Filter and filterNot in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the filter function do in Kotlin?

The filter function returns a list containing only the elements that satisfy a given condition (predicate).

beginner

What is the difference between filter and filterNot in Kotlin?

filter keeps elements that match the condition, while filterNot keeps elements that do NOT match the condition.

beginner

How would you use filterNot to remove all even numbers from a list?

Use filterNot { it % 2 == 0 } to keep only odd numbers by removing even numbers.

beginner

Given val numbers = listOf(1, 2, 3, 4, 5), what is the result of numbers.filter { it > 3 }?

The result is [4, 5] because only numbers greater than 3 are kept.

intermediate

Can filter and filterNot be used with any Kotlin collection?

Yes, both functions can be used with any Kotlin collection like lists, sets, and sequences.

What does filterNot { it % 2 == 0 } do to a list of integers?

A Keeps only odd numbers

B Keeps only even numbers

C Removes odd numbers

D Removes all numbers

It removes elements where the condition is true (even numbers), so only odd numbers remain.

Which function keeps elements that satisfy the condition?

A filterNot

B map

C reduce

D filter

filter keeps elements matching the condition.

What will listOf(1,2,3).filter { it > 5 } return?

A [] (empty list)

B Error

C [5]

D [1, 2, 3]

No elements are greater than 5, so the result is an empty list.

If you want to remove elements that are less than 10, which function and condition would you use?

A filter { it < 10 }

B filterNot { it < 10 }

C filter { it > 10 }

D filterNot { it > 10 }

filterNot removes elements where the condition is true, so it removes elements less than 10.

Are filter and filterNot functions destructive (do they change the original list)?

A Yes, they modify the original list

B Only filter modifies the list

C No, they return a new list

D Only filterNot modifies the list

Both return new lists and do not change the original collection.

Explain how filter and filterNot work in Kotlin with an example.

Think about keeping or removing elements based on a condition.

Describe a real-life situation where you might use filter and filterNot in a Kotlin program.

Imagine sorting or selecting items from a list based on a rule.

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