Vararg parameters in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with vararg parameter

What is the output of this Kotlin code using a vararg parameter?

Kotlin

fun printAll(vararg messages: String) {
    for (m in messages) print("$m ")
}

fun main() {
    printAll("Hi", "there", "Kotlin")
}

A [Hi, there, Kotlin]

B Hi,there,Kotlin

C

Hi
there
Kotlin

D Hi there Kotlin

💡 Hint

Remember that the function prints each message followed by a space.

The function loops through each string in the vararg and prints it with a trailing space, so the output is all words separated by spaces on the same line.

🧠 Conceptual

intermediate

Vararg parameter usage in Kotlin

Which statement about vararg parameters in Kotlin is true?

A Vararg parameters must always be the first parameter in the function.

B A function can have multiple vararg parameters.

C Vararg parameters allow passing zero or more arguments of the specified type.

D You cannot pass an array to a vararg parameter.

💡 Hint

Think about how many arguments you can pass to a vararg parameter.

Vararg parameters let you pass zero or more arguments of the specified type. Only one vararg parameter is allowed per function, and it must be the last parameter. You can pass an array using the spread operator.

❓ Predict Output

advanced

Output with vararg and spread operator

What is the output of this Kotlin code snippet?

Kotlin

fun sumAll(vararg numbers: Int): Int {
    var sum = 0
    for (n in numbers) sum += n
    return sum
}

fun main() {
    val nums = intArrayOf(1, 2, 3)
    println(sumAll(*nums, 4))
}

A Compilation error

B 10

C 7

D 6

💡 Hint

The spread operator (*) passes array elements as individual arguments.

The array nums contains 1, 2, 3. Using *nums passes these as separate arguments along with 4, so sumAll sums 1+2+3+4 = 10.

🔧 Debug

advanced

Identify the error with vararg usage

What error does this Kotlin code produce?

Kotlin

fun printNumbers(prefix: String, vararg nums: Int) {
    for (n in nums) print("$prefix$n ")
}

fun main() {
    printNumbers(prefix = "#", 1, 2, 3)
}

A Prints: #1 #2 #3

B Runtime error: prefix not found

C Compilation error: missing argument for prefix

D Compilation error: vararg parameter must be the last parameter

💡 Hint

Check the order of parameters when using vararg.

In Kotlin, the vararg parameter must be the last parameter in the function declaration. Here, prefix comes before vararg, which is correct, so the code compiles and prints the output.

🚀 Application

expert

Count total characters using vararg strings

What is the value of totalChars after running this Kotlin code?

Kotlin

fun countChars(vararg words: String): Int {
    return words.sumOf { it.length }
}

fun main() {
    val totalChars = countChars("Kotlin", "is", "fun")
}

A 11

B 10

C 12

D Compilation error

💡 Hint

Add the lengths of all strings passed as arguments.

The strings are "Kotlin" (6 letters), "is" (2 letters), and "fun" (3 letters). Total is 6 + 2 + 3 = 11.

Practice

1.

What does the vararg keyword do in Kotlin functions?

easy

A. Allows passing a variable number of arguments to a function

B. Declares a function that returns multiple values

C. Defines a function with no parameters

D. Specifies a function that accepts only one argument

Solution

1. Step 1: Understand the purpose of vararg

   The vararg keyword lets a function accept zero or more arguments of the same type without needing an array or list explicitly.

2. Step 2: Compare options with this meaning

   Only Allows passing a variable number of arguments to a function correctly describes this behavior, while others describe unrelated function features.

3. Final Answer:

   Allows passing a variable number of arguments to a function -> Option A

4. Quick Check:

   Vararg = variable arguments [OK]

Hint: Vararg means many arguments, not just one [OK]

Common Mistakes:

• Thinking vararg returns multiple values
• Confusing vararg with no parameters
• Assuming vararg limits to one argument

2.

Which of the following is the correct syntax to declare a function with a vararg parameter in Kotlin?

fun printAll(____ numbers: Int) { for (n in numbers) println(n) }

easy

A. var

B. varargs

C. vararg

D. args

Solution

1. Step 1: Recall the exact keyword for variable arguments

   Kotlin uses vararg (singular) to declare variable number of arguments.

2. Step 2: Check each option

   Only vararg matches the correct keyword; others are incorrect or invalid syntax.

3. Final Answer:

   vararg -> Option C

4. Quick Check:

   Keyword is vararg, not varargs [OK]

Hint: Remember: vararg is singular keyword [OK]

Common Mistakes:

• Using 'varargs' instead of 'vararg'
• Omitting the keyword entirely
• Using 'var' or 'args' which are invalid here

3.

What is the output of this Kotlin code?

fun sumAll(vararg numbers: Int): Int { var sum = 0; for (n in numbers) sum += n; return sum }

fun main() {
  println(sumAll(1, 2, 3))
  println(sumAll())
}

medium

A. Error and error

B. 6 and error

C. Error and 0

D. 6 and 0

Solution

1. Step 1: Understand the function behavior with vararg

   The function sums all integers passed. If no arguments, the loop does not run, sum stays 0.

2. Step 2: Calculate outputs for calls

   sumAll(1,2,3) = 1+2+3 = 6; sumAll() = 0 (no arguments)

3. Final Answer:

   6 and 0 -> Option D

4. Quick Check:

   Sum of inputs or zero if none [OK]

Hint: Vararg can be empty, sum then zero [OK]

Common Mistakes:

• Assuming empty vararg causes error
• Forgetting sum starts at zero
• Confusing vararg with list requiring explicit array

4.

Identify the error in this Kotlin function declaration:

fun printNames(vararg names: String, age: Int) {
  for (name in names) println(name)
  println(age)
}

medium

A. Function cannot have more than one parameter

B. Vararg parameter must be the last parameter

C. Vararg parameter cannot be of type String

D. Vararg parameters must be declared with 'varargs'

Solution

1. Step 1: Recall vararg parameter position rule

   Kotlin requires the vararg parameter to be the last in the parameter list.

2. Step 2: Check the function parameters

   Here, vararg names is before age, which is invalid syntax.

3. Final Answer:

   Vararg parameter must be the last parameter -> Option B

4. Quick Check:

   Vararg last parameter rule [OK]

Hint: Vararg must always be last parameter [OK]

Common Mistakes:

• Placing vararg parameter before others
• Thinking vararg can be anywhere
• Using 'varargs' instead of 'vararg'

5.

Given this function:

fun combine(vararg parts: String): String {
  return parts.joinToString(separator = ", ")
}

Which call correctly passes an existing array val arr = arrayOf("a", "b", "c") to combine?

hard

A. combine(*arr)

B. combine(arr)

C. combine(arr.toList())

D. combine(arr as String)

Solution

1. Step 1: Understand how to pass an array to vararg

   Kotlin requires the spread operator * to pass an array as vararg arguments.

2. Step 2: Evaluate each option

   combine(*arr) uses *arr, correctly spreading the array. combine(arr) passes the array as a single argument, not vararg. combine(arr.toList()) passes a list, not vararg. combine(arr as String) is invalid cast.

3. Final Answer:

   combine(*arr) -> Option A

4. Quick Check:

   Use * to spread array for vararg [OK]

Hint: Use * to spread array into vararg [OK]

Common Mistakes:

• Passing array without * operator
• Passing list instead of array
• Trying invalid casts to String