Function declaration syntax in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple function call

What is the output of this Kotlin code?

Kotlin

fun greet(name: String): String {
    return "Hello, $name!"
}

fun main() {
    println(greet("Alice"))
}

A Hello, Alice!

B greet(Alice)

C Hello, name!

D Error: Missing return statement

💡 Hint

Look at how the function returns a greeting string using the input name.

The function greet takes a string parameter and returns a greeting message. Calling greet("Alice") returns "Hello, Alice!" which is printed.

❓ Predict Output

intermediate

Function with default parameter value

What will this Kotlin program print?

Kotlin

fun multiply(x: Int, y: Int = 2): Int {
    return x * y
}

fun main() {
    println(multiply(5))
}

A 10

B 7

C Error: Missing argument for y

D 25

💡 Hint

Check the default value of parameter y.

The function multiply has a default value of 2 for y. Calling multiply(5) uses y=2, so 5*2=10 is printed.

❓ Predict Output

advanced

Single-expression function output

What does this Kotlin code print?

Kotlin

fun square(n: Int) = n * n

fun main() {
    println(square(4))
}

A n * n

B 4

C Error: Missing return type

D 16

💡 Hint

Single-expression functions return the expression value automatically.

The function square returns n multiplied by n. For input 4, it returns 16.

❓ Predict Output

advanced

Function with no return value

What is the output of this Kotlin program?

Kotlin

fun printMessage(message: String): Unit {
    println("Message: $message")
}

fun main() {
    printMessage("Hi there")
}

A Error: Function must return a value

B Unit

C Message: Hi there

D Hi there

💡 Hint

Unit means the function returns no meaningful value but can print output.

The function prints the message with prefix "Message:". The output is that line.

❓ Predict Output

expert

Function with named arguments and default values

What will this Kotlin code print?

Kotlin

fun formatText(text: String, prefix: String = "", suffix: String = "") = "$prefix$text$suffix"

fun main() {
    println(formatText(suffix = "!", text = "Hello"))
}

A !Hello

B Hello!

C Hello

D Error: Named arguments must be in order

💡 Hint

Named arguments can be passed in any order in Kotlin.

The function concatenates prefix, text, and suffix. Here prefix is default empty, suffix is "!", text is "Hello", so output is "Hello!".

Practice

1. Which of the following is the correct way to declare a simple function named greet in Kotlin that takes no parameters and returns no value?

easy

A. function greet() { println("Hello") }

B. fun greet() { println("Hello") }

C. def greet() { println("Hello") }

D. fun greet { println("Hello") }

Solution

1. Step 1: Identify the correct keyword for function declaration

   Kotlin uses the keyword fun to declare functions. Options using function or def are incorrect.

2. Step 2: Check the function syntax for parameters and braces

   The function must have parentheses even if there are no parameters, so fun greet { println("Hello") } is incorrect because it misses parentheses.

3. Final Answer:

   fun greet() { println("Hello") } -> Option B

4. Quick Check:

   fun + name + () + body = B [OK]

Hint: Remember: Kotlin functions start with 'fun' and need parentheses [OK]

Common Mistakes:

• Using 'function' or 'def' instead of 'fun'
• Omitting parentheses for parameter list
• Missing curly braces for function body

2. Which of the following is the correct syntax to declare a function add that takes two Int parameters and returns their sum as an Int?

easy

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

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

C. fun add(a: Int, b: Int) -> Int { return a + b }

D. fun add(a: Int, b: Int): Int = a + b

Solution

1. Step 1: Check parameter declaration syntax

   Parameters must be declared as name: Type, so fun add(a Int, b Int): Int { return a + b } is incorrect because it misses colons.

2. Step 2: Verify return type and function body syntax

   fun add(a: Int, b: Int): Int = a + b uses the correct syntax with an expression body and return type. fun add(a: Int, b: Int) -> Int { return a + b } uses '->' which is invalid in Kotlin. fun add(a: Int, b: Int): Int { a + b } misses the return keyword in a block body, so it won't return the sum.

3. Final Answer:

   fun add(a: Int, b: Int): Int = a + b -> Option D

4. Quick Check:

   fun + parameters with colons + : ReturnType + = expression = A [OK]

Hint: Use colons for types and '=' for single-expression functions [OK]

Common Mistakes:

• Omitting colons between parameter names and types
• Using '->' instead of ':' for return type
• Missing 'return' in block body functions

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

fun multiply(x: Int, y: Int): Int {
    return x * y
}

fun main() {
    println(multiply(3, 4))
}

medium

A. 12

B. 34

C. 7

D. Error

Solution

1. Step 1: Understand the multiply function

   The function takes two integers and returns their product using return x * y.

2. Step 2: Calculate the function call multiply(3, 4)

   3 multiplied by 4 equals 12, so the function returns 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply parameters and check return value [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Assuming syntax error due to missing expression body

4. Identify the error in the following Kotlin function declaration:

fun divide(a: Int, b: Int): Int {
    return a / b

medium

A. Missing closing brace '}'

B. Incorrect return type

C. Parameters should not have types

D. Function name is invalid

Solution

1. Step 1: Check function syntax completeness

   The function body starts with '{' but there is no closing '}', so the code is incomplete.

2. Step 2: Verify other syntax parts

   The return type is correct, parameters have types, and the function name is valid, so no other errors.

3. Final Answer:

   Missing closing brace '}' -> Option A

4. Quick Check:

   All blocks need matching braces [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Forgetting to close function body with '}'
• Misplacing colons in parameters
• Using invalid function names

5. You want to declare a Kotlin function formatName that takes two parameters: firstName and lastName (both Strings), and returns a single String in the format "Last, First". Which of the following is the correct declaration and implementation?

hard

A. fun formatName(firstName: String, lastName: String): Int = "$lastName, $firstName"

B. fun formatName(firstName String, lastName String) = lastName + ", " + firstName

C. fun formatName(firstName: String, lastName: String): String { return "$lastName, $firstName" }

D. fun formatName(firstName: String, lastName: String): String { "$lastName, $firstName" }

Solution

1. Step 1: Check parameter and return type syntax

   Parameters must use name: Type and return type must match. A uses : Int but returns String (type mismatch). B misses colons (firstName String invalid).

2. Step 2: Verify function body syntax

   C has block body { "$lastName, $firstName" } without return, returns Unit not String. D correctly uses block body with return and string interpolation.

3. Final Answer:

   fun formatName(firstName: String, lastName: String): String { return "$lastName, $firstName" } -> Option C

4. Quick Check:

   Correct types + block body with return + interpolation = D [OK]

Hint: Use return keyword in block body functions for clarity [OK]

Common Mistakes:

• Omitting return keyword in block body
• Missing return type declaration
• Incorrect string concatenation or interpolation