Collections (List, Map, Set) in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mutable list of strings named fruits.

Android Kotlin

val fruits = mutableListOf[1]

Drag options to blanks, or click blank then click option'

A{"Apple", "Banana"}

B<String>()

C["Apple", "Banana"]

D("Apple", "Banana")

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] which is not valid syntax for list creation in Kotlin.

Using parentheses () without specifying the type.

Use mutableListOf() to create an empty mutable list of strings in Kotlin.

2fill in blank

medium

Complete the code to add the string "Orange" to the fruits list.

Android Kotlin

fruits.[1]("Orange")

Drag options to blanks, or click blank then click option'

Apush

Binsert

Cappend

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using append() which is not a Kotlin list method.

Using push() which is used in other languages like JavaScript.

In Kotlin, use the add() method to add an element to a mutable list.

3fill in blank

hard

Fix the error in the code to create a map with keys as strings and values as integers.

Android Kotlin

val scores = mapOf<String, Int>[1]("Alice" to 10, "Bob" to 8)

Drag options to blanks, or click blank then click option'

A(

B[

C{

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] which causes syntax errors.

Using curly braces {} which are for lambda expressions.

Use parentheses () to pass pairs to mapOf in Kotlin.

4fill in blank

hard

Fill both blanks to create a set of integers and add the number 5 to it.

Android Kotlin

val numbers = mutableSetOf[1](1, 2, 3)
numbers.[2](5)

Drag options to blanks, or click blank then click option'

A<Int>

Badd

Cappend

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Using append() which is not a Kotlin method for sets.

Omitting the type specification which can cause type inference issues.

Use to specify the type and add() to add an element to a mutable set.

5fill in blank

hard

Fill all three blanks to create a map from strings to integers, add a new pair, and retrieve a value.

Android Kotlin

val ages = mutableMapOf[1]("John" to 25, "Jane" to 30)
ages.[2]("Mike", 20)
val age = ages[[3]]

Drag options to blanks, or click blank then click option'

A<String, Int>

Bput

C"Mike"

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using add() instead of put() to add to a map.

Using a variable instead of the string key when retrieving the value.

Use to specify map types, put() to add a pair, and the key "Mike" to retrieve the value.

Practice

1. Which Kotlin collection type should you use to store a list of names where order matters and duplicates are allowed?

easy

A. List

B. Map

C. Set

D. Array

Solution

1. Step 1: Understand collection types

   List keeps items in order and allows duplicates, Map stores key-value pairs, Set stores unique items without duplicates.

2. Step 2: Match requirement to collection

   Since order matters and duplicates are allowed, List fits best.

3. Final Answer:

   List -> Option A

4. Quick Check:

   Order + duplicates = List [OK]

Hint: Remember: List = ordered + duplicates allowed [OK]

Common Mistakes:

• Choosing Set which disallows duplicates
• Choosing Map which stores key-value pairs
• Confusing Array with List in Kotlin

2. Which of the following is the correct syntax to create a mutable map in Kotlin?

easy

A. val map = mutableListOf()

B. val map = mapOf()

C. val map = mutableMapOf()

D. val map = setOf()

Solution

1. Step 1: Identify mutable map syntax

   mutableMapOf creates a mutable map; mapOf creates an immutable map.

2. Step 2: Check other options

   mutableListOf creates a list, setOf creates a set; both are incorrect for map.

3. Final Answer:

   val map = mutableMapOf() -> Option C

4. Quick Check:

   Mutable map uses mutableMapOf [OK]

Hint: Mutable map uses mutableMapOf() function [OK]

Common Mistakes:

• Using mapOf which is immutable
• Using mutableListOf instead of mutableMapOf
• Using setOf which creates a set, not a map

3. What will be the output of this Kotlin code?

val set = setOf(1, 2, 2, 3)
println(set.size)

medium

A. 3

B. 2

C. 4

D. Error

Solution

1. Step 1: Understand set behavior

   Set stores unique elements only, so duplicates are removed.

2. Step 2: Count unique elements

   Elements are 1, 2, 3 so size is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Set removes duplicates, size = 3 [OK]

Hint: Set size counts unique elements only [OK]

Common Mistakes:

• Counting duplicates in set size
• Assuming set keeps duplicates
• Expecting runtime error for duplicates

4. Identify the error in this Kotlin code snippet:

val map = mapOf("a" to 1, "b" to 2)
map["c"] = 3

medium

A. map["c"] = 3 is correct syntax

B. Syntax error in map declaration

C. Key "c" already exists

D. mapOf creates an immutable map, cannot add elements

Solution

1. Step 1: Check mapOf mutability

   mapOf creates an immutable map, so you cannot add or change elements.

2. Step 2: Analyze assignment

   map["c"] = 3 tries to add a new key-value pair, which is not allowed on immutable map.

3. Final Answer:

   mapOf creates an immutable map, cannot add elements -> Option D

4. Quick Check:

   mapOf = immutable map, no additions allowed [OK]

Hint: mapOf is immutable; use mutableMapOf to add items [OK]

Common Mistakes:

• Assuming mapOf is mutable
• Thinking syntax is wrong
• Ignoring immutability of mapOf

5. Given a list of words, how can you create a map that counts how many times each word appears? Choose the correct Kotlin code snippet.

hard

A. val counts = words.filter { it.length > 1 }

B. val counts = words.groupingBy { it }.eachCount()

C. val counts = words.distinct()

D. val counts = words.toSet().map { it.length }

Solution

1. Step 1: Understand the goal

   We want a map with words as keys and their counts as values.

2. Step 2: Analyze options

   val counts = words.groupingBy { it }.eachCount() uses groupingBy and eachCount to count occurrences correctly. Others do not count frequencies.

3. Final Answer:

   val counts = words.groupingBy { it }.eachCount() -> Option B

4. Quick Check:

   groupingBy + eachCount counts items [OK]

Hint: Use groupingBy and eachCount to count list items [OK]

Common Mistakes:

• Using toSet which removes duplicates but doesn't count
• Using map to get lengths instead of counts
• Filtering instead of counting