R purrr reduce 通过迭代应用二元函数将列表减少为单个值

reduce(.x, .f, ..., .init, .dir = c("forward", "backward"))

reduce2(.x, .y, .f, ..., .init)

# Before:
reduce_right(1:3, f)

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

accumulate() 用于返回归约的所有中间值的版本。

# Reducing `+` computes the sum of a vector while reducing `*`
# computes the product:
1:3 |> reduce(`+`)
#> [1] 6
1:10 |> reduce(`*`)
#> [1] 3628800

# By ignoring the input vector (nxt), you can turn output of one step into
# the input for the next. This code takes 10 steps of a random walk:
reduce(1:10, \(acc, nxt) acc + rnorm(1), .init = 0)
#> [1] 5.293988

# When the operation is associative, the direction of reduction
# does not matter:
reduce(1:4, `+`)
#> [1] 10
reduce(1:4, `+`, .dir = "backward")
#> [1] 10

# However with non-associative operations, the reduced value will
# be different as a function of the direction. For instance,
# `list()` will create left-leaning lists when reducing from the
# right, and right-leaning lists otherwise:
str(reduce(1:4, list))
#> List of 2
#>  $ :List of 2
#>   ..$ :List of 2
#>   .. ..$ : int 1
#>   .. ..$ : int 2
#>   ..$ : int 3
#>  $ : int 4
str(reduce(1:4, list, .dir = "backward"))
#> List of 2
#>  $ : int 1
#>  $ :List of 2
#>   ..$ : int 2
#>   ..$ :List of 2
#>   .. ..$ : int 3
#>   .. ..$ : int 4

# reduce2() takes a ternary function and a second vector that is
# one element smaller than the first vector:
paste2 <- function(x, y, sep = ".") paste(x, y, sep = sep)
letters[1:4] |> reduce(paste2)
#> [1] "a.b.c.d"
letters[1:4] |> reduce2(c("-", ".", "-"), paste2)
#> [1] "a-b.c-d"

x <- list(c(0, 1), c(2, 3), c(4, 5))
y <- list(c(6, 7), c(8, 9))
reduce2(x, y, paste)
#> [1] "0 2 6 4 8" "1 3 7 5 9"


# You can shortcircuit a reduction and terminate it early by
# returning a value wrapped in a done(). In the following example
# we return early if the result-so-far, which is passed on the LHS,
# meets a condition:
paste3 <- function(out, input, sep = ".") {
  if (nchar(out) > 4) {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> reduce(paste3)
#> [1] "a.b.c"

# Here the early return branch checks the incoming inputs passed on
# the RHS:
paste4 <- function(out, input, sep = ".") {
  if (input == "j") {
    return(done(out))
  }
  paste(out, input, sep = sep)
}
letters |> reduce(paste4)
#> [1] "a.b.c.d.e.f.g.h.i"