scan

Learn intermediate results with scan.

Running accumulate with scan

scan is like fold but it collects each intermediate result in a list. It is useful for seeing intermediate states.

Example: listOf(1, 2, 3).scan(0) { acc, n -> acc + n } returns [0, 1, 3, 6]. The initial value is included; each element adds an intermediate sum. While fold yields only the result, scan shows the whole run. This pattern is ideal for running totals and cumulative computations.

scan vs fold

fold returns only the final value. scan returns all intermediate values including the initial one. So a scan result has one more element than the input.

Example: listOf(1, 2, 3).fold(0) { acc, n -> acc + n } gives 6. The same operation with scan gives [0, 1, 3, 6]. Use scan when you want to track progress.

Exercise Compute a running total

In main assign the result of listOf(1, 2, 3, 4).scan(0) { acc, n -> acc + n } to a variable and print it with println.

Show example solution

fun main() {
    val running = listOf(1, 2, 3, 4).scan(0) { acc, n ->
        acc + n
    }
    println(running)
}

Test yourself

What does listOf(1, 2, 3).scan(0) { acc, n -> acc + n } return?
- [0, 1, 3, 6]
- [1, 3, 6]
- [0, 1, 2, 3]
- [6]
Show answer

[0, 1, 3, 6] — scan includes the initial value and all intermediate sums.
Match the scan parts

scan(0)

Initial value
{ acc, n -> }

Lambda
[0, 1, 3, 6]

Intermediate results
fold(0)

Only final value

Complete the scan call

val running = listOf(1, 2, 3).____(0) { acc, n -> acc + n }

Show answer

val running = listOf(1, 2, 3).scan(0) { acc, n -> acc + n }

Next lesson: flatMap and flatten

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