Infix functions for readable calls in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an infix function named shout for the String class.

Kotlin

infix fun String.[1]() = this.uppercase() + "!"

Drag options to blanks, or click blank then click option'

Ashout

Bprint

Clowercase

Drepeat

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that is not descriptive of the action.

Forgetting to use the infix keyword.

The infix function is named shout to match the intended usage.

2fill in blank

medium

Complete the code to call the infix function shout on the string "hello".

Kotlin

val loud = "hello" [1] shout()

Drag options to blanks, or click blank then click option'

Ainfix

B.

C->

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using the infix keyword in the call instead of the declaration.

Using invalid operators like -> or !!.

Infix functions are called without dots or parentheses, but since shout has no parameters, parentheses are optional. Here, the dot is needed to call the function normally.

3fill in blank

hard

Fix the error in the infix function declaration by completing the code.

Kotlin

infix fun Int.[1](times: Int): Int = this * times

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AmultiplyBy

Brepeat

Ctimes

Dmultiply

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that conflicts with existing Kotlin functions.

Choosing a name that is not intuitive for multiplication.

The function name times is a common and readable infix function name for multiplication.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their uppercase form only if the word length is greater than 3.

Kotlin

val result = words.associateWith { it.[1]() }.filter { it.key.length [2] 3 }

Drag options to blanks, or click blank then click option'

Auppercase

B>

C<

Dlowercase

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase() instead of uppercase().

Using the wrong comparison operator.

The uppercase() function converts strings to uppercase, and the filter keeps words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a map from uppercase keys to their lengths, only for words longer than 4 characters.

Kotlin

val map = words.filter { it.length [1] 4 }.associateBy( [2] ) { it.[3] }

Drag options to blanks, or click blank then click option'

A>

Bit.uppercase()

Clength

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong comparison operator.

Using the word itself as key instead of uppercase.

Using a function name instead of a property for length.

Filter words longer than 4, use uppercase words as keys, and map to their length.

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