Question

I want to write a function that, given a vector v computes the product of all the entries in v. (There is a function in R that does this, but I want to write one myself.)

I tried however how can I get for product of any elements in a vector?

product <- function(v){
    out <- 1
    for(i in 1:length(v)){
       out <- out*v[i]
    }
    out
}

Answer 1

If you use ... as the argument to your function, you can pass it several objects or just one. Inside the function, you can convert to a list and use Reduce to apply a function (*) recursively to the list. If you combine list, unlist and as.list you can make this very general. The following will work with a vector, or with 2 or more numbers, or a mixture of vectors and single numbers.

> product <- function(...) Reduce("*", as.list(unlist(list(...))))
> product(2, 7, 3)
[1] 42
> product(c(2, 7, 3))
[1] 42
> product(2, c(7, 3))
[1] 42