Function declaration syntax in Swift - Mini Project: Build & Apply

Function declaration syntax

📖 Scenario: Imagine you are creating a simple calculator app that can greet users and perform basic math operations.

🎯 Goal: You will learn how to declare functions in Swift to greet users and add two numbers.

📋 What You'll Learn

Declare a function called greetUser that prints a greeting message.

Declare a function called addNumbers that takes two Int parameters and returns their sum.

Call both functions to see their output.

💡 Why This Matters

🌍 Real World

Functions help organize code into reusable blocks, making apps easier to build and maintain.

💼 Career

Knowing how to declare and use functions is essential for any Swift developer working on iOS or macOS apps.

1

Create a simple function to greet the user

Declare a function called greetUser that prints the message "Hello, welcome to the calculator!".

Swift

# Declare the greetUser function that prints a welcome message
// Your code here

Hint

Use the func keyword followed by the function name and parentheses. Inside curly braces, use print to show the message.

2

Add a function to sum two numbers

Declare a function called addNumbers that takes two Int parameters named a and b, and returns their sum as an Int.

Swift

func greetUser() {
    print("Hello, welcome to the calculator!")
}

// Declare addNumbers function that takes two Int parameters and returns an Int
// Your code here

Hint

Remember to specify parameter types and the return type after an arrow ->. Use return to send back the sum.

3

Call the functions to see their output

Call the greetUser() function and then call addNumbers(a: 5, b: 7) storing the result in a variable called result.

Swift

func greetUser() {
    print("Hello, welcome to the calculator!")
}

func addNumbers(a: Int, b: Int) -> Int {
    return a + b
}

// Call greetUser and store the sum of 5 and 7 in result
// Your code here

Hint

Call functions by writing their name followed by parentheses. Assign the returned value to a variable using let.

4

Print the result of the addition

Print the value stored in the variable result using print.

Swift

func greetUser() {
    print("Hello, welcome to the calculator!")
}

func addNumbers(a: Int, b: Int) -> Int {
    return a + b
}

greetUser()
let result = addNumbers(a: 5, b: 7)

// Print the result variable
// Your code here

Hint

Use print(result) to show the sum on the screen.

Practice

1. Which keyword is used to declare a function in Swift?

easy

A. def

B. func

C. function

D. method

Solution

1. Step 1: Recall Swift function declaration

   In Swift, functions always start with the keyword func.

2. Step 2: Compare options

   Only func uses func, the correct keyword for Swift functions.

3. Final Answer:

   func -> Option B

4. Quick Check:

   Swift functions start with func [OK]

Hint: Remember: Swift functions always start with 'func' keyword [OK]

Common Mistakes:

• Using 'function' instead of 'func'
• Confusing Swift with Python's 'def'
• Using 'method' which is not a keyword

2. Which of the following is the correct syntax to declare a function named greet that takes no parameters and returns nothing?

easy

A. func greet() -> void {}

B. function greet() {}

C. func greet {}

D. func greet() {}

Solution

1. Step 1: Check function declaration syntax

   In Swift, a function with no parameters and no return type is declared as func name() {}.

2. Step 2: Analyze each option

   func greet() {} matches the correct syntax. function greet() {} uses 'function' which is invalid. func greet {} misses parentheses. func greet() -> void {} uses 'void' which is not used in Swift.

3. Final Answer:

   func greet() {} -> Option D

4. Quick Check:

   Empty parameter list needs () and no return type means no arrow [OK]

Hint: Empty parameters always need () after function name [OK]

Common Mistakes:

• Omitting parentheses for parameters
• Using 'function' instead of 'func'
• Using 'void' as return type in Swift

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

func add(a: Int, b: Int) -> Int {
    return a + b
}

print(add(a: 3, b: 4))

medium

A. a + b

B. 34

C. 7

D. Error: Missing return statement

Solution

1. Step 1: Understand function parameters and return

   The function add takes two integers and returns their sum.

2. Step 2: Calculate the function call result

   Calling add(a: 3, b: 4) returns 3 + 4 = 7, which is printed.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   Sum of 3 and 4 is 7 [OK]

Hint: Add numbers inside function and print result [OK]

Common Mistakes:

• Concatenating numbers as strings
• Forgetting to return a value
• Misunderstanding parameter labels

4. Identify the error in the following Swift function declaration:

func multiply(a: Int, b Int) -> Int {
    return a * b
}

medium

A. Missing colon between parameter name and type for b

B. Missing return keyword

C. Function name is invalid

D. Return type should be String

Solution

1. Step 1: Check parameter syntax

   Each parameter must have a colon between its name and type. Here, b Int is missing a colon.

2. Step 2: Confirm other parts are correct

   The return keyword is present, function name is valid, and return type Int is appropriate.

3. Final Answer:

   Missing colon between parameter name and type for b -> Option A

4. Quick Check:

   Parameters need colon between name and type [OK]

Hint: Check colons between parameter names and types [OK]

Common Mistakes:

• Omitting colon in parameter list
• Forgetting return keyword
• Using wrong return type

5. You want to declare a Swift function named describePerson that takes a String parameter name and an Int parameter age, and returns a String describing the person like "Name is Age years old." Which is the correct function declaration?

hard

A. func describePerson(name: String, age: Int) -> String {
return "\(name) is \(age) years old."
}

B. func describePerson(name String, age Int) -> String {
return "\(name) is \(age) years old."
}

C. func describePerson(name: String, age: Int) {
print("\(name) is \(age) years old.")
}

D. func describePerson(name: String, age: Int) -> Void {
return "\(name) is \(age) years old."
}

Solution

1. Step 1: Check parameter syntax and return type

   Parameters must have colons between name and type. The function returns a String, so return type must be String.

2. Step 2: Verify function body matches return type

   func describePerson(name: String, age: Int) -> String { return "\(name) is \(age) years old." } returns a String with correct string interpolation. func describePerson(name String, age Int) -> String { return "\(name) is \(age) years old." } misses colons. func describePerson(name: String, age: Int) { print("\(name) is \(age) years old.") } has no return type and uses print instead of return. func describePerson(name: String, age: Int) -> Void { return "\(name) is \(age) years old." } uses Void return type but returns a String, which is invalid.

3. Final Answer:

   func describePerson(name: String, age: Int) -> String { return "\(name) is \(age) years old." } -> Option A

4. Quick Check:

   Correct parameters and return type with string interpolation [OK]

Hint: Use colons for parameters and match return type with returned value [OK]

Common Mistakes:

• Omitting colons in parameters
• Using print instead of return for returning values
• Mismatching return type and returned value