Default parameter values in Swift - Mini Project: Build & Apply

Using Default Parameter Values in Swift Functions

📖 Scenario: You are creating a simple greeting app that can say hello to users. Sometimes, users provide their name, and sometimes they don't. You want the app to greet users properly in both cases.

🎯 Goal: Build a Swift function that uses default parameter values to greet users by name or with a generic greeting if no name is given.

📋 What You'll Learn

Create a function called greet that takes one parameter name of type String.

Set a default value for the name parameter to "Guest".

Call the greet function twice: once without any argument and once with the argument "Alice".

Print the greeting message inside the greet function.

💡 Why This Matters

🌍 Real World

Default parameter values are useful in apps where some inputs are optional, like greeting users who may or may not provide their names.

💼 Career

Understanding default parameters helps you write cleaner, more flexible functions, a skill important for Swift developers building user-friendly apps.

1

Create the greet function with a name parameter

Write a Swift function called greet that takes one parameter named name of type String. Inside the function, print the message "Hello, \(name)!".

Swift

# Your code here

Hint

Define a function with func, add a parameter name: String, and use print with string interpolation.

2

Add a default value to the name parameter

Modify the greet function so that the name parameter has a default value of "Guest".

Swift

func greet(name: String = "Guest") {
    print("Hello, \(name)!")
}
# Your code here

Hint

Add = "Guest" after the parameter type to set a default value.

3

Call greet without an argument

Call the greet function without passing any argument.

Swift

func greet(name: String = "Guest") {
    print("Hello, \(name)!")
}
greet()
# Your code here

Hint

Just write greet() to call the function with the default parameter.

4

Call greet with the argument "Alice" and print both greetings

Call the greet function with the argument "Alice" after the call without arguments. The program should print both greetings.

Swift

func greet(name: String = "Guest") {
    print("Hello, \(name)!")
}
greet()
greet("Alice")
# Your code here

Hint

Call greet("Alice") after greet() to see both greetings.

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