Infix functions for readable calls in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an infix function in Kotlin?

An infix function is a special kind of function that allows you to call it using infix notation, which means you can write it between its receiver and argument without dots or parentheses, making the code more readable.

beginner

How do you declare an infix function in Kotlin?

You declare an infix function by adding the keyword infix before the function name. It must be a member function or an extension function with a single parameter and no varargs or default values.

intermediate

Example: What does this Kotlin code do?

infix fun Int.times(str: String) = str.repeat(this)

This code defines an infix function times for Int that repeats the given string the number of times equal to the integer. For example, 2 times "Hi" returns "HiHi".

intermediate

What are the requirements for a function to be used as infix in Kotlin?

It must be a member or extension function, have a single parameter, not accept varargs or default parameter values, and be marked with the infix keyword.

beginner

Why use infix functions? Give a real-life analogy.

Infix functions make code look like natural language, improving readability. Like saying "I give you" instead of "I.give(you)", it feels more like a simple conversation.

Which keyword is used to declare an infix function in Kotlin?

A infix

B inline

C override

D fun

The keyword infix is used before the function name to declare an infix function.

What is a requirement for the parameter of an infix function?

A It must be nullable

B It can have multiple parameters

C It must be a vararg parameter

D It must be a single parameter without default values

Infix functions must have exactly one parameter without default values or varargs.

How would you call an infix function named connect on an object a with argument b?

A connect(a, b)

B a.connect(b)

C a connect b

D connect a b

Infix functions allow calling like a connect b without dots or parentheses.

Can an infix function be a top-level function (not inside a class or object)?

A Only if marked inline

B No, it must be a member or extension function

C Yes, always

D Only if it has no parameters

Infix functions must be member or extension functions, not top-level standalone functions.

What is the benefit of using infix functions?

A Improves code readability by allowing natural language style calls

B Makes functions run faster

C Allows multiple parameters

D Automatically handles null safety

Infix functions improve readability by letting you write calls that look like natural language.

Explain what an infix function is and how to declare one in Kotlin.

Think about how you write functions that look like natural language.

Describe a simple example of an infix function and how it improves code readability.

Use a real-life analogy or simple string repetition example.

Practice

1. What is the main purpose of using infix functions in Kotlin?

easy

A. To call functions without dots and parentheses for more readable code

B. To allow functions to have multiple parameters

C. To make functions run faster

D. To restrict functions to only private visibility

Solution

1. Step 1: Understand the role of infix functions

   Infix functions allow calling a function without using dots and parentheses, making code more readable.

2. Step 2: Compare options with this purpose

   Only To call functions without dots and parentheses for more readable code correctly describes this purpose; others mention unrelated features.

3. Final Answer:

   To call functions without dots and parentheses for more readable code -> Option A

4. Quick Check:

   Infix functions improve readability = D [OK]

Hint: Infix means calling without dots and parentheses [OK]

Common Mistakes:

• Thinking infix allows multiple parameters
• Believing infix changes function speed
• Confusing infix with visibility modifiers

2. Which of the following is the correct syntax to declare an infix function in Kotlin?

easy

A. fun add infix(x: Int): Int = this + x

B. infix fun add(x: Int): Int

C. fun infix add(x: Int, y: Int): Int = x + y

D. infix fun add(x: Int): Int = this + x

Solution

1. Step 1: Recall infix function syntax

   An infix function must be declared with the keyword infix before fun, have exactly one parameter, and a body.

2. Step 2: Check each option

   infix fun add(x: Int): Int = this + x correctly uses infix fun add(x: Int): Int = this + x. fun infix add(x: Int, y: Int): Int = x + y has two parameters, invalid. infix fun add(x: Int): Int misses function body. fun add infix(x: Int): Int = this + x has wrong keyword order.

3. Final Answer:

   infix fun add(x: Int): Int = this + x -> Option D

4. Quick Check:

   Infix keyword before fun, one parameter only = A [OK]

Hint: Infix keyword comes before fun, one parameter only [OK]

Common Mistakes:

• Declaring multiple parameters in infix function
• Placing infix keyword after fun
• Omitting function body

3. What is the output of the following Kotlin code?

class Calculator {
    infix fun add(x: Int): Int = x + 10
}

fun main() {
    val calc = Calculator()
    println(calc add 5)
}

medium

A. SyntaxError

B. 5

C. 15

D. 10

Solution

1. Step 1: Understand the infix function call

   The infix function add adds 10 to the parameter x. Calling calc add 5 means calc.add(5).

2. Step 2: Calculate the result

   5 + 10 = 15, so the printed output is 15.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Infix calls work like normal calls without dots [OK]

Common Mistakes:

• Confusing infix call with normal method call syntax
• Expecting parameter to be ignored
• Thinking infix changes return value

4. Identify the error in the following Kotlin code using an infix function:

class Person {
    infix fun greet(name: String, age: Int) {
        println("Hello $name, age $age")
    }
}

fun main() {
    val p = Person()
    p greet ("Alice", 30)
}

medium

A. Missing parentheses in function call

B. Infix functions cannot have more than one parameter

C. Infix functions must return a value

D. Class Person cannot have infix functions

Solution

1. Step 1: Check infix function parameter rules

   Infix functions must have exactly one parameter. Here, greet has two parameters, which is invalid.

2. Step 2: Verify other options

   Parentheses are optional in infix calls. Infix functions can return Unit (no value). Classes can have infix functions. So only Infix functions cannot have more than one parameter is correct.

3. Final Answer:

   Infix functions cannot have more than one parameter -> Option B

4. Quick Check:

   One parameter max for infix = A [OK]

Hint: Infix functions take exactly one parameter [OK]

Common Mistakes:

• Trying to use multiple parameters in infix function
• Thinking parentheses are always required
• Believing infix functions must return a value

5. Given the following Kotlin code, what will be the output?

class Box(val value: Int) {
    infix fun combine(other: Box): Box = Box(this.value + other.value)
}

fun main() {
    val box1 = Box(3)
    val box2 = Box(7)
    val result = box1 combine box2 combine Box(5)
    println(result.value)
}

hard

A. 15

B. 10

C. SyntaxError

D. 35

Solution

1. Step 1: Understand chaining of infix calls

   The expression box1 combine box2 combine Box(5) is evaluated left to right as (box1 combine box2) combine Box(5).

2. Step 2: Calculate each combine call

   First combine: 3 + 7 = 10, returns Box(10). Second combine: 10 + 5 = 15, returns Box(15). Printing result.value outputs 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   3+7=10, 10+5=15 [OK]

Hint: Infix calls chain left to right like normal calls [OK]

Common Mistakes:

• Assuming combine adds values incorrectly
• Thinking infix calls don't chain
• Confusing order of operations