GroupBy for categorization in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the groupBy function do in Kotlin?

It groups elements of a collection into a map, where each key is a category and the value is a list of elements belonging to that category.

beginner

How do you specify the category for grouping when using groupBy?

You provide a lambda function that returns the key for each element, which determines the category it belongs to.

beginner

Example: What is the result of listOf(1, 2, 3, 4).groupBy { it % 2 }?

A map grouping numbers by even or odd: {0=[2, 4], 1=[1, 3]} where 0 means even and 1 means odd.

intermediate

Can groupBy be used with custom objects?

Yes, you can group custom objects by any property or computed value by providing a lambda that returns the grouping key.

intermediate

What type does groupBy return?

It returns a Map<K, List<T>> where K is the key type from the lambda and T is the element type.

What is the output type of groupBy in Kotlin?

A Map<Key, List<Element>>

B List<Key>

C Set<Element>

D Array<Element>

groupBy returns a map where each key maps to a list of elements in that group.

Which lambda expression groups a list of strings by their first letter?

A list.groupBy { it.first() }

B list.groupBy { it.length }

C list.groupBy { it.toUpperCase() }

D list.groupBy { it.last() }

Grouping by the first letter uses it.first() in the lambda.

If you want to group numbers by whether they are positive or negative, which key selector is best?

A groupBy { it % 2 }

B groupBy { it == 0 }

C groupBy { it.toString() }

D groupBy { it > 0 }

it > 0 returns true or false, grouping numbers by positive or not.

Can groupBy be used on an empty list?

A No, it throws an exception.

B Yes, it returns an empty map.

C Yes, it returns null.

D No, it returns an empty list.

Grouping an empty list returns an empty map without error.

What happens if multiple elements have the same key in groupBy?

A An error is thrown.

B Only the first element is kept.

C They are collected into the list for that key.

D Only the last element is kept.

All elements with the same key are grouped together in a list.

Explain how the groupBy function works in Kotlin and give a simple example.

Think about sorting items into labeled boxes based on a rule.

Describe a real-life scenario where you could use groupBy to organize data.

Imagine sorting your friends by their favorite color.

Practice

1. What does the groupBy function do in Kotlin?

easy

A. It sorts a list in ascending order.

B. It organizes a list into a map based on a key selector function.

C. It removes duplicate elements from a list.

D. It converts a list into a set.

Solution

1. Step 1: Understand the purpose of groupBy

   The groupBy function takes a list and groups its elements by a key, returning a map.

2. Step 2: Compare with other functions

   Sorting, removing duplicates, and converting to set do different things than grouping by a key.

3. Final Answer:

   It organizes a list into a map based on a key selector function. -> Option B

4. Quick Check:

   groupBy = organize by key [OK]

Hint: Remember: groupBy creates categories as map keys [OK]

Common Mistakes:

• Confusing groupBy with sorting functions
• Thinking groupBy removes duplicates
• Mixing groupBy with set conversion

2. Which of the following is the correct syntax to group a list of strings by their first character in Kotlin?

easy

A. val result = list.groupBy { it.first() }

B. val result = list.groupBy(it -> it.first())

C. val result = list.groupBy { it.first }

D. val result = list.groupBy { first(it) }

Solution

1. Step 1: Review Kotlin lambda syntax

   Kotlin uses curly braces with it for single-parameter lambdas: { it.first() }.

2. Step 2: Check each option

   val result = list.groupBy { it.first() } uses correct Kotlin syntax. val result = list.groupBy(it -> it.first()) uses arrow syntax which is invalid here. val result = list.groupBy { it.first } omits parentheses on first() method call, causing compile error. val result = list.groupBy { first(it) } calls undefined function first().

3. Final Answer:

   val result = list.groupBy { it.first() } -> Option A

4. Quick Check:

   Use curly braces and it.first() [OK]

Hint: Use curly braces and 'it' for lambda in groupBy [OK]

Common Mistakes:

• Using arrow syntax inside groupBy lambda
• Calling undefined functions like first(it)
• Omitting parentheses on first() method call

3. What is the output of the following Kotlin code?

val numbers = listOf(1, 2, 3, 4, 5, 6)
val grouped = numbers.groupBy { if (it % 2 == 0) "even" else "odd" }
println(grouped)

medium

A. {odd=[1, 3, 5], even=[2, 4, 6]}

B. {1=[1], 2=[2], 3=[3], 4=[4], 5=[5], 6=[6]}

C. {true=[2, 4, 6], false=[1, 3, 5]}

D. SyntaxError

Solution

1. Step 1: Understand the grouping condition

   The lambda groups numbers by "even" if divisible by 2, else "odd".

2. Step 2: Group numbers accordingly

   Numbers 2,4,6 go to "even"; 1,3,5 go to "odd".

3. Final Answer:

   {odd=[1, 3, 5], even=[2, 4, 6]} -> Option A

4. Quick Check:

   Group by even/odd labels [OK]

Hint: Check grouping keys carefully for expected output [OK]

Common Mistakes:

• Confusing keys with values in output
• Expecting boolean keys instead of strings
• Thinking groupBy sorts the list

4. Identify the error in this Kotlin code snippet:

val words = listOf("apple", "banana", "apricot")
val grouped = words.groupBy { it[0] }
println(grouped)

medium

A. Indexing with it[0] is invalid for strings

B. Must convert characters to strings before grouping

C. groupBy requires a function, not a lambda

D. No error; code runs and groups by first character

Solution

1. Step 1: Check indexing on strings

   In Kotlin, strings support indexing with [0] to get first character.

2. Step 2: Validate groupBy usage

   The lambda { it[0] } is valid and groups words by their first character.

3. Final Answer:

   No error; code runs and groups by first character -> Option D

4. Quick Check:

   Strings support indexing; groupBy accepts lambdas [OK]

Hint: Remember strings can be indexed like arrays in Kotlin [OK]

Common Mistakes:

• Thinking indexing strings causes error
• Believing groupBy needs named functions only
• Assuming characters must be converted to strings

5. Given a list of people represented as pairs Pair<String, Int> where the first element is the name and the second is the age, how would you group them by age category "minor" (age < 18) or "adult" (age >= 18) using groupBy?

hard

A. people.groupBy { it.first }

B. people.groupBy { it.first.length }

C. people.groupBy { if (it.second < 18) "minor" else "adult" }

D. people.groupBy { it.second }

Solution

1. Step 1: Identify grouping key based on age

   We want to group by "minor" if age < 18, else "adult".

2. Step 2: Use groupBy with conditional key

   Access age with it.second and apply if-else to return category string.

3. Final Answer:

   people.groupBy { if (it.second < 18) "minor" else "adult" } -> Option C

4. Quick Check:

   Group by age category using conditional [OK]

Hint: Use if-else inside groupBy lambda for categories [OK]

Common Mistakes:

• Grouping by name or name length instead of age
• Grouping by exact age instead of categories
• Confusing first and second in Pair