Collection size and emptiness checks in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the list is empty.

Kotlin

if (myList.[1]()) {
    println("List is empty")
}

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

AisEmpty

Bcount

Csize

DisNotEmpty

Attempts:

3 left

💡 Hint

Common Mistakes

Using size == 0 instead of isEmpty()

Using isNotEmpty() which checks the opposite

The isEmpty() function returns true if the collection has no elements.

2fill in blank

medium

Complete the code to check if the list has exactly 3 elements.

Kotlin

if (myList.[1] == 3) {
    println("List has 3 elements")
}

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

Acapacity

Bcount

Clength

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using length which is for arrays or strings

Using capacity which is unrelated

The size property gives the number of elements in the collection.

3fill in blank

hard

Fix the error in the code to check if the list is not empty.

Kotlin

if (myList.[1]()) {
    println("List has elements")
}

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

AhasElements

BisNotEmpty

CnotEmpty

DisEmpty

Attempts:

3 left

💡 Hint

Common Mistakes

Using isEmpty() which checks the opposite

Using non-existent functions like notEmpty or hasElements

The isNotEmpty() function returns true if the collection has one or more elements.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, but only for words longer than 3 characters.

Kotlin

val lengths = words.filter { word -> word.length > 3 }.associate { word -> [1] to [2] }

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

Aword

Bword.length

Cword.size

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using word.size which is not a valid property for String

Using 'length' as a variable instead of word.length

We map each word to its length property for words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a filtered list of words with length greater than 4, then map to their uppercase forms.

Kotlin

val result = words.filter { it.[1] > [2] }.map { it.[3]() }

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

Alength

B4

Cuppercase

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size instead of length for string length

Using uppercase without parentheses

We filter words where length is greater than 4, then map each word to its uppercase form using uppercase().

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()