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)
}