Sorted and sortedBy in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using sorted()?

Consider the following Kotlin code that sorts a list of numbers. What will be printed?

Kotlin

val numbers = listOf(5, 3, 8, 1)
val sortedNumbers = numbers.sorted()
println(sortedNumbers)

A [8, 5, 3, 1]

B [1, 5, 3, 8]

C [5, 3, 8, 1]

D [1, 3, 5, 8]

💡 Hint

The sorted() function returns a new list with elements in ascending order.

The sorted() function returns a new list with the elements sorted in ascending order. So the original list [5, 3, 8, 1] becomes [1, 3, 5, 8].

❓ Predict Output

intermediate

What does sortedBy return for this list of strings?

Given this Kotlin code, what will be the output?

Kotlin

val fruits = listOf("apple", "banana", "pear", "kiwi")
val sortedByLength = fruits.sortedBy { it.length }
println(sortedByLength)

A ["kiwi", "pear", "apple", "banana"]

B ["pear", "kiwi", "apple", "banana"]

C ["banana", "apple", "pear", "kiwi"]

D ["apple", "banana", "pear", "kiwi"]

💡 Hint

sortedBy sorts elements based on the value returned by the lambda.

sortedBy sorts the list by the length of each string in ascending order. Lengths: pear(4), kiwi(4), apple(5), banana(6). Since sortedBy uses a stable sort, pear comes before kiwi. So the sorted list is ["pear", "kiwi", "apple", "banana"].

🔧 Debug

advanced

Why does this Kotlin code cause a compilation error?

Look at this Kotlin code snippet. Why does it cause a compilation error?

Kotlin

val numbers = listOf(1, 2, 3)
val sorted = numbers.sortedBy { it > 2 }
println(sorted)

A sortedBy expects a selector returning a Comparable, but 'it > 2' returns Boolean which is not Comparable

B sortedBy cannot be used on lists of integers

C The lambda syntax is incorrect; it should be 'it -> it > 2'

D sortedBy requires the list to be mutable

💡 Hint

Check the return type of the lambda passed to sortedBy.

The lambda passed to sortedBy must return a value that implements Comparable. The expression 'it > 2' returns a Boolean, which does not implement Comparable, causing a compilation error.

🧠 Conceptual

advanced

How does sorted() differ from sortedBy() in Kotlin?

Which statement best describes the difference between sorted() and sortedBy()?

A sorted() modifies the original list; sortedBy() returns a new list

B sorted() sorts only numbers; sortedBy() sorts only strings

C sorted() sorts elements naturally; sortedBy() sorts elements using a selector function

D sorted() sorts in descending order; sortedBy() sorts in ascending order

💡 Hint

Think about how you tell Kotlin what to sort by.

sorted() sorts elements according to their natural order (like numbers ascending). sortedBy() lets you provide a function to select a value to sort by, useful for complex objects or custom criteria.

❓ Predict Output

expert

What is the output of this Kotlin code using sortedBy with nulls?

Consider this Kotlin code that sorts a list with nullable strings. What will be printed?

Kotlin

val names: List<String?> = listOf("Anna", null, "Bob", "", null)
val sortedNames = names.sortedBy { it?.length ?: -1 }
println(sortedNames)

A [null, null, "", "Bob", "Anna"]

B ["", "Bob", "Anna", null, null]

C [null, "", null, "Bob", "Anna"]

D ["Anna", "Bob", "", null, null]

💡 Hint

Look at how nulls are handled with the Elvis operator ?: in the selector.

The selector returns length of the string or -1 if null. So nulls get -1, which is smallest, so they come first. Then empty string (length 0), then "Bob" (3), then "Anna" (4).