Efficient fold / reduce / accumulate / combine of a vector

This function comes from the future.apply package.

fold(x, f, left = TRUE, unname = TRUE, threshold = 1000L)

Arguments

x	A vector.
f	A binary function, i.e. a function take takes two arguments.
left	If TRUE, vector is combined from the left (the first element), otherwise the right (the last element).
unname	If TRUE, function f is called as f(unname(y), x[[ii]]), otherwise as f(y, x[[ii]]), which may introduce name "y".
threshold	An integer (>= 2) specifying the length where the recursive divide'and'conquer call will stop and incremental building of the partial value is performed. Using threshold = +Inf will disable recursive folding.

Value

A vector.

Details

In order for recursive folding to give the same results as non-recursive folding, binary function f must be associative with itself, i.e. f(f(x[[1]], x[[2]]), x[[3]]) equals f(x[[1]], f(x[[2]]), x[[3]]).

This function is a more efficient (memory and speed) of base::Reduce(f, x, right = !left, accumulate = FALSE), especially when x is long.