Fold and reduce operations in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this fold operation?

Consider the following Kotlin code using fold. What will be printed?

Kotlin

val numbers = listOf(1, 2, 3, 4)
val result = numbers.fold(10) { acc, num -> acc - num }
println(result)

A 14

B 0

C 4

D -10

💡 Hint

Remember that fold starts with the initial value and applies the operation from left to right.

The fold starts with 10. Then 10 - 1 = 9, 9 - 2 = 7, 7 - 3 = 4, 4 - 4 = 0. The output is 0.

❓ Predict Output

intermediate

What is the output of this reduce operation?

Look at this Kotlin code using reduce. What will it print?

Kotlin

val numbers = listOf(2, 3, 4)
val result = numbers.reduce { acc, num -> acc * num }
println(result)

A 24

B 9

C 20

D 14

💡 Hint

Reduce starts with the first element and multiplies all elements.

Reduce multiplies 2 * 3 = 6, then 6 * 4 = 24. So the output is 24.

❓ Predict Output

advanced

What error does this reduce call cause?

What happens when you run this Kotlin code?

Kotlin

val emptyList = emptyList<Int>()
val result = emptyList.reduce { acc, num -> acc + num }
println(result)

A No output, prints 0

B Prints null

C Throws NullPointerException

D Throws UnsupportedOperationException

💡 Hint

Reduce requires at least one element in the list.

Calling reduce on an empty list throws UnsupportedOperationException because there is no initial element to start with.

❓ Predict Output

advanced

What is the output of this foldRight operation?

What will this Kotlin code print?

Kotlin

val letters = listOf("a", "b", "c")
val result = letters.foldRight("") { letter, acc -> acc + letter }
println(result)

A ""

B "abc"

C "cba"

D "cab"

💡 Hint

foldRight starts from the right end of the list.

foldRight applies the operation starting from the right: acc starts as "". Then acc + "c" = "c", then "c" + "b" = "cb", then "cb" + "a" = "cba".

🧠 Conceptual

expert

How many elements are in the resulting list after this fold?

Given this Kotlin code, how many elements does result contain?

Kotlin

val numbers = listOf(1, 2, 3)
val result = numbers.fold(mutableListOf<Int>()) { acc, num ->
    acc.add(num * 2)
    acc
}
println(result.size)

A 3

B 0

C 1

D Throws an exception

💡 Hint

fold can accumulate results in a mutable collection.

The fold starts with an empty mutable list. For each number, it adds the doubled value. After processing 3 numbers, the list has 3 elements.