Collection size and emptiness checks in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Check if a list is empty using Kotlin

Given the Kotlin list val fruits = listOf("apple", "banana", "cherry"), what is the output of fruits.isEmpty()?

Kotlin

val fruits = listOf("apple", "banana", "cherry")
println(fruits.isEmpty())

A false

B true

C null

D Compilation error

💡 Hint

Think about whether the list has any items inside.

The isEmpty() function returns true if the list has no elements. Since fruits has three items, it returns false.

❓ query_result

intermediate

Count elements in a Kotlin set

What is the output of the following Kotlin code?

val numbers = setOf(1, 2, 3, 4, 5)
println(numbers.size)

Kotlin

val numbers = setOf(1, 2, 3, 4, 5)
println(numbers.size)

A 0

B 4

C Compilation error

D 5

💡 Hint

Count how many unique numbers are in the set.

The size property returns the number of elements in the set. Here, there are 5 unique numbers, so the output is 5.

📝 Syntax

advanced

Identify the correct way to check if a Kotlin map is empty

Which of the following Kotlin expressions correctly checks if the map val map = mapOf("a" to 1, "b" to 2) is empty?

Kotlin

val map = mapOf("a" to 1, "b" to 2)

A map.isEmpty()

B map.empty()

C map.size == null

D map.isEmpty

💡 Hint

Look for the standard Kotlin function to check emptiness.

The correct function to check if a collection is empty in Kotlin is isEmpty(). The other options are either invalid or incorrect syntax.

❓ query_result

advanced

Result of filtering a list and checking size

What is the output of this Kotlin code?

val nums = listOf(1, 2, 3, 4, 5)
val filtered = nums.filter { it > 3 }
println(filtered.size)

Kotlin

val nums = listOf(1, 2, 3, 4, 5)
val filtered = nums.filter { it > 3 }
println(filtered.size)

A 3

B 2

C 5

D 0

💡 Hint

Count how many numbers are greater than 3.

The filter keeps numbers greater than 3: 4 and 5. So the filtered list has 2 elements, and filtered.size is 2.

🧠 Conceptual

expert

Understanding Kotlin collection emptiness and size properties

Consider a Kotlin collection val col = listOf(). Which statement is true about col.isEmpty() and col.size?

A <code>col.isEmpty()</code> returns false and <code>col.size</code> returns 0

B <code>col.isEmpty()</code> returns true and <code>col.size</code> returns null

C <code>col.isEmpty()</code> returns true and <code>col.size</code> returns 0

D <code>col.isEmpty()</code> returns false and <code>col.size</code> returns null

💡 Hint

Think about what an empty list means for size and emptiness.

An empty list has no elements, so isEmpty() returns true. The size property returns the number of elements, which is 0 for an empty list.

Practice

1. Which Kotlin function checks if a collection has no elements?

easy

A. count()

B. isNotEmpty()

C. size()

D. isEmpty()

Solution

1. Step 1: Understand the purpose of isEmpty()

   This function returns true if the collection has zero elements.

2. Step 2: Compare with other functions

   isNotEmpty() checks for one or more elements, count() and size return the number of elements but do not return a boolean.

3. Final Answer:

   isEmpty() -> Option D

4. Quick Check:

   Check empty collection with isEmpty() = true [OK]

Hint: Use isEmpty() to check for no elements [OK]

Common Mistakes:

• Confusing isEmpty() with isNotEmpty()
• Using count() to check emptiness
• Calling size() as a function instead of property

2. Which of the following is the correct syntax to check if a list named items is not empty in Kotlin?

easy

A. if (items.isNotEmpty()) { ... }

B. if (items.count > 0) { ... }

C. if (items.size() > 0) { ... }

D. if (items.isEmpty) { ... }

Solution

1. Step 1: Identify correct function call syntax

   In Kotlin, isNotEmpty() is a function and requires parentheses.

2. Step 2: Check other options for syntax errors

   isEmpty without parentheses is invalid, size is a property, not a function, so size() is wrong, and count is a function, so count > 0 is invalid.

3. Final Answer:

   if (items.isNotEmpty()) { ... } -> Option A

4. Quick Check:

   Use isNotEmpty() with parentheses [OK]

Hint: Remember isNotEmpty() needs parentheses [OK]

Common Mistakes:

• Using property syntax without parentheses for functions
• Calling size as a function
• Using count without parentheses

3. What is the output of this Kotlin code?

val numbers = listOf(1, 2, 3, 4)
println(numbers.size)
println(numbers.isEmpty())
println(numbers.isNotEmpty())

medium

A. 4
true
false

B. 3
true
false

C. 4
false
true

D. Error: size is not a property

Solution

1. Step 1: Check the size of the list

   The list has 4 elements, so numbers.size returns 4.

2. Step 2: Evaluate emptiness checks

   numbers.isEmpty() returns false because the list is not empty.
   numbers.isNotEmpty() returns true because the list has elements.

3. Final Answer:

   4 false true -> Option C

4. Quick Check:

   Size = 4, empty? false, not empty? true [OK]

Hint: Remember size counts elements; isEmpty() returns false if not empty [OK]

Common Mistakes:

• Thinking size is zero-based index
• Confusing isEmpty() and isNotEmpty() results
• Assuming size is a function, causing error

4. Identify the error in this Kotlin code snippet:

val data = listOf<String>()
if (data.isEmpty) {
    println("No data")
}

medium

A. Missing parentheses after isEmpty

B. listOf<String>() cannot be empty

C. println syntax is incorrect

D. data should be mutable

Solution

1. Step 1: Check usage of isEmpty

   isEmpty is a function and must be called with parentheses: isEmpty().

2. Step 2: Verify other parts of the code

   listOf<String>() creates an empty list correctly, println syntax is correct, and mutability is not required for checking emptiness.

3. Final Answer:

   Missing parentheses after isEmpty -> Option A

4. Quick Check:

   Functions need parentheses even if no arguments [OK]

Hint: Always add () when calling functions like isEmpty() [OK]

Common Mistakes:

• Using property syntax for functions
• Thinking empty list creation is invalid
• Assuming mutability is needed for checks

5. You have a list of user names and want to print how many users are online only if the list is not empty. Which Kotlin code snippet correctly does this?

hard

A. if (users.isEmpty()) println("Online users: ${users.size}")

B. if (users.size > 0) println("Online users: ${users.size}")

C. if (users.count == 0) println("Online users: ${users.count}")

D. if (users.isNotEmpty) println("Online users: ${users.size}")

Solution

1. Step 1: Check condition for non-empty list

   Using users.size > 0 correctly checks if the list has one or more elements.

2. Step 2: Verify printing statement

   The print statement uses string interpolation to show the number of users online.

3. Step 3: Analyze incorrect options

   isEmpty() checks for empty, so if (users.isEmpty()) println("Online users: ${users.size}") prints when empty, which is wrong.
   count is a function, so count == 0 is invalid.
   isNotEmpty without parentheses is incorrect syntax.

4. Final Answer:

   if (users.size > 0) println("Online users: ${users.size}") -> Option B

5. Quick Check:

   Check size > 0 for non-empty list [OK]

Hint: Use size > 0 or isNotEmpty() with parentheses [OK]

Common Mistakes:

• Using isEmpty() to check for non-empty
• Calling count without parentheses
• Omitting parentheses on isNotEmpty()