Iterating collections with forEach in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using forEach?

Consider the following Kotlin code snippet:

val numbers = listOf(1, 2, 3)
numbers.forEach { print(it * 2) }

What will be printed when this code runs?

Kotlin

val numbers = listOf(1, 2, 3)
numbers.forEach { print(it * 2) }

A 246

B 123

C 6

D Error: forEach cannot be used on List

💡 Hint

Remember that forEach applies the given action to each element in the list.

The forEach function runs the lambda for each element. Here, it prints each number multiplied by 2 without spaces, so the output is 246.

🧠 Conceptual

intermediate

Which statement about Kotlin's forEach is true?

Choose the correct statement about the forEach function in Kotlin collections.

A <code>forEach</code> can be used to modify the elements of a mutable list directly.

B <code>forEach</code> is a terminal operation that applies a given action to each element.

C <code>forEach</code> returns a new list with transformed elements.

D <code>forEach</code> can only be used with arrays, not lists.

💡 Hint

Think about what forEach does with the elements and its return type.

forEach applies the given action to each element and returns Unit. It does not transform or return a new collection.

🔧 Debug

advanced

What error does this Kotlin code raise?

Examine the code below:

val items = listOf("a", "b", "c")
items.forEach { it = it.uppercase() }

What error will this code produce?

Kotlin

val items = listOf("a", "b", "c")
items.forEach { it = it.uppercase() }

A TypeError: Cannot assign to 'it' because it is a val

B SyntaxError: Missing lambda arrow

C NullPointerException

D No error, prints uppercase letters

💡 Hint

Consider whether the lambda parameter it can be reassigned.

The lambda parameter it is read-only (val). Trying to assign to it causes a compilation error.

❓ Predict Output

advanced

What is the output of this Kotlin code using forEachIndexed?

Consider this Kotlin code:

val list = listOf("x", "y", "z")
list.forEachIndexed { index, value -> print("$index$value") }

What will be printed?

Kotlin

val list = listOf("x", "y", "z")
list.forEachIndexed { index, value -> print("$index$value") }

A 012

B xyz

C Error: forEachIndexed not found

D 0x1y2z

💡 Hint

Remember forEachIndexed provides both index and element.

The code prints the index followed by the element for each item, resulting in 0x1y2z.

📝 Syntax

expert

Which option causes a compilation error in Kotlin forEach usage?

Which of the following Kotlin code snippets will cause a compilation error?

A

val nums = listOf(1, 2, 3)
nums.forEach { println(it + 1) }

B

val nums = listOf(1, 2, 3)
nums.forEach { val it = it + 1; println(it) }

C

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

D

val nums = listOf(1, 2, 3)
nums.forEach { number -> println(number) }

💡 Hint

Check the lambda syntax in Kotlin carefully.

Kotlin does not use the arrow syntax it -> for lambdas. The correct syntax uses curly braces with parameters before the arrow or implicit it.

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