] as Int?)
print(anyDict["str"] as String?)
print(anyDict["dbl"] as Double?)
print(anyDict["missing"] as Int?)

A

Optional(42)
Optional("hello")
Optional(3.14)
nil

B

42
hello
3.14
0

C Compile-time error due to generic subscript

D

Optional(42)
Optional("hello")
Optional(3.14)
Optional(0)

💡 Hint

The subscript casts Any to requested type and returns optional.

Each print prints the optional returned by the generic subscript. Existing keys return Optional values, missing key returns nil.