Vararg parameters in Kotlin - Mini Project: Build & Apply

Using Vararg Parameters in Kotlin

📖 Scenario: You are building a simple Kotlin program to handle a list of exam scores for students. Sometimes, you want to pass multiple scores at once to calculate their average.

🎯 Goal: Create a Kotlin function that uses vararg parameters to accept multiple exam scores and calculate their average.

📋 What You'll Learn

Create a function named averageScore that accepts multiple Int scores using vararg.

Inside the function, calculate the average of all scores passed.

Call the function with exactly these scores: 70, 85, 90, 75.

Print the average score as a Double.

💡 Why This Matters

🌍 Real World

Vararg parameters let you write flexible functions that can take any number of inputs, like scores, names, or prices, without needing to create arrays manually.

💼 Career

Understanding vararg parameters is useful for Kotlin developers working on apps that process lists of data dynamically, such as user inputs, sensor readings, or batch operations.

1

Create a function with vararg parameter

Write a Kotlin function named averageScore that takes a vararg parameter called scores of type Int. The function should return a Double. For now, just return 0.0 inside the function.

Kotlin

fun averageScore(vararg scores: Int): Double {
    // Your code here
    return 0.0
}

Hint

Use vararg before the parameter name to accept multiple values.

2

Calculate the sum of all scores

Inside the averageScore function, create a variable named sum that adds all values in scores using a for loop.

Kotlin

fun averageScore(vararg scores: Int): Double {
    var sum = 0
    // Your code here to add all scores to sum
    return 0.0
}

Hint

Use a for loop to add each score to sum.

3

Calculate and return the average

Modify the averageScore function to return the average of all scores as a Double. Use sum and the size of scores.

Kotlin

fun averageScore(vararg scores: Int): Double {
    var sum = 0
    for (score in scores) {
        sum += score
    }
    // Your code here to calculate and return average
    return 0.0
}

Hint

Convert sum to Double before dividing by scores.size.

4

Call the function and print the result

Call the averageScore function with the scores 70, 85, 90, and 75. Print the returned average.

Kotlin

fun averageScore(vararg scores: Int): Double {
    var sum = 0
    for (score in scores) {
        sum += score
    }
    return sum.toDouble() / scores.size
}

// Your code here to call averageScore and print the result

Hint

Use println to show the average returned by averageScore.

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