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fold and reduce

Learn aggregation with fold and reduce.

fold and reduce

fold performs an associative operation with an initial value. Example: numbers.fold(0) { acc, n -> acc + n } computes the sum. acc is the accumulator and n is the current element. The initial value makes the empty list case safe.

reduce takes no initial value; the first element acts as the start. Example: numbers.reduce { a, b -> a + b }. On an empty list reduce throws because there is no starting element. For operations like a sum that always need a start, prefer fold.

fold versus reduce

The main difference between fold and reduce is the initial value. fold takes an explicit start, which can even have a different type than the list. Example: numbers.fold("") { acc, n -> acc + n.toString() } joins numbers into a string.

reduce uses the first element as the start, so the result type must match the list type. On an empty list reduce throws a NoSuchElementException. For a safe sum use fold(0); when the first element is the natural start, choose reduce.

Practice Sum with fold, find max with reduce

Define val numbers = listOf(3, 1, 4, 1, 5). Compute the sum with fold(0) { acc, n -> acc + n }. Find the largest element with reduce { a, b -> if (a > b) a else b }. Print the result as println("Sum: $sum, Max: $max").

Starter code

Main.kt
fun main() {
    val numbers = listOf(3, 1, 4, 1, 5)
    // Compute sum with fold, max with reduce, and print
}

Sample solution

Main.kt
fun main() {
    val numbers = listOf(3, 1, 4, 1, 5)
    val sum = numbers.fold(0) { acc, n -> acc + n }
    val max = numbers.reduce { a, b -> if (a > b) a else b }
    println("Sum: $sum, Max: $max")
}

Expected output

Sum: 14, Max: 5

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the fold call

Question 
numbers.____(0) { acc, n -> acc + n }
Answer 
numbers.fold(0) { acc, n -> acc + n }

Explanation: fold(0) computes the sum starting from zero.

2 Match the fold and reduce concepts

Term Meaning
fold(0) Has initial value
reduce First element starts
acc Accumulator
{ acc, n -> } Operation

3 What does reduce do on an empty list?

  • Throws an exception Correct answer
  • Returns zero
  • Returns null
  • Returns an empty list

Explanation: reduce throws NoSuchElementException on an empty list.

All lessons in Intermediate

1 Classes in depth

  1. Primary constructor
  2. Secondary constructor
  3. Properties
  4. lateinit and lazy
  5. companion object
  6. Summary - Classes

2 Inheritance

  1. open
  2. override
  3. super
  4. abstract
  5. interface
  6. Summary - Inheritance

3 Data class

  1. data class
  2. copy
  3. equals and hashCode
  4. toString
  5. Destructuring
  6. Summary - Data class

4 Project: Helper library

  1. Module design
  2. String helpers
  3. Number helpers
  4. Date helpers
  5. Putting it together

5 Sealed class

  1. sealed class
  2. when with sealed
  3. Sealed and data
  4. Sealed and state
  5. Sealed and result
  6. Summary - Sealed class

6 Object declaration

  1. object
  2. companion object
  3. Object expression
  4. companion and interface
  5. object vs class
  6. Summary - Object declaration

7 Extension functions

  1. Extension functions
  2. Extension properties
  3. Nullable extensions
  4. companion extensions
  5. Standard library
  6. Summary - Extension functions

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Generics

  1. Generic class
  2. Generic functions
  3. Variance
  4. Type constraints
  5. Reified
  6. Summary - Generics

10 Higher-order functions

  1. Function types
  2. Lambda
  3. it
  4. inline
  5. Scope functions
  6. Summary - Higher-order functions

11 Project: Library app

  1. Model design
  2. Library class
  3. Checking out
  4. Search and filter
  5. Putting it together

12 Advanced collections

  1. map and filter
  2. fold and reduce
  3. groupBy and partition
  4. sortedBy and sortedWith
  5. associate and flatMap
  6. Summary - Advanced collections

13 Delegation

  1. by
  2. Delegated properties
  3. by map
  4. vetoable
  5. Custom delegates
  6. Summary - Delegation

14 Value class

  1. value class
  2. Type safety
  3. Methods
  4. Generic value class
  5. value class vs data class
  6. Summary - Value class

15 Coroutine basics

  1. What are coroutines?
  2. launch
  3. suspend
  4. runBlocking
  5. async/await
  6. Summary - Coroutine basics

16 Flow basics

  1. Flow
  2. flow builder
  3. collect
  4. map and filter
  5. flowOn
  6. Summary - Flow basics

17 File I/O

  1. File
  2. readText
  3. writeText
  4. readLines
  5. appendText
  6. Summary - File I/O

18 Packages and modules

  1. package
  2. import
  3. internal
  4. Gradle
  5. kotlinx
  6. Summary - Packages

19 Testing

  1. JUnit
  2. @Test
  3. Assertions
  4. @BeforeEach
  5. MockK
  6. Summary - Testing

20 Intermediate final

  1. Project design
  2. OOP model
  3. Collections
  4. File I/O
  5. Putting it together