Apply the function to each nth element in the vector

I have a vector in R:

> v <- c(5,10,15,20,25,30,35,40,45,50)

I want to apply a function to every Nth element of a vector and let it return a new vector For example, suppose I want to multiply each third element of a vector by 2 Then I'll get

> v
[1] 5 10 30 20 25 60 35 40 90 50

I try to extract elements using vector logic:

> v[c(rep(FALSE,2),TRUE)]
[1] 15 30 45

I've figured out how to access elements, I can do things for them, but I don't know how to get them back to my original vector v

Solution

We need to allocate

v[c(FALSE,FALSE,TRUE)] <- v[c(FALSE,TRUE)]*2