Nil coalescing operator (??) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nil coalescing with optional Int

What is the output of this Swift code snippet?

Swift

let a: Int? = nil
let b = a ?? 10
print(b)

A Compilation error

B nil

C 0

D 10

💡 Hint

Remember, the nil coalescing operator returns the left value if it is not nil, otherwise the right value.

Since 'a' is nil, the expression 'a ?? 10' returns 10. So, the printed output is 10.

❓ Predict Output

intermediate

Nil coalescing with non-nil optional

What will be printed by this Swift code?

Swift

let name: String? = "Alice"
let displayName = name ?? "Guest"
print(displayName)

A Alice

B nil

C Guest

D Compilation error

💡 Hint

If the optional has a value, the nil coalescing operator returns that value.

The optional 'name' contains "Alice", so 'name ?? "Guest"' evaluates to "Alice".

❓ Predict Output

advanced

Chained nil coalescing operator output

What is the output of this Swift code?

Swift

let x: Int? = nil
let y: Int? = 5
let z = x ?? y ?? 10
print(z)

A Compilation error

B nil

C 5

D 10

💡 Hint

The nil coalescing operator can be chained to check multiple optionals.

Since 'x' is nil, it checks 'y' which is 5, so 'z' becomes 5.

❓ Predict Output

advanced

Nil coalescing with different types

What error or output occurs when running this Swift code?

Swift

let a: Int? = nil
let b = a ?? "default"
print(b)

A nil

B Compilation error

C default

D Optional("")

💡 Hint

The nil coalescing operator requires both sides to have the same type.

The left side is Int? and the right side is String, so the compiler reports a type mismatch error.

🧠 Conceptual

expert

Behavior of nil coalescing with function returning optional

Consider the following Swift code. What is the value of 'result' after execution?

Swift

func fetchValue() -> Int? {
    return nil
}

let result = fetchValue() ?? 42

A 42

B nil

C Compilation error

D Optional(42)

💡 Hint

The function returns nil, so the nil coalescing operator uses the default value.

Since fetchValue() returns nil, 'result' is assigned the default value 42.

Practice

1. What does the nil coalescing operator ?? do in Swift?

easy

A. It unwraps an optional without checking for nil.

B. It provides a default value if an optional is nil.

C. It converts a non-optional to an optional.

D. It forces a runtime error if the optional is nil.

Solution

1. Step 1: Understand optional values

   Optionals may contain a value or nil (no value).

2. Step 2: Role of nil coalescing operator

   The operator ?? returns the optional's value if it exists; otherwise, it returns the default value provided.

3. Final Answer:

   It provides a default value if an optional is nil. -> Option B

4. Quick Check:

   Nil coalescing = default value if nil [OK]

Hint: Remember: ?? means "use this if nil" [OK]

Common Mistakes:

• Thinking ?? unwraps without nil check
• Confusing ?? with forced unwrapping (!)
• Assuming ?? converts types

2. Which of the following is the correct syntax to use the nil coalescing operator in Swift?

easy

A. let value = optionalValue !! defaultValue

B. let value = optionalValue ?: defaultValue

C. let value = optionalValue ? defaultValue

D. let value = optionalValue ?? defaultValue

Solution

1. Step 1: Recall the syntax of nil coalescing

   The nil coalescing operator is written as ?? between an optional and a default value.

2. Step 2: Match correct syntax

   let value = optionalValue ?? defaultValue uses ?? correctly: let value = optionalValue ?? defaultValue.

3. Final Answer:

   let value = optionalValue ?? defaultValue -> Option D

4. Quick Check:

   Correct syntax uses ?? [OK]

Hint: Look for double question marks ?? in syntax [OK]

Common Mistakes:

• Using ?: which is from other languages
• Using single ? without ??
• Using !! which is invalid

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

var name: String? = nil
let displayName = name ?? "Guest"
print(displayName)

medium

A. Guest

B. Optional("Guest")

C. nil

D. Error: Cannot print optional

Solution

1. Step 1: Check the optional value

   The variable name is nil.

2. Step 2: Apply nil coalescing operator

   The expression name ?? "Guest" returns "Guest" because name is nil.

3. Final Answer:

   Guest -> Option A

4. Quick Check:

   nil ?? "Guest" = "Guest" [OK]

Hint: If optional is nil, ?? returns the default value [OK]

Common Mistakes:

• Expecting nil to print
• Thinking print shows Optional(...)
• Confusing ?? with forced unwrap

4. Identify the error in this Swift code snippet using the nil coalescing operator:

let age: Int? = 25
let userAge = age ??
print(userAge)

medium

A. Optional variable cannot be used with ??

B. print statement syntax is incorrect

C. Missing default value after ?? operator

D. age should not be optional

Solution

1. Step 1: Check nil coalescing usage

   The operator ?? must be followed by a default value to use if the optional is nil.

2. Step 2: Identify missing default value

   In the code, age ?? is incomplete because no default value is provided after ??.

3. Final Answer:

   Missing default value after ?? operator -> Option C

4. Quick Check:

   ?? needs a value after it [OK]

Hint: Always provide a value after ?? [OK]

Common Mistakes:

• Leaving ?? without default value
• Thinking optional can't be used with ??
• Misplacing print statement

5. Given the dictionary let scores: [String: Int?] = ["Alice": 85, "Bob": nil], which expression correctly uses the nil coalescing operator to get Bob's score or 0 if nil?

hard

A. let bobScore = scores["Bob"] ?? nil ?? 0

B. let bobScore = scores["Bob"]! ?? 0

C. let bobScore = scores["Bob"] ?? Optional(0)

D. let bobScore = scores["Bob"] ?? 0

Solution

1. Step 1: Understand dictionary value type

   The dictionary stores optional Int values: Int? inside the dictionary optional lookup, so scores["Bob"] returns Int??.

2. Step 2: Use nil coalescing correctly

   Chaining scores["Bob"] ?? nil ?? 0 first flattens the double optional to Int? (returns inner value or nil), then applies ?? 0 to provide 0 if nil, yielding Int. For "Bob", some(nil) ?? nil = nil, then nil ?? 0 = 0.

3. Final Answer:

   let bobScore = scores["Bob"] ?? nil ?? 0 -> Option A

4. Quick Check:

   ?? nil ?? 0 handles [Key: Opt<V>] [OK]

Hint: Chain ?? nil ?? 0 for dict of optionals [OK]

Common Mistakes:

• Forcing unwrap before ?? causing crash
• Direct ?? 0 causes type mismatch
• Optional(0) doesn't handle inner nil