sequence.fold(base, function) → value sequence.fold(base, function).optArg("emit", function)[.optArg("final_emit", function)] → sequence
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
:
combiningFunction(accumulator | base, element) → newAccumulator
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:
If provided, the emitting function must return a list.
emit(previousAccumulator, 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.
finalEmit(accumulator | base) → array
Example: Concatenate words from a list.
r.table("words").orderBy("id").fold("", (acc, word) -> acc.add(r.branch(r.eq(acc, ""), "", ", ")).add(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, (acc, row) -> r.add(acc, 1) ).optArg("emit", (acc, row, new_acc) -> r.branch(r.mod(new_acc, 2).eq(0), r.array(row), r.array()) ).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( r.hashMap("name", "bob") ).orderBy("date").g("weight").fold(r.array(), (acc, row) -> r.add(r.array(row), acc).limit(5) ).optArg("emit", (acc, row, new_acc) -> r.branch(new_acc.size().eq(5), r.array(new_acc.avg()), r.array()) ).run(conn);
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https://rethinkdb.com/api/java/fold/