Infix functions for readable calls in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of infix function call

What is the output of this Kotlin code using an infix function?

Kotlin

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

fun main() {
    val result = 3 times "Hi"
    println(result)
}

A HiHiHi

B Hi 3 times

C 3Hi

D Error: infix function cannot be called like this

💡 Hint

Remember infix functions allow calling like 'a times b' instead of 'a.times(b)'.

The infix function 'times' repeats the string the number of times specified by the integer. So 3 times "Hi" produces "HiHiHi".

❓ Predict Output

intermediate

Result of chained infix calls

What is the output of this Kotlin code with chained infix calls?

Kotlin

infix fun String.append(s: String) = this + s
infix fun String.exclaim(times: Int) = this + "!".repeat(times)

fun main() {
    val result = "Hello" append " World" exclaim 3
    println(result)
}

A Hello World!!!

B Error: cannot chain infix functions without parentheses

C Hello World exclaim 3

D Hello World!3

💡 Hint

Infix functions can be chained if the return type matches the next function's receiver.

The first infix call appends " World" to "Hello", then the second adds three exclamation marks, resulting in "Hello World!!!".

🔧 Debug

advanced

Identify the error in infix function usage

What error does this Kotlin code produce?

Kotlin

infix fun Int.add(other: Int) = this + other

fun main() {
    val sum = 5 add
    println(sum)
}

A TypeError: Cannot add Int and Unit

B SyntaxError: Missing argument for infix function

C Compilation error: Infix function call must have exactly one argument

D No error, prints 5

💡 Hint

Infix functions require exactly one argument when called.

The call '5 add' is missing the argument for the infix function 'add', causing a compilation error.

🧠 Conceptual

advanced

Understanding infix function constraints

Which of the following is NOT a requirement for defining an infix function in Kotlin?

A It must have exactly one parameter

B It must return Unit (void)

C It must be marked with the 'infix' keyword

D It must be a member function or an extension function

💡 Hint

Think about what return types infix functions can have.

Infix functions can return any type, not just Unit. The other options are required.

❓ Predict Output

expert

Output of infix function with nullable receiver

What is the output of this Kotlin code?

Kotlin

infix fun String?.orDefault(default: String) = this ?: default

fun main() {
    val a: String? = null
    val b: String? = "Kotlin"
    println(a orDefault "Default")
    println(b orDefault "Default")
}

A Compilation error: infix function cannot have nullable receiver

B

null
Kotlin

C

Default
Default

D

Default
Kotlin

💡 Hint

The Elvis operator ?: returns the left side if not null, otherwise the right side.

For 'a' which is null, the default is returned. For 'b' which is "Kotlin", it returns 'b'.

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