Functions and lambdas in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function in Kotlin?

A function is a reusable block of code that performs a specific task. It can take inputs, called parameters, and can return a result.

beginner

How do you declare a simple function in Kotlin that adds two numbers?

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

intermediate

What is a lambda expression in Kotlin?

A lambda is a small anonymous function that can be passed as a value. It is often used for short pieces of code like callbacks or operations on collections.

intermediate

How do you write a lambda that multiplies a number by 2?

val double = { x: Int -> x * 2 }

intermediate

What is the difference between a named function and a lambda in Kotlin?

A named function has a name and can be called anywhere by that name. A lambda is anonymous and usually used as a value passed to other functions.

How do you declare a function in Kotlin that returns no value?

A fun greet() { println("Hi") }

B fun greet(): Int { println("Hi") }

C fun greet() -> Unit { println("Hi") }

D function greet() { println("Hi") }

In Kotlin, a function that returns no value has return type Unit, which can be omitted. So fun greet() { ... } is correct.

What symbol separates parameters and the body in a lambda expression?

A :

B ->

C =

D =>

In Kotlin lambdas, the arrow -> separates the parameter list from the body.

Which of these is a valid lambda that takes no parameters and returns 5?

A { -> 5 }

B fun() = 5

C { x -> 5 }

D { 5 }

If a lambda has no parameters, you can omit the arrow and just write { 5 }.

How do you call a function named 'sum' with arguments 3 and 4?

A sum(3, 4)

B sum 3, 4

C call sum(3, 4)

D sum.call(3,4)

Functions in Kotlin are called by their name followed by parentheses with arguments inside.

What is the type of the lambda val square = { x: Int -> x * x }?

A Unit

B Int

C (Int) -> Int

D Function

The lambda takes an Int and returns an Int, so its type is (Int) -> Int.

Explain how to declare and use a simple function in Kotlin with parameters and a return value.

Think about how you tell the app to do something with inputs and give back a result.

Describe what a lambda expression is and give an example of when you might use one in an Android app.

Imagine you want to quickly tell a button what to do when clicked without writing a full function.

Practice

1. What is a lambda function in Kotlin?
val sum = { a: Int, b: Int -> a + b }

easy

A. A function that must have a name and cannot be stored

B. An unnamed function that can be stored in a variable

C. A special class for storing data

D. A type of loop for repeating code

Solution

1. Step 1: Understand lambda syntax

   The code { a: Int, b: Int -> a + b } defines a function without a name.

2. Step 2: Identify lambda characteristics

   This function is stored in the variable sum, showing it is unnamed but reusable.

3. Final Answer:

   An unnamed function that can be stored in a variable -> Option B

4. Quick Check:

   Lambda = unnamed function stored in variable [OK]

Hint: Lambdas have no name and are assigned to variables [OK]

Common Mistakes:

• Thinking lambdas must have names
• Confusing lambdas with classes
• Assuming lambdas are loops

2. Which of the following is the correct syntax to declare a lambda that takes no parameters and returns a String "Hello"?

easy

A. val greet = fun() { return "Hello" }

B. val greet = () -> "Hello"

C. val greet = { -> "Hello"

D. val greet = { "Hello" }

Solution

1. Step 1: Review lambda syntax for no parameters

   Lambdas with no parameters use { expression } syntax.

2. Step 2: Check each option

   val greet = { "Hello" } correctly uses { "Hello" }. val greet = { -> "Hello" missing closing brace, B is function type syntax, D is a named function.

3. Final Answer:

   val greet = { "Hello" } -> Option D

4. Quick Check:

   No-parameter lambda = { value } [OK]

Hint: Use { expression } for lambdas with no parameters [OK]

Common Mistakes:

• Missing closing brace in lambda
• Confusing lambda with function type syntax
• Using fun keyword inside lambda

3. What is the output of this Kotlin code?

val multiply = { x: Int, y: Int -> x * y }
println(multiply(3, 4))

medium

A. 7

B. 34

C. 12

D. Error

Solution

1. Step 1: Understand the lambda function

   The lambda { x: Int, y: Int -> x * y } multiplies two integers.

2. Step 2: Calculate the result of multiply(3, 4)

   3 * 4 equals 12, so the printed output is 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs inside lambda to get output [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Expecting syntax error

4. Identify the error in this Kotlin lambda declaration:

val greet = { name: String ->
  println("Hello, " + name)
}

medium

A. No error, this is valid syntax

B. Cannot use println inside lambda

C. Missing return type declaration

D. Lambda must return a value, println returns Unit

Solution

1. Step 1: Check lambda syntax

   The lambda takes a String parameter and calls println, which is allowed.

2. Step 2: Confirm return type flexibility

   Lambdas can return Unit (like void), so no return type declaration is needed.

3. Final Answer:

   No error, this is valid syntax -> Option A

4. Quick Check:

   Lambda with println is valid [OK]

Hint: Lambdas can return Unit; println is allowed [OK]

Common Mistakes:

• Expecting return type declaration
• Thinking println is disallowed
• Confusing Unit with error

5. You want to filter a list of numbers to keep only even numbers using a lambda. Which code correctly does this?

val numbers = listOf(1, 2, 3, 4, 5, 6)
val evens = numbers.filter( /* fill here */ )
println(evens)

hard

A. numbers.filter { it % 2 == 0 }

B. numbers.filter { x -> x / 2 == 0 }

C. numbers.filter { it % 2 = 0 }

D. numbers.filter { it & 2 == 0 }

Solution

1. Step 1: Understand filtering with lambda

   The filter lambda should return true for even numbers using modulo operator.

2. Step 2: Evaluate each option

   numbers.filter { it % 2 == 0 } correctly uses it % 2 == 0. numbers.filter { x -> x / 2 == 0 } uses division instead of modulo, C uses assignment (=) instead of comparison (==), A uses bitwise AND incorrectly.

3. Final Answer:

   numbers.filter { it % 2 == 0 } -> Option A

4. Quick Check:

   Filter even numbers with modulo == 0 [OK]

Hint: Use it % 2 == 0 to check even numbers in filter [OK]

Common Mistakes:

• Using = instead of == for comparison
• Using division instead of modulo
• Misusing bitwise operators