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.