Default parameter values in Swift

Introduction

Default parameter values let you give a starting value to a function's input. This way, if you don't provide that input, the function still works smoothly.

When you want a function to work even if some inputs are missing.

When you want to avoid writing many versions of the same function.

When you want to provide common or typical values automatically.

When you want to make your code easier to use and read.

Syntax

Swift

func functionName(parameterName: Type = defaultValue) {
    // function body
}

You write the default value after an equals sign = in the parameter list.

If you call the function without that parameter, the default value is used.

Examples

This function says hello. If you don't give a name, it uses "Friend".

Swift

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

This function multiplies a number by a multiplier. If you don't give the multiplier, it doubles the number.

Swift

func multiply(number: Int, by multiplier: Int = 2) -> Int {
    return number * multiplier
}

Sample Program

This program shows how the greet function works with and without giving a name.

Swift

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

greet()
greet(name: "Alice")

Important Notes

Default parameters must come after parameters without defaults.

You can mix parameters with and without default values in the same function.

Summary

Default parameter values make functions easier to call by providing fallback inputs.

They help avoid writing many similar functions.

Use them to make your code cleaner and friendlier.

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