Higher-order function declaration in Kotlin - Mini Project: Build & Apply

Higher-order function declaration in Kotlin

📖 Scenario: Imagine you are building a simple calculator app that can perform different operations like addition and multiplication. You want to write a function that can take another function as input to decide which operation to perform.

🎯 Goal: Build a Kotlin program that declares a higher-order function which accepts two numbers and a function to perform an operation on those numbers.

📋 What You'll Learn

Create a higher-order function called calculate that takes two Int parameters and a function parameter that itself takes two Int and returns an Int.

Create two simple functions called add and multiply that each take two Int parameters and return their sum and product respectively.

Use the calculate function to perform addition and multiplication by passing add and multiply as arguments.

Print the results of both operations.

💡 Why This Matters

🌍 Real World

Higher-order functions are used in many apps to customize behavior, like sorting lists, handling user input, or performing calculations based on user choices.

💼 Career

Understanding higher-order functions is important for Kotlin developers because it helps write clean, reusable, and flexible code, which is highly valued in software development jobs.

1

Create the add and multiply functions

Write two functions called add and multiply. Each should take two Int parameters named a and b. The add function should return the sum of a and b. The multiply function should return the product of a and b.

Kotlin

# Your code here

Hint

Define two functions with the exact names and parameters. Use return a + b for addition and return a * b for multiplication.

2

Declare the higher-order function calculate

Create a function called calculate that takes three parameters: two Int parameters named x and y, and a function parameter named operation which itself takes two Int parameters and returns an Int. The calculate function should return the result of calling operation(x, y).

Kotlin

fun add(a: Int, b: Int): Int {
    return a + b
}

fun multiply(a: Int, b: Int): Int {
    return a * b
}

# Your code here

Hint

Define calculate with two Int parameters and one function parameter with type (Int, Int) -> Int. Return the result of calling operation(x, y).

3

Use calculate with add and multiply

Create two variables called sumResult and productResult. Assign sumResult the result of calling calculate with arguments 5, 3, and the add function. Assign productResult the result of calling calculate with arguments 5, 3, and the multiply function.

Kotlin

fun add(a: Int, b: Int): Int {
    return a + b
}

fun multiply(a: Int, b: Int): Int {
    return a * b
}

fun calculate(x: Int, y: Int, operation: (Int, Int) -> Int): Int {
    return operation(x, y)
}

# Your code here

Hint

Use the function reference operator :: to pass add and multiply to calculate.

4

Print the results

Write two println statements to print sumResult and productResult.

Kotlin

fun add(a: Int, b: Int): Int {
    return a + b
}

fun multiply(a: Int, b: Int): Int {
    return a * b
}

fun calculate(x: Int, y: Int, operation: (Int, Int) -> Int): Int {
    return operation(x, y)
}

val sumResult = calculate(5, 3, ::add)
val productResult = calculate(5, 3, ::multiply)

# Your code here

Hint

Use println(sumResult) and println(productResult) to show the results.

Practice

1. What is a higher-order function in Kotlin?

easy

A. A function that only returns integers

B. A function that takes another function as a parameter or returns a function

C. A function that cannot be called directly

D. A function that has no parameters

Solution

1. Step 1: Understand the definition of higher-order functions

   Higher-order functions are functions that can take other functions as parameters or return them as results.

2. Step 2: Match the definition with the options

   A function that takes another function as a parameter or returns a function correctly describes this behavior, while others describe unrelated function properties.

3. Final Answer:

   A function that takes another function as a parameter or returns a function -> Option B

4. Quick Check:

   Higher-order function = function with function parameter or return [OK]

Hint: Remember: higher-order means function uses another function [OK]

Common Mistakes:

• Thinking higher-order means function returns only numbers
• Confusing higher-order with functions without parameters
• Assuming higher-order functions cannot be called directly

2. Which of the following is the correct syntax to declare a higher-order function in Kotlin that takes a function parameter returning Int?

easy

A. fun operate(x: Int, f: Int): Int { return f(x) }

B. fun operate(x: Int, f: (Int) Int): Int { return f(x) }

C. fun operate(x: Int, f: (Int) -> Int): Int { return f(x) }

D. fun operate(x: Int, f: (Int) -> Int) { return f(x) }

Solution

1. Step 1: Identify correct function type syntax

   Kotlin uses (Type) -> ReturnType to declare function types. fun operate(x: Int, f: (Int) -> Int): Int { return f(x) } uses (Int) -> Int correctly.

2. Step 2: Check function return and parameter usage

   fun operate(x: Int, f: (Int) -> Int): Int { return f(x) } returns Int and calls f(x) properly. fun operate(x: Int, f: Int): Int { return f(x) } treats f as Int, not a function. fun operate(x: Int, f: (Int) Int): Int { return f(x) } misses arrow. fun operate(x: Int, f: (Int) -> Int) { return f(x) } lacks return type.

3. Final Answer:

   fun operate(x: Int, f: (Int) -> Int): Int { return f(x) } -> Option C

4. Quick Check:

   Function type syntax = (Int) -> Int [OK]

Hint: Look for (Type) -> ReturnType syntax for function parameters [OK]

Common Mistakes:

• Omitting the arrow '->' in function type
• Using wrong parameter type instead of function type
• Missing return type in function declaration

3. What is the output of this Kotlin code?

fun applyTwice(x: Int, f: (Int) -> Int): Int {
    return f(f(x))
}

fun increment(n: Int) = n + 1

fun main() {
    println(applyTwice(3, ::increment))
}

medium

A. 5

B. 4

C. 6

D. 3

Solution

1. Step 1: Understand applyTwice function behavior

   applyTwice calls the function f twice on x: f(f(x)). Here, f is increment which adds 1.

2. Step 2: Calculate the output step-by-step

   First increment(3) = 4, then increment(4) = 5, so applyTwice(3, increment) returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Increment twice on 3 = 5 [OK]

Hint: Apply the function twice stepwise to find output [OK]

Common Mistakes:

• Applying function only once
• Confusing function call syntax
• Ignoring the nested function call

4. Identify the error in this Kotlin higher-order function declaration:

fun transform(x: Int, f: Int -> Int): Int {
    return f(x)
}

medium

A. Missing parentheses around function type parameter

B. Return type of transform is missing

C. Function parameter name is invalid

D. Function body syntax is incorrect

Solution

1. Step 1: Check function type syntax in parameter

   Function types must be enclosed in parentheses like (Int) -> Int. Here, f: Int -> Int is missing parentheses.

2. Step 2: Verify other parts of the function

   Return type Int is present, parameter name f is valid, and function body syntax is correct.

3. Final Answer:

   Missing parentheses around function type parameter -> Option A

4. Quick Check:

   Function type needs parentheses: (Int) -> Int [OK]

Hint: Always wrap function types in parentheses (Type) -> ReturnType [OK]

Common Mistakes:

• Omitting parentheses around function type
• Confusing parameter name with type
• Missing return type declaration

5. How would you declare a Kotlin higher-order function named compose that takes two functions f and g (both from Int to Int) and returns a new function from Int to Int that applies g then f?

hard

A. fun compose(f: (Int) -> Int, g: (Int) -> Int): Int = f(g(0))

B. fun compose(f: Int, g: Int): Int = { x -> f(g(x)) }

C. fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int = f(g)

D. fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int = { x -> f(g(x)) }

Solution

1. Step 1: Understand function composition signature

   compose takes two functions f and g, both (Int) -> Int, and returns a function (Int) -> Int.

2. Step 2: Check the lambda expression for composition

   The returned function applies g first, then f: { x -> f(g(x)) }. fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int = { x -> f(g(x)) } matches this exactly.

3. Final Answer:

   fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int = { x -> f(g(x)) } -> Option D

4. Quick Check:

   Compose = return lambda applying g then f [OK]

Hint: Return a lambda applying g then f: { x -> f(g(x)) } [OK]

Common Mistakes:

• Returning wrong type instead of function
• Using incorrect parameter types
• Trying to call functions instead of returning a lambda