Efficient Fold, Reduce, Accumulate, Combine of a Vector
fold(x, f, left = TRUE, unname = TRUE, threshold = 1000L)A vector.
A binary function, i.e. a function take takes two arguments.
If TRUE, vector is combined from the left (the first element),
otherwise the right (the last element).
If TRUE, function f is called as
f(unname(y), x[[ii]]), otherwise as f(y, x[[ii]]),
which may introduce name "y".
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.
A vector.
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
Reduce(f, x, right = !left, accumulate = FALSE),
especially when x is long.