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.