Default parameter values in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with default parameters

What is the output of this Swift code when calling greet() without any arguments?

Swift

func greet(name: String = "Friend", greeting: String = "Hello") {
    print("\(greeting), \(name)!")
}
greet()

A Error: Missing arguments

B Hello, !

C Friend, Hello!

D Hello, Friend!

💡 Hint

Check the default values assigned to the parameters in the function definition.

The function greet has default values for both parameters. Calling greet() uses those defaults, printing "Hello, Friend!".

❓ Predict Output

intermediate

Function call with one default parameter overridden

What will be printed when calling greet(name: "Alice") with the function below?

Swift

func greet(name: String = "Friend", greeting: String = "Hello") {
    print("\(greeting), \(name)!")
}
greet(name: "Alice")

A Hello, Alice!

B Alice, Hello!

C Error: Missing argument for greeting

D Hello, Friend!

💡 Hint

Only the name parameter is provided; the other uses its default.

The name parameter is set to "Alice" explicitly, while greeting uses its default "Hello".

❓ Predict Output

advanced

Output with default parameters and argument labels

What is the output of this Swift code?

Swift

func calculateArea(width: Int = 10, height: Int = 5) -> Int {
    return width * height
}
print(calculateArea(height: 3))

A 50

B Error: Missing argument for width

C 30

D 15

💡 Hint

The width parameter uses its default value.

The call provides height: 3 but omits width, so width defaults to 10. The area is 10 * 3 = 30.

❓ Predict Output

advanced

Default parameter with mutable variable

What will be printed after this code runs?

Swift

var count = 5
func increment(by amount: Int = count) -> Int {
    return amount + 1
}
count = 10
print(increment())

A Error: Cannot use variable 'count' as default parameter

B 11

C 5

D 6

💡 Hint

Swift requires default parameter values to be compile-time constants; variables are not allowed.

Swift does not allow using variables as default parameter values because they must be compile-time constants. This code will not compile.

🧠 Conceptual

expert

Why default parameter values must be compile-time constants

Why does Swift require default parameter values to be compile-time constants or literals?

A Because Swift does not support functions with optional parameters

B Because default values are evaluated once at compile time and cannot depend on runtime variables

C Because default values are ignored during function calls

D Because default values must be mutable to change during execution

💡 Hint

Think about when the default value is set and how it relates to program execution.

Swift evaluates default parameter values at compile time to ensure consistent behavior and avoid runtime errors from changing values.

Practice

1. What is the main purpose of using default parameter values in Swift functions?

easy

A. To make functions run faster

B. To provide fallback values when arguments are not passed

C. To force the caller to always provide arguments

D. To declare variables inside the function

Solution

1. Step 1: Understand default parameters

   Default parameter values allow a function to use a preset value if no argument is provided.

2. Step 2: Identify the purpose

   This makes calling the function easier and avoids errors from missing arguments.

3. Final Answer:

   To provide fallback values when arguments are not passed -> Option B

4. Quick Check:

   Default parameters = fallback values [OK]

Hint: Default values fill in missing arguments automatically [OK]

Common Mistakes:

• Thinking default values speed up code
• Believing default values force arguments
• Confusing default parameters with variable declarations

2. Which of the following is the correct way to declare a Swift function with a default parameter value?

easy

A. func greet(name: String) { name = "Guest"; print("Hello, \(name)!") }

B. func greet(name: String) = "Guest" { print("Hello, \(name)!") }

C. func greet(name: String default "Guest") { print("Hello, \(name)!") }

D. func greet(name: String = "Guest") { print("Hello, \(name)!") }

Solution

1. Step 1: Review Swift default parameter syntax

   In Swift, default values are assigned using an equals sign after the parameter type.

2. Step 2: Check each option

   func greet(name: String = "Guest") { print("Hello, \(name)!") } uses correct syntax: name: String = "Guest". Others have syntax errors.

3. Final Answer:

   func greet(name: String = "Guest") { print("Hello, \(name)!") } -> Option D

4. Quick Check:

   Default parameter syntax = type = value [OK]

Hint: Use = after type to set default value [OK]

Common Mistakes:

• Placing default value outside parentheses
• Using = after parameter name without type
• Trying to assign default inside function body

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

func multiply(_ a: Int, by b: Int = 2) -> Int {
    return a * b
}

print(multiply(5))

medium

A. Error: Missing argument for parameter 'b'

B. 5

C. 10

D. 25

Solution

1. Step 1: Understand function parameters

   The function multiply takes two parameters: 'a' and 'b' with a default value of 2.

2. Step 2: Evaluate the call multiply(5)

   Only 'a' is provided as 5, so 'b' uses default 2. Result is 5 * 2 = 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   5 * default 2 = 10 [OK]

Hint: If argument missing, default value is used [OK]

Common Mistakes:

• Expecting error for missing argument
• Using first argument for both parameters
• Confusing default value with multiplication factor

4. Identify the error in this Swift function declaration:

func greet(name: String = "User", _ age: Int) {
    print("Hello \(name), you are \(age) years old.")
}

medium

A. Default parameter must be last in the parameter list

B. Missing return type

C. Cannot use default values with String type

D. Function body syntax error

Solution

1. Step 1: Check parameter order rules

   In Swift, parameters with default values must come after parameters without defaults.

2. Step 2: Analyze given function

   'name' has a default but appears before 'age' which has no default. This causes a compile error.

3. Final Answer:

   Default parameter must be last in the parameter list -> Option A

4. Quick Check:

   Default params last = correct order [OK]

Hint: Place parameters with defaults after others [OK]

Common Mistakes:

• Putting default parameters before non-default ones
• Assuming return type is mandatory for void functions
• Confusing type restrictions on default values

5. You want to write a Swift function formatMessage that takes a text and an optional prefix with default value "Info:". It should return a string combining prefix and text separated by a space. Which is the correct function and call to get output "Info: Hello"?

hard

A. func formatMessage(text: String, prefix: String = "Info:") -> String { return "\(prefix) \(text)" }
print(formatMessage(text: "Hello"))

B. func formatMessage(prefix: String = "Info:", text: String) -> String { return "\(text) \(prefix)" }
print(formatMessage(text: "Hello"))

C. func formatMessage(text: String, prefix: String) -> String { return "\(prefix) \(text)" }
print(formatMessage(text: "Hello"))

D. func formatMessage(text: String, prefix: String = "Info:") -> String { return "\(text) \(prefix)" }
print(formatMessage(text: "Hello"))

Solution

1. Step 1: Define function with default parameter last

   Parameter with default 'prefix' must come after 'text' without default.

2. Step 2: Check function call and output

   Calling with only 'text' uses default prefix "Info:" and returns "Info: Hello".

3. Final Answer:

   func formatMessage(text: String, prefix: String = "Info:") -> String { return "\(prefix) \(text)" } print(formatMessage(text: "Hello")) -> Option A

4. Quick Check:

   Default param last + call with one arg = correct output [OK]

Hint: Default params last, call with missing arg uses default [OK]

Common Mistakes:

• Placing default parameter before non-default
• Calling function without required parameters
• Swapping order of prefix and text in output