List.Accumulate

Syntax

List.Accumulate(
 list as list,
 seed as any,
 accumulator as function
) as any

About

Accumulates a summary value from the items in the specified list using the accumulator.

• list: The list to iterate.
• seed: An initial accumulated value.
• accumulator: A function that takes the current state and the current item and returns the new state.

Example 1

Accumulates the summary value from the items in the list.

Usage

let
 Source = List.Accumulate(
 {1, 2, 3, 4, 5},
 0,
 (runningSum, nextNumber) => runningSum + nextNumber
 )
in
 Source

Output

15

Example 2

Concatenate each word in the list with a space between, but don't include a space at the beginning.

Usage

let
 Source = List.Accumulate(
 {"The", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog."},
 null,
 (fullTextSoFar, nextPart) =>
 Text.Combine({fullTextSoFar, nextPart}, " ")
 )
in
 Source

Output

"The quick brown fox jumps over the lazy dog."

Example 3

Build a list of process completion times from a start date and a list of process run times.

Usage

let
 #"Process Duration" =
 {
 #duration(0,1,0,0),
 #duration(0,2,0,0),
 #duration(0,3,0,0)
 },
 #"Start Time" = #datetime(2025, 9, 8, 19, 0, 0),
 #"Process Timeline" = List.Accumulate(
 #"Process Duration",
 {#"Start Time"},
 (accumulatedTimes, nextDuration) =>
 accumulatedTimes & {List.Last(accumulatedTimes) + nextDuration}
 )
in
 #"Process Timeline"

Output

{
 #datetime(2025, 9, 8, 19, 0, 0),
 #datetime(2025, 9, 8, 20, 0, 0),
 #datetime(2025, 9, 8, 22, 0, 0),
 #datetime(2025, 9, 9, 1, 0, 0)
}