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.

Exercise 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").

Show example solution

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")
}

Test yourself

Complete the fold call

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

Show answer

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

Match the fold and reduce concepts

fold(0)

Has initial value
reduce

First element starts
acc

Accumulator
{ acc, n -> }

Operation
What does reduce do on an empty list?
- Throws an exception
- Returns zero
- Returns null
- Returns an empty list
Show answer

Throws an exception — reduce throws NoSuchElementException on an empty list.

Next lesson: groupBy and partition

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