Fold and reduce operations in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the sum of all numbers in the list using fold.

Kotlin

val numbers = listOf(1, 2, 3, 4)
val sum = numbers.fold([1]) { acc, num -> acc + num }
println(sum)

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

A0

B1

Cnull

D""

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as initial value will add 1 extra to the sum.

Using null causes a runtime error.

The fold function starts with an initial value. For summing numbers, the initial value should be 0.

2fill in blank

medium

Complete the code to find the product of all numbers in the list using reduce.

Kotlin

val numbers = listOf(2, 3, 4)
val product = numbers.reduce { acc, num -> acc [1] num }
println(product)

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' will sum instead of multiply.

Using '-' or '/' will give wrong results or errors.

The reduce function combines elements. To get the product, use the multiplication operator '*'.

3fill in blank

hard

Fix the error in the code to correctly concatenate all strings in the list using fold.

Kotlin

val words = listOf("Kotlin", "is", "fun")
val sentence = words.fold([1]) { acc, word -> acc + " " + word }
println(sentence.trim())

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

A1

B""

C0

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null causes a NullPointerException.

Using 0 or 1 causes type mismatch errors.

For string concatenation, the fold initial value should be an empty string "".

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, but only include words longer than 3 characters.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val lengths = words.associateWith { [1] }
val filtered = lengths.filter { it.value [2] 3 }
println(filtered)

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

Ait.length

B>

C<

Dit.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using it.size is invalid for String.

Using '<' filters shorter words, not longer.

Use it.length to get the length of each word. Filter words with length greater than 3 using '>'.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, including only words longer than 4 characters.

Kotlin

val words = listOf("tree", "house", "car", "elephant")
val result = words.filter { it.length [1] 4 }
    .associateBy([2]) { [3] }
println(result)

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

A>

Bit.uppercase()

Cit.length

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' filters shorter words.

Using it.lowercase() instead of uppercase.

Swapping key and value in associateBy.

Filter words with length greater than 4 using '>'. Use it.uppercase() as the key and it.length as the value in associateBy.

Practice

1. What is the main difference between fold and reduce in Kotlin?

easy

A. fold starts with an initial value; reduce starts with the first list item.

B. fold only works on numbers; reduce works on any type.

C. fold modifies the original list; reduce creates a new list.

D. fold is faster than reduce in all cases.

Solution

1. Step 1: Understand fold behavior

   fold requires an initial value and then applies the operation starting from that value.

2. Step 2: Understand reduce behavior

   reduce uses the first element of the list as the initial value and then applies the operation to the rest.

3. Final Answer:

   fold starts with an initial value; reduce starts with the first list item. -> Option A

4. Quick Check:

   fold initial value vs reduce first item = C [OK]

Hint: Remember: fold needs your start, reduce uses first item [OK]

Common Mistakes:

• Thinking fold modifies the original list
• Assuming reduce needs an initial value
• Believing fold only works on numbers

2. Which of the following is the correct syntax to use fold on a list listOf(1, 2, 3) to sum all elements starting from 10?

easy

A. listOf(1, 2, 3).fold(10, (acc, item) -> acc + item)

B. listOf(1, 2, 3).fold { acc, item -> acc + item }

C. listOf(1, 2, 3).fold(10) { acc, item -> acc + item }

D. listOf(1, 2, 3).fold(10, acc + item)

Solution

1. Step 1: Check fold function signature

   fold takes an initial value and a lambda with two parameters: accumulator and current item.

2. Step 2: Match syntax with options

   listOf(1, 2, 3).fold(10) { acc, item -> acc + item } correctly uses fold(10) { acc, item -> acc + item }. Others miss initial value or have wrong lambda syntax.

3. Final Answer:

   listOf(1, 2, 3).fold(10) { acc, item -> acc + item } -> Option C

4. Quick Check:

   fold(initial) with lambda = D [OK]

Hint: fold needs initial value and lambda with two params [OK]

Common Mistakes:

• Omitting the initial value in fold
• Using incorrect lambda syntax
• Passing expression instead of lambda

3. What is the output of this Kotlin code?

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

medium

A. Error

B. 9

C. 14

D. 24

Solution

1. Step 1: Understand reduce operation

   reduce starts with first element (2) as accumulator, then multiplies by next items.

2. Step 2: Calculate step-by-step

   2 * 3 = 6, then 6 * 4 = 24.

3. Final Answer:

   24 -> Option D

4. Quick Check:

   2 * 3 * 4 = 24 [OK]

Hint: Multiply stepwise starting from first item [OK]

Common Mistakes:

• Adding instead of multiplying
• Starting from zero instead of first item
• Expecting a list output

4. Identify the error in this Kotlin code using reduce:

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

medium

A. Throws exception because list is empty

B. No error, output is 0

C. Syntax error in lambda

D. Returns null

Solution

1. Step 1: Check reduce behavior on empty list

   reduce requires at least one element; empty list causes runtime exception.

2. Step 2: Understand exception cause

   Since list is empty, no first element exists to start reduction, so it throws NoSuchElementException.

3. Final Answer:

   Throws exception because list is empty -> Option A

4. Quick Check:

   reduce on empty list = Exception [OK]

Hint: reduce fails on empty lists, fold needs initial value [OK]

Common Mistakes:

• Assuming reduce returns 0 on empty list
• Thinking lambda syntax is wrong
• Expecting null instead of exception

5. Given a list of strings listOf("a", "b", "c"), which Kotlin expression using fold produces the string "abc-"?

hard

A. listOf("a", "b", "c").fold("-") { acc, s -> acc + s }

B. listOf("a", "b", "c").fold("") { acc, s -> acc + s + "-" }

C. listOf("a", "b", "c").reduce { acc, s -> acc + s + "-" }

D. listOf("a", "b", "c").fold("") { acc, s -> s + acc + "-" }

Solution

1. Step 1: Understand fold with initial empty string

   Starting with empty string, each item is appended with a dash after it.

2. Step 2: Check each step of fold

   "" + "a" + "-" = "a-"; then "a-" + "b" + "-" = "a-b-"; then "a-b-" + "c" + "-" = "abc-".

3. Final Answer:

   listOf("a", "b", "c").fold("") { acc, s -> acc + s + "-" } -> Option B

4. Quick Check:

   fold with empty start and append = A [OK]

Hint: fold with empty start and add dash after each item [OK]

Common Mistakes:

• Using reduce which starts with first item, missing initial empty string
• Appending dash before instead of after
• Wrong initial value causing different output