Functions (revision)

Other languages

Before we go on, here's some tips for users of other programming languages this year:

Functions that accept parameters

Functions can accept multiple parameters, and each parameter must have a specified type.

func greet(name: String, age: Int) -> String {
    return "Hello, \(name)! You are \(age) years old."
}

let message = greet(name: "Alice", age: 17)
print(message) // Output: Hello, Alice! You are 17 years old.

📝 Task

• Create a function introduce(name:age:city:) that takes a name, age, and city, returning a formatted introduction string.
• Call the function and print its result.

Specifying parameter names when calling

By default, parameter names must be used when calling a function:

func multiply(number1 a: Int, number2 b: Int) -> Int {
    return a * b
}

let product = multiply(number1: 3, number2: 5)  // Must use names!
let product2 = multiply(3, 5)  // Swift refuses to compile this code

📝 Task

• Write a function subtract(minuend:subtrahend:) that subtracts the second number from the first.
• Call it using named parameters and print the result.

Using _ to omit parameter names

If we don’t want to require parameter names at the call site, we use _:

func subtract(_ a: Int, _ b: Int) -> Int {
    return a - b
}

let difference = subtract(10, 4)  // No parameter labels needed

📝 Task

• Convert the subtract(minuend:subtrahend:) function from Task 4 to allow omission of parameter names.
• Call the function and print the result.

Functions that return values

In Swift, functions can return values using the -> operator followed by the return type.

func square(of number: Int) -> Int {
    return number * number
}

let result = square(of: 4)  // result is 16

📝 Task

• Write a function cube(of:) that returns the cube of a given integer.
• Call the function with different numbers and print the results.

Returning optional values

Sometimes, a function may not always have a valid result. In such cases, use an [optional]() return type (?).

func divide(_ a: Int, by b: Int) -> Int? {
    if b == 0 {
        return nil  // Return nil when division by zero occurs
    }
    return a / b
}

if let result = divide(10, by: 2) {
    print("Result is \(result)")  // Prints: Result is 5
} else {
    print("Invalid division")
}

📝 Task

• Modify the divide function above to return Double? instead of Int?.
• Use optional binding (if let) to safely unwrap the result and print it.
• Try calling the function with divide(10, by: 0) and handle the nil case.

Using functions with map, filter, and reduce

Swift’s higher-order functions like map, filter, and reduce work well with named functions, provided the function signatures match their expected parameter and return types.

Using map with functions

map applies a function to each element in an array to create a new array.

For example, the double function below expects a single Int; using map with the numbers list would also have a single Int (i.e. the number in numbers.map { number in ...); therefore, the double function can be supplied directly to the call to map.

func double(_ number: Int) -> Int {
    return number * 2
}

let numbers = [1, 2, 3, 4]
let doubledNumbers = numbers.map(double)
print(doubledNumbers)  // Output: [2, 4, 6, 8]

📝 Task

• Write a function squareNumber(_:) that returns the square of a number.
• Use it with map to square all numbers in an array.
• Print the transformed array.

Using filter with functions

filter keeps elements that meet a condition:

func isEven(_ number: Int) -> Bool {
    return number % 2 == 0
}

let evenNumbers = numbers.filter(isEven)
print(evenNumbers)  // Output: [2, 4]

📝 Task

• Write a function isPositive(_:) that returns true if a number is positive.
• Use it with filter to remove negative numbers from an array.
• Print the filtered array.

Using reduce to combine values

reduce is used to combine all elements into a single value:

func sum(_ total: Int, _ number: Int) -> Int {
    return total + number
}

let totalSum = numbers.reduce(0, sum)
print(totalSum)  // Output: 10

📝 Task

• Write a function multiply(_:_:) that multiplies two numbers.
• Use reduce to find the product of all numbers in an array.
• Print the final result.