reduce_right: Reduce from the right (retired)

Description

reduce_right() is soft-deprecated as of purrr 0.3.0. Please use the .dir argument of reduce() instead. Note that the algorithm has changed. Whereas reduce_right() computed f(f(3, 2), 1), reduce(.dir = \"backward\") computes f(1, f(2, 3)). This is the standard way of reducing from the right.

To update your code with the same reduction as reduce_right(), simply reverse your vector and use a left reduction:

# Before:
reduce_right(1:3, f)
# After:
reduce(rev(1:3), f)

reduce2_right() is deprecated as of purrr 0.3.0 without replacement. It is not clear what algorithmic properties should a right reduction have in this case. Please reach out if you know about a use case for a right reduction with a ternary function.

Usage

reduce_right(.x, .f, ..., .init)
reduce2_right(.x, .y, .f, ..., .init)
accumulate_right(.x, .f, ..., .init)

Arguments

.x

A list or atomic vector.

.f

For reduce(), a 2-argument function. The function will be passed the accumulated value as the first argument and the "next" value as the second argument.

For reduce2(), a 3-argument function. The function will be passed the accumulated value as the first argument, the next value of .x as the second argument, and the next value of .y as the third argument.

The reduction terminates early if .f returns a value wrapped in a done().

...

Additional arguments passed on to the reduce function.

We now generally recommend against using ... to pass additional (constant) arguments to .f. Instead use a shorthand anonymous function:

# Instead of
x |> reduce(f, 1, 2, collapse = ",")
# do:
x |> reduce(\(x, y) f(x, y, 1, 2, collapse = ","))

This makes it easier to understand which arguments belong to which function and will tend to yield better error messages.

.init

If supplied, will be used as the first value to start the accumulation, rather than using .x[[1]]. This is useful if you want to ensure that reduce returns a correct value when .x is empty. If missing, and .x is empty, will throw an error.

.y

For reduce2() an additional argument that is passed to .f. If init is not set, .y should be 1 element shorter than .x.