Filter and filterNot in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter even numbers from the list.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.filter { it [1] 2 == 0 }
println(evens)

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

A%

B*

C/

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using division (/) instead of modulo (%) causes wrong filtering.

Using multiplication (*) or addition (+) operators here is incorrect.

The filter function keeps elements where the condition is true. Using % 2 == 0 selects even numbers.

2fill in blank

medium

Complete the code to filter out odd numbers using filterNot.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.filterNot { it [1] 2 != 0 }
println(evens)

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

A-

B*

C+

D%

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication or addition operators instead of modulo.

Confusing filter and filterNot logic.

The filterNot function excludes elements where the condition is true. Using % 2 != 0 excludes odd numbers, keeping evens.

3fill in blank

hard

Fix the error in the code to filter words longer than 3 characters.

Kotlin

val words = listOf("cat", "dog", "elephant", "fox")
val longWords = words.filter { it.length [1] 3 }
println(longWords)

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

A<=

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than (<) instead of greater than (>).

Using equality (==) which only selects words of length exactly 3.

To get words longer than 3 characters, use > operator comparing length to 3.

4fill in blank

hard

Fill both blanks to create a map of word lengths for words shorter than 5 characters.

Kotlin

val words = listOf("apple", "bat", "cat", "dog", "elephant")
val shortWordLengths = words.filter { it.length [1] 5 }.associateWith { it.[2] }
println(shortWordLengths)

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

A<

Blength

C>

DtoString

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than (>) instead of less than (<) in filter.

Using toString instead of length for mapping.

Filter words with length less than 5 using <. Then map each word to its length using length.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths for words not starting with 'a'.

Kotlin

val words = listOf("apple", "bat", "cat", "dog", "ant")
val result = words.filterNot { it.startsWith([1]) }.associateWith { it.[2] }.mapKeys { it.key.[3]() }
println(result)

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

A"a"

Blength

Cuppercase

DtoUpperCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase instead of toUpperCase() function.

Forgetting quotes around "a" in startsWith.

Using wrong property for length.

Filter out words starting with "a" using startsWith("a"). Map values to their length. Change keys to uppercase using toUpperCase().

Practice

1. What does the filter function do in Kotlin collections?

easy

A. Sorts the collection in ascending order

B. Removes all items from the collection

C. Keeps items where the condition is false

D. Keeps items where the condition is true

Solution

1. Step 1: Understand the purpose of filter

   The filter function selects elements that satisfy a given condition (true).

2. Step 2: Compare with other options

   Options B and D describe different operations, and C describes filterNot, not filter.

3. Final Answer:

   Keeps items where the condition is true -> Option D

4. Quick Check:

   filter = keep true items [OK]

Hint: Remember: filter keeps true, filterNot keeps false [OK]

Common Mistakes:

• Confusing filter with filterNot
• Thinking filter removes all items
• Mixing filter with sorting functions

2. Which of the following is the correct syntax to filter even numbers from a list numbers in Kotlin?

easy

A. numbers.filter { it / 2 == 0 }

B. numbers.filterNot { it % 2 == 0 }

C. numbers.filter { it % 2 == 0 }

D. numbers.filter { it % 2 != 0 }

Solution

1. Step 1: Identify the condition for even numbers

   Even numbers satisfy it % 2 == 0.

2. Step 2: Choose correct filter syntax

   filter keeps items where condition is true, so numbers.filter { it % 2 == 0 } keeps even numbers.

3. Final Answer:

   numbers.filter { it % 2 == 0 } -> Option C

4. Quick Check:

   filter with even condition = numbers.filter { it % 2 == 0 } [OK]

Hint: Use filter with condition true to keep desired items [OK]

Common Mistakes:

• Using filterNot instead of filter
• Using division instead of modulo
• Confusing condition signs (== vs !=)

3. What is the output of this Kotlin code?

val nums = listOf(1, 2, 3, 4, 5)
val result = nums.filterNot { it > 3 }
println(result)

medium

A. [1, 2, 3]

B. [4, 5]

C. [1, 2, 3, 4, 5]

D. []

Solution

1. Step 1: Understand filterNot condition

   filterNot { it > 3 } keeps items where it > 3 is false, so items ≤ 3.

2. Step 2: Apply condition to list

   Numbers 1, 2, 3 are ≤ 3, so they are kept; 4 and 5 are removed.

3. Final Answer:

   [1, 2, 3] -> Option A

4. Quick Check:

   filterNot > 3 = keep ≤ 3 [OK]

Hint: filterNot keeps items where condition is false [OK]

Common Mistakes:

• Confusing filterNot with filter
• Including numbers greater than 3
• Expecting empty list output

4. Find the error in this Kotlin code that tries to filter out odd numbers:

val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.filterNot { it % 2 == 0 }
println(evens)

medium

A. The list is empty after filtering

B. It incorrectly keeps odd numbers instead of even

C. filterNot cannot be used with modulo operator

D. Syntax error in lambda expression

Solution

1. Step 1: Analyze the filterNot condition

   filterNot { it % 2 == 0 } keeps items where it % 2 == 0 is false, so it keeps odd numbers.

2. Step 2: Check intended output

   The code tries to get even numbers but actually keeps odd numbers, so logic is reversed.

3. Final Answer:

   It incorrectly keeps odd numbers instead of even -> Option B

4. Quick Check:

   filterNot with even condition keeps odd [OK]

Hint: filterNot reverses condition; use filter to keep true items [OK]

Common Mistakes:

• Using filterNot instead of filter for positive condition
• Assuming filterNot removes items instead of keeping false
• Misreading modulo condition

5. Given a list val words = listOf("apple", "banana", "", "cherry", ""), which Kotlin expression returns a list of only the non-empty words using filter and filterNot correctly?

hard

A. words.filter { it.isNotEmpty() }

B. words.filterNot { it.isNotEmpty() }

C. words.filter { it.isEmpty() }

D. words.filterNot { it.length > 0 }

Solution

1. Step 1: Identify condition for non-empty strings

   Non-empty strings satisfy it.isNotEmpty().

2. Step 2: Choose correct filter usage

   filter keeps items where condition is true, so words.filter { it.isNotEmpty() } returns only non-empty words.

3. Final Answer:

   words.filter { it.isNotEmpty() } -> Option A

4. Quick Check:

   filter with isNotEmpty = keep non-empty [OK]

Hint: Use filter with positive condition to keep desired items [OK]

Common Mistakes:

• Using filterNot with isNotEmpty to keep empty strings
• Confusing isEmpty and isNotEmpty
• Expecting filterNot to remove empty strings