Nil coalescing operator deep usage in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Nil Coalescing with Optional Chaining

What is the output of this Swift code snippet?

Swift

struct User {
    var name: String?
}

let user: User? = User(name: nil)
let displayName = user?.name ?? "Guest"
print(displayName)

A nil

B "Guest"

C "Optional(nil)"

D Compilation error

💡 Hint

Remember that optional chaining returns an optional, and nil coalescing provides a default value.

The optional chaining user?.name returns nil because name is nil. The nil coalescing operator then provides "Guest" as the default value.

❓ Predict Output

intermediate

Nested Nil Coalescing Operators

What will be printed by this Swift code?

Swift

let a: Int? = nil
let b: Int? = 5
let c: Int? = nil

let result = a ?? b ?? c ?? 10
print(result)

A 5

B nil

C 10

D Compilation error

💡 Hint

The nil coalescing operator evaluates from left to right until it finds a non-nil value.

a is nil, so it checks b which is 5 (non-nil), so result is 5.

🔧 Debug

advanced

Unexpected Behavior with Nil Coalescing and Function Calls

Consider this Swift code. What is the output and why?

Swift

func getValue() -> Int? {
    print("getValue called")
    return nil
}

let x: Int? = nil
let y = x ?? getValue() ?? 42
print(y)

A getValue called\nnil

B 42

C getValue called\n42

D Compilation error

💡 Hint

Check when the function getValue() is called in the nil coalescing chain.

Since x is nil, getValue() is called and prints "getValue called". It returns nil, so the final value is 42.

📝 Syntax

advanced

Correct Usage of Nil Coalescing with Optional Binding

Which option correctly uses nil coalescing to assign a non-optional Int from an optional Int?

A let value = optionalInt ?? 0

B let value = optionalInt ? 0

C let value = optionalInt || 0

D let value = optionalInt ?: 0

💡 Hint

The nil coalescing operator in Swift is ??

Option A uses the correct nil coalescing operator ?? to provide a default value.

🚀 Application

expert

Complex Nil Coalescing in Dictionary Lookup

Given this Swift code, what is the value of finalValue after execution?

Swift

let dict: [String: Int?] = ["a": 1, "b": nil]

let finalValue = dict["b"] ?? (10 as Int?)
print(finalValue ?? -1)

A -1

B 10

C 1

D nil

💡 Hint

Remember the dictionary value is an optional Int optional, so dict["b"] returns Int??

dict["b"] returns an optional Int? which is .some(nil). The nil coalescing operator unwraps only the outer optional, so finalValue is nil.

Practice

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

easy

A. It returns the value of an optional if it is not nil, otherwise returns a default value.

B. It forces unwraps an optional and crashes if nil.

C. It converts a non-optional value into an optional.

D. It checks if two optionals are equal.

Solution

1. Step 1: Understand the purpose of the nil coalescing operator

   The operator ?? is used to provide a default value when an optional is nil.

2. Step 2: Compare with other options

   Options A, C, and D describe different behaviors unrelated to ??.

3. Final Answer:

   It returns the value of an optional if it is not nil, otherwise returns a default value. -> Option A

4. Quick Check:

   Nil coalescing operator = default value fallback [OK]

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

Common Mistakes:

• Confusing ?? with forced unwrapping (!)
• Thinking ?? converts types
• Assuming ?? compares optionals

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 correct syntax for nil coalescing

   The correct syntax uses two question marks: ?? between the optional and the default value.

2. Step 2: Eliminate incorrect options

   Options B, C, and D use invalid or incorrect operators.

3. Final Answer:

   let value = optionalValue ?? defaultValue -> Option D

4. Quick Check:

   Syntax for nil coalescing = ?? [OK]

Hint: Look for exactly two question marks ?? [OK]

Common Mistakes:

• Using ?: which is from other languages
• Using single ? instead of ??
• Adding extra question marks ???

3. What is the output of the following Swift code?

let a: Int? = nil
let b: Int? = 5
let c: Int? = nil
let result = a ?? b ?? c ?? 10
print(result)

medium

A. 5

B. 10

C. nil

D. Compilation error

Solution

1. Step 1: Evaluate the chained nil coalescing

   The expression a ?? b ?? c ?? 10 checks each optional from left to right and returns the first non-nil value.

2. Step 2: Check each optional's value

   a is nil, so move to b which is 5 (non-nil), so the result is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   First non-nil value = 5 [OK]

Hint: Chain ?? to pick first non-nil value [OK]

Common Mistakes:

• Assuming nil is returned if first is nil
• Ignoring chaining order
• Thinking default 10 is always returned

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

let x: Int? = nil
let y: Int = 3
let z = x ?? y ?? 5
print(z)

medium

A. x must be non-optional

B. No error, code runs fine

C. y must be optional to use ??

D. Cannot use ?? with non-optional y

Solution

1. Step 1: Understand the types involved in nil coalescing

   The operator ?? requires the right side to be the same type as the optional or a non-optional fallback value.

2. Step 2: Analyze the expression x ?? y ?? 5

   Here, x is Int?, y is Int, but y ?? 5 is invalid because y is not optional, so you cannot use ?? on it.

3. Final Answer:

   Cannot use ?? with non-optional y -> Cannot use ?? with non-optional y

4. Quick Check:

   Right side of ?? must be optional or value [OK]

Hint: Right side of ?? must be optional or value, not non-optional with ?? [OK]

Common Mistakes:

• Using ?? on non-optional values
• Confusing optional and non-optional types
• Assuming ?? works like || operator

5. Given the following Swift code, what will be the value of finalValue?

func getValue() -> Int? { return nil }
let a: Int? = nil
let b: Int? = getValue()
let c: Int? = 7
let finalValue = a ?? b ?? c ?? 0

hard

A. nil

B. 0

C. 7

D. Compilation error

Solution

1. Step 1: Evaluate each optional in the chain

   a is nil, b is nil (from getValue()), c is 7 (non-nil).

2. Step 2: Apply nil coalescing operator

   The expression a ?? b ?? c ?? 0 returns the first non-nil value, which is 7.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   First non-nil in chain = 7 [OK]

Hint: Chain ?? to find first non-nil, else default [OK]

Common Mistakes:

• Assuming function return nil means error
• Ignoring that c is non-nil
• Thinking default 0 is returned always