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    ReQL command: fold

    Command syntax

    sequence.fold(base, function) → value

    sequence.fold(base, function, emit=function[, final_emit=function]) → sequence

    Description

    Apply a function to a sequence in order, maintaining state via an accumulator. The fold command returns either a single value or a new sequence.

    In its first form, fold operates like reduce, returning a value by applying a combining function to each element in a sequence. The combining function takes two parameters: the previous reduction result (the accumulator) and the current element. However, fold has the following differences from reduce:

    • it is guaranteed to proceed through the sequence from first element to last.
    • it passes an initial base value to the function with the first element in place of the previous reduction result.

    combining_function(accumulator | base, element) → new_accumulator

    In its second form, fold operates like concat_map, returning a new sequence rather than a single value. When an emit function is provided, fold will:

    • proceed through the sequence in order and take an initial base value, as above.
    • for each element in the sequence, call both the combining function and a separate emitting function. The emitting function takes three parameters: the previous reduction result (the accumulator), the current element, and the output from the combining function (the new value of the accumulator).

    If provided, the emitting function must return a list.

    emit(previous_accumulator, element, accumulator) → array

    A finalEmit function may also be provided, which will be called at the end of the sequence. It takes a single parameter: the result of the last reduction through the iteration (the accumulator), or the original base value if the input sequence was empty. This function must return a list, which will be appended to fold’s output stream.

    final_emit(accumulator | base) → array

    Example: Concatenate words from a list.

    r.table('words').order_by('id').fold('',
        lambda acc, word: acc + r.branch(acc == '', '', ', ') + word
    ).run(conn)
    

    (This example could be implemented with reduce, but fold will preserve the order when words is a RethinkDB table or other stream, which is not guaranteed with reduce.)

    Example: Return every other row in a table.

    r.table('even_things').fold(0,
        lambda acc, row: acc + 1,
        emit=lambda acc, row, new_acc: r.branch((new_acc % 2 == 0), [row], [])
    ).run(conn)
    

    The first function increments the accumulator each time it’s called, starting at 0; the second function, the emitting function, alternates between returning a single-item list containing the current row or an empty list. The fold command will return a concatenated list of each emitted value.

    Example: Compute a five-day running average for a weight tracker.

    r.table('tracker').filter({'name': 'bob'}).order_by('date')['weight'].fold(
        [],
        lambda acc, row: ([row] + acc).limit(5),
        emit=lambda acc, row, new_acc: r.branch(new_acc.size() == 5, [new_acc.avg()], [])
    ).run(conn)
    

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