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.

Practice

1. What is the main benefit of using generic subscripts in Swift?

easy

A. They allow subscripts to accept different types as keys or indexes.

B. They make subscripts run faster by using fixed types only.

C. They restrict subscripts to only integer indexes.

D. They automatically convert all keys to strings.

Solution

1. Step 1: Understand generic subscripts purpose

   Generic subscripts let you write one subscript that works with many types of keys or indexes.

2. Step 2: Compare options with this purpose

   Only They allow subscripts to accept different types as keys or indexes. correctly states that generic subscripts accept different types as keys or indexes.

3. Final Answer:

   They allow subscripts to accept different types as keys or indexes. -> Option A

4. Quick Check:

   Generic subscripts = flexible key types [OK]

Hint: Generic subscripts = flexible keys/indexes [OK]

Common Mistakes:

• Thinking generic subscripts improve speed
• Assuming subscripts only accept integers
• Believing keys auto-convert to strings

2. Which of the following is the correct syntax to declare a generic subscript in Swift?

easy

A. subscript(index: T) -> String { get }

B. func subscript<T>(index: T) -> String { get }

C. subscript<T>(index: Int) -> T { get }

D. subscript<T>(index: T) -> String { get }

Solution

1. Step 1: Recall generic subscript syntax

   Generic subscripts use subscript<T>(index: T) -> ReturnType syntax.

2. Step 2: Check each option

   subscript<T>(index: T) -> String { get } matches the correct syntax. func subscript<T>(index: T) -> String { get } wrongly uses func. subscript(index: T) -> String { get } misses generic parameter declaration. subscript<T>(index: Int) -> T { get } has mismatched types.

3. Final Answer:

   subscript<T>(index: T) -> String { get } -> Option D

4. Quick Check:

   Generic subscript syntax = subscript<T> [OK]

Hint: Generic subscript uses subscript<T>(index: T) syntax [OK]

Common Mistakes:

• Using func instead of subscript
• Omitting generic parameter <T>
• Mismatching parameter and return types

3. What will be the output of this Swift code?

struct Box {
    private var values = [String: Any]()
    subscript<T>(key: String) -> T? {
        get { values[key] as? T }
        set { values[key] = newValue }
    }
}

var box = Box()
box["age"] = 30
box["name"] = "Alice"
print(box["age"] as Int?, box["name"] as String?)

medium

A. Optional(30) Optional("Alice")

B. 30 Alice

C. nil nil

D. Compilation error

Solution

1. Step 1: Understand subscript behavior

   The subscript stores values in a dictionary with String keys and Any values, casting on get.

2. Step 2: Analyze stored values and prints

   "age" stores 30 (Int), "name" stores "Alice" (String). The print casts them back to Int? and String?, so output shows optionals.

3. Final Answer:

   Optional(30) Optional("Alice") -> Option A

4. Quick Check:

   Generic subscript returns optional cast values [OK]

Hint: Generic subscript returns optional cast values [OK]

Common Mistakes:

• Expecting non-optional output
• Confusing dictionary storage with direct types
• Assuming compilation error due to Any type

4. Identify the error in this generic subscript code and choose the fix:

struct Container {
    private var data = [Int: String]()
    subscript<T>(key: T) -> String? {
        get { data[key] }
        set { data[key] = newValue }
    }
}

medium

A. Change dictionary to [T: String] to match generic key type.

B. Add type constraint to subscript declaration.

C. Change subscript to subscript(key: Int) -> String? to match dictionary key type.

D. No error; code works as is.

Solution

1. Step 1: Check dictionary key type vs subscript key type

   Dictionary keys are Int, but subscript uses generic T which can be any type.

2. Step 2: Identify mismatch causing error

   Accessing data[key] fails because key is T, not necessarily Int. Fix is to restrict subscript key to Int.

3. Final Answer:

   Change subscript to subscript(key: Int) -> String? to match dictionary key type. -> Option C

4. Quick Check:

   Subscript key type must match dictionary key type [OK]

Hint: Subscript key type must match dictionary key type [OK]

Common Mistakes:

• Assuming generic T works without constraints
• Trying to change dictionary type without reason
• Ignoring type mismatch errors

5. Given a struct that stores values with keys of any Hashable type, how can you implement a generic subscript to safely get and set values?
Choose the best approach.

hard

A. Use separate dictionaries for each key type and switch inside the subscript.

B. Use a dictionary with keys of type AnyHashable and cast keys inside the subscript.

C. Use a dictionary with keys of type String only and convert all keys to String.

D. Use a generic dictionary [T: Any] without constraints and force cast keys.

Solution

1. Step 1: Understand requirement for any Hashable key

   To store values with keys of any Hashable type, use a dictionary with keys of type AnyHashable.

2. Step 2: Implement generic subscript with safe casting

   Use generic subscript with key of type T: Hashable, convert key to AnyHashable for dictionary access, ensuring type safety.

3. Final Answer:

   Use a dictionary with keys of type AnyHashable and cast keys inside the subscript. -> Option B

4. Quick Check:

   AnyHashable dictionary + generic subscript = safe flexible keys [OK]

Hint: Use AnyHashable keys for flexible generic subscripts [OK]

Common Mistakes:

• Using multiple dictionaries instead of one
• Converting all keys to String losing type info
• Force casting keys causing runtime errors