Sorted and sortedBy in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sorted() function do in Kotlin?

It returns a new list with the elements sorted in natural ascending order without changing the original list.

beginner

How does sortedBy differ from sorted()?

sortedBy sorts elements based on a selector function you provide, allowing custom sorting criteria.

beginner

Example: What is the result of listOf(3, 1, 2).sorted()?

The result is [1, 2, 3], a new list sorted in ascending order.

beginner

Example: How to sort a list of strings by their length using sortedBy?

Use list.sortedBy { it.length } to get a list sorted by string length.

beginner

Does sorted() or sortedBy modify the original list?

No, both return a new sorted list and leave the original list unchanged.

What does sorted() return when called on a list?

A A new list sorted in ascending order

B The original list sorted in place

C A list sorted in descending order

D An error if the list is empty

sorted() returns a new list sorted in ascending order without changing the original.

Which function lets you sort a list by a custom property?

A sorted()

B sort()

C filter()

D sortedBy()

sortedBy() sorts elements using a selector function you provide.

What will listOf("apple", "kiwi", "banana").sortedBy { it.length } return?

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

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

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

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

The list is sorted by string length: "kiwi" (4), "apple" (5), "banana" (6).

Does sorted() change the original list?

A Yes, it sorts the original list

B No, it returns a new sorted list

C Only if the list is mutable

D Only if you assign it back

sorted() always returns a new sorted list and does not modify the original.

Which of these is true about sortedBy?

A It sorts elements by a key you specify

B It only works on numbers

C It sorts elements randomly

D It sorts elements by their natural order

sortedBy sorts elements based on a key selector function you provide.

Explain how sorted() and sortedBy() work in Kotlin and when you would use each.

Think about sorting numbers vs sorting by a property like length or age.

Describe a real-life example where sortedBy is more useful than sorted().

Imagine sorting a list of fruits by how long their names are.

Practice

1. What does the sorted() function do in Kotlin?

easy

A. It sorts the original list in place and returns nothing.

B. It arranges elements randomly in a new list.

C. It returns a new list with elements arranged in natural ascending order.

D. It filters elements based on a condition.

Solution

1. Step 1: Understand the purpose of sorted()

   The sorted() function creates a new list where elements are arranged in natural ascending order (numbers from smallest to largest, strings alphabetically).

2. Step 2: Check if original list changes

   The original list remains unchanged because sorted() returns a new sorted list without modifying the original.

3. Final Answer:

   It returns a new list with elements arranged in natural ascending order. -> Option C

4. Quick Check:

   sorted() = new ascending list [OK]

Hint: sorted() always returns a new ascending list [OK]

Common Mistakes:

• Thinking sorted() changes the original list
• Confusing sorted() with filtering
• Assuming sorted() arranges elements randomly

2. Which of the following is the correct syntax to sort a list of strings by their length using sortedBy in Kotlin?

easy

A. list.sortedBy { it.length }

B. list.sortedBy.length()

C. list.sortedBy(it.length)

D. list.sortedBy { length(it) }

Solution

1. Step 1: Identify correct lambda syntax for sortedBy

   The sortedBy function takes a lambda expression that returns the property to sort by. The correct syntax is list.sortedBy { it.property }.

2. Step 2: Check each option

   list.sortedBy { it.length } uses { it.length } which correctly accesses the length property of each string. Other options have syntax errors or incorrect usage.

3. Final Answer:

   list.sortedBy { it.length } -> Option A

4. Quick Check:

   sortedBy uses lambda with it.property [OK]

Hint: Use curly braces and 'it' to access property in sortedBy [OK]

Common Mistakes:

• Using parentheses instead of curly braces
• Calling length() as a function incorrectly
• Passing property outside lambda

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

val numbers = listOf(5, 3, 8, 1)
val sortedNumbers = numbers.sortedBy { it % 3 }
println(sortedNumbers)

medium

A. [1, 3, 5, 8]

B. [3, 6, 9, 12]

C. [1, 5, 3, 8]

D. [3, 1, 5, 8]

Solution

1. Step 1: Calculate the sorting key for each element

   Calculate it % 3 for each number: 5 % 3 = 2, 3 % 3 = 0, 8 % 3 = 2, 1 % 3 = 1.

2. Step 2: Sort numbers by these keys

   Sort by keys in ascending order: 0 (3), 1 (1), 2 (5), 2 (8). So sorted list is [3, 1, 5, 8].

3. Final Answer:

   [3, 1, 5, 8] -> Option D

4. Quick Check:

   sortedBy { it % 3 } = [3, 1, 5, 8] [OK]

Hint: Calculate sorting key for each element before sorting [OK]

Common Mistakes:

• Sorting by values instead of keys
• Confusing modulo results
• Assuming sortedBy sorts original list

4. Identify the error in this Kotlin code snippet:

val fruits = listOf("apple", "banana", "cherry")
val sortedFruits = fruits.sortedBy(it.length)
println(sortedFruits)

medium

A. sortedBy cannot be used on lists of strings.

B. Missing curly braces around the lambda expression in sortedBy.

C. The list should be mutable to use sortedBy.

D. The println statement is incorrect.

Solution

1. Step 1: Check syntax of sortedBy usage

   The sortedBy function requires a lambda expression inside curly braces, like { it.length }. The code uses parentheses without braces, which is incorrect.

2. Step 2: Verify other options

   sortedBy works on lists of strings, the list can be immutable, and println is correct. So only the lambda syntax is wrong.

3. Final Answer:

   Missing curly braces around the lambda expression in sortedBy. -> Option B

4. Quick Check:

   sortedBy needs { } for lambda [OK]

Hint: Always use curly braces for lambda in sortedBy [OK]

Common Mistakes:

• Using parentheses instead of curly braces for lambda
• Thinking sortedBy needs mutable list
• Misunderstanding println usage

5. Given a list of data class objects data class Person(val name: String, val age: Int), how would you sort the list first by age ascending, then by name alphabetically using Kotlin's sorting functions?

hard

A. persons.sortedWith(compareBy({ it.age }, { it.name }))

B. persons.sortedBy { it.age + it.name }

C. persons.sortedBy { it.name }.sortedBy { it.age }

D. persons.sortedByDescending { it.age }.sortedBy { it.name }

Solution

1. Step 1: Understand multi-level sorting in Kotlin

   To sort by multiple properties, use sortedWith(compareBy(...)) passing lambdas for each property in priority order.

2. Step 2: Analyze options

   persons.sortedWith(compareBy({ it.age }, { it.name })) correctly uses compareBy with age first, then name. persons.sortedBy { it.age + it.name } incorrectly adds age and name (different types). persons.sortedBy { it.name }.sortedBy { it.age } sorts twice but second sort overrides first. persons.sortedByDescending { it.age }.sortedBy { it.name } sorts age descending, not ascending.

3. Final Answer:

   persons.sortedWith(compareBy({ it.age }, { it.name })) -> Option A

4. Quick Check:

   Use sortedWith(compareBy(...)) for multi-level sort [OK]

Hint: Use sortedWith(compareBy(...)) for sorting by multiple keys [OK]

Common Mistakes:

• Adding different types in sortedBy
• Chaining sortedBy loses previous sort
• Using sortedByDescending when ascending needed