Iterating collections with forEach in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print each number in the list using forEach.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
numbers.forEach { [1] -> println([1]) }

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

Anumber

Bitem

Cnum

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a keyword or invalid variable name as the lambda parameter.

Leaving the lambda parameter blank.

The lambda parameter name can be any valid identifier. Here, number is used to represent each element.

2fill in blank

medium

Complete the code to print each fruit in uppercase using forEach.

Kotlin

val fruits = listOf("apple", "banana", "cherry")
fruits.forEach { [1] -> println([1].uppercase()) }

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

Aitem

Bfruits

Cfruit

Df

Attempts:

3 left

💡 Hint

Common Mistakes

Using the plural 'fruits' as the lambda parameter.

Using a variable name not matching the context.

The lambda parameter fruit represents each element in the list fruits.

3fill in blank

hard

Fix the error in the forEach lambda parameter to correctly print each name.

Kotlin

val names = listOf("Anna", "Bob", "Cara")
names.forEach { [1] -> println([1]) }

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

Alist

Bname

Cn

Dnames

Attempts:

3 left

💡 Hint

Common Mistakes

Using the collection name 'names' as the lambda parameter.

Using unrelated variable names.

The lambda parameter should be a single element, so name is correct. Using names (plural) causes confusion and errors.

4fill in blank

hard

Fill both blanks to create a map of word lengths for words longer than 3 characters.

Kotlin

val words = listOf("cat", "house", "tree", "a", "elephant")
val lengths = words.filter { it.length [1] 3 }.associateWith { [2].length }

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

A>

Bword

C<

Dit

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the filter condition.

Using 'word' instead of 'it' inside associateWith.

The filter uses it.length > 3 to select words longer than 3 characters. The associateWith uses it.length to map each word to its length.

5fill in blank

hard

Fill all three blanks to create a map of uppercase keys and their lengths for words starting with 'a'.

Kotlin

val words = listOf("apple", "banana", "apricot", "cherry", "avocado")
val result = words.filter { [1].startsWith("a") }.associateBy([2].uppercase()) { [3].length }

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

Ait

Bword

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using 'it' in associateBy lambdas when a named parameter is expected.

In the filter, it represents each word. In associateBy, word.uppercase() is the key, and word.length is the value.

Practice

1. What does the forEach function do when used on a list in Kotlin?

easy

A. It runs a block of code for each item in the list.

B. It creates a new list with modified items.

C. It removes items from the list.

D. It sorts the list in ascending order.

Solution

1. Step 1: Understand the purpose of forEach

   The forEach function is designed to execute a given block of code on every element in a collection.

2. Step 2: Compare options with forEach behavior

   Only It runs a block of code for each item in the list. correctly describes this behavior. Options B, C, and D describe other functions like map, remove, and sort.

3. Final Answer:

   It runs a block of code for each item in the list. -> Option A

4. Quick Check:

   forEach = run code on each item [OK]

Hint: forEach runs code on every element, not changing the list [OK]

Common Mistakes:

• Confusing forEach with map that creates new lists
• Thinking forEach modifies the list itself
• Assuming forEach sorts or filters the list

2. Which of the following is the correct syntax to print each element of a list numbers using forEach in Kotlin?

easy

A. numbers.forEach -> println(it)

B. forEach(numbers) { println(it) }

C. numbers.forEach { println(it) }

D. numbers.forEach(println(it))

Solution

1. Step 1: Recall the correct forEach syntax

   In Kotlin, forEach is called on the collection with a lambda block: collection.forEach { item -> ... }. The shorthand uses it for the current item.

2. Step 2: Check each option's syntax

   numbers.forEach { println(it) } matches the correct syntax. forEach(numbers) { println(it) } wrongly uses forEach as a function with the list as argument. numbers.forEach(println(it)) incorrectly calls println inside forEach parentheses. numbers.forEach -> println(it) uses an invalid arrow syntax.

3. Final Answer:

   numbers.forEach { println(it) } -> Option C

4. Quick Check:

   Correct forEach syntax = numbers.forEach { println(it) } [OK]

Hint: Use collection.forEach { println(it) } to print items [OK]

Common Mistakes:

• Calling forEach as a standalone function
• Passing println(it) directly inside forEach parentheses
• Using arrow (->) outside lambda block

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

val fruits = listOf("apple", "banana", "cherry")
fruits.forEach { println(it.length) }

medium

A. 5 6 6

B. 5
6
6

C. apple banana cherry

D. 3 6 6

Solution

1. Step 1: Understand the code behavior

   The list fruits contains three strings: "apple", "banana", and "cherry". The forEach prints the length of each string.

2. Step 2: Calculate lengths and output format

   "apple" has length 5, "banana" has length 6, "cherry" has length 6. Each println prints on a new line, so output is: 5 6 6

3. Final Answer:

   5 6 6 -> Option B

4. Quick Check:

   Print lengths with println = lines of 5,6,6 [OK]

Hint: println prints each item on a new line [OK]

Common Mistakes:

• Assuming output is on one line separated by spaces
• Confusing string length with number of vowels
• Printing the strings instead of their lengths

4. Identify the error in this Kotlin code snippet:

val nums = listOf(1, 2, 3)
nums.forEach(i -> println(i))

medium

A. Calling println inside forEach is not allowed

B. Missing parentheses after forEach

C. Using val instead of var for list

D. Using arrow (->) instead of curly braces for lambda

Solution

1. Step 1: Check lambda syntax in Kotlin

   Kotlin uses curly braces {} for lambdas, not arrow notation like -> outside braces. The arrow is used inside the lambda to separate parameters from body.

2. Step 2: Identify the incorrect usage

   The code uses i -> println(i) directly after forEach without braces, which is invalid syntax. Correct form is nums.forEach { i -> println(i) }.

3. Final Answer:

   Using arrow (->) instead of curly braces for lambda -> Option D

4. Quick Check:

   Lambda needs braces {} around arrow -> [OK]

Hint: Use { param -> code } for lambdas, not (param -> code) [OK]

Common Mistakes:

• Using arrow notation without braces
• Confusing Kotlin lambda syntax with other languages
• Thinking val vs var affects forEach usage

5. Given a list of integers nums, how can you use forEach to print only the even numbers?

hard

A. nums.forEach { if (it % 2 == 0) println(it) }

B. nums.forEach { println(it % 2 == 0) }

C. nums.forEach { if (it / 2 == 0) println(it) }

D. nums.forEach { println(it) if (it % 2 == 0) }

Solution

1. Step 1: Understand the condition for even numbers

   An even number has zero remainder when divided by 2, so it % 2 == 0 checks if a number is even.

2. Step 2: Use if inside forEach to print only even numbers

   nums.forEach { if (it % 2 == 0) println(it) } correctly uses an if statement inside the lambda to print only when the condition is true. Other options either print booleans or have incorrect syntax.

3. Final Answer:

   nums.forEach { if (it % 2 == 0) println(it) } -> Option A

4. Quick Check:

   Use if inside forEach to filter printing [OK]

Hint: Use if (it % 2 == 0) inside forEach to print evens [OK]

Common Mistakes:

• Printing boolean results instead of numbers
• Using division instead of modulo for even check
• Placing if after println causing syntax error