Is the FoldLeft function available in R?

Question

I would like to know if there is an implementation of the foldLeft function (and foldRight?) in R.

The language is supposed to be "rather" functional oriented and hence I think there should be something like this, but I could not find it in the documentation.

To me, foldLeft function applies on a list and has the following signature:

foldLeft[B](z : B)(f : (B, A) => B) : B

It is supposed to return the following result:

f(... (f(f(z, a0), a1) ...), an) if the list is [a0, a1, ..., an].

(I use the definition of the Scala List API)

Does anybody know if such a function exists in R?

Answer 1

?Reduce. Usage Reduce(f, x, init, right = FALSE, accumulate = FALSE)

Answer 2

If you want a vector of results, this will work:

foldl = function(f, v, x) {w = v; for (i in 1 : length(v)) { x = w[[i]] = f(x, v[[i]]) }; w }

Now you can redefine cumsum as

cumsum(v) = foldl(function(x,y) { x+y }, v, 0)

To improve it you should handle missing values like Reduce does.