How does the overloading of the assignment operator in combination of the length function work?

Question

How does the mutating implementation of length() actually work?

Example: Given a vector v, how does this set the length to 12?

length(v) <- 12

Can I create my own function that can overload an operator in the same way?

Example: Set every other element to 7

everyOther(v) <- 7

Answer 1

Those assignment functions are just that, functions. They can be written in the following form (note the backticks - they must be used), where fname distinguishes the function name.

`fname<-` <- function(x, value) { ... }

So your everyOther assignment function can be written as

`everyOther<-` <- function(x, value) {
    x[c(FALSE, TRUE)] <- value
    x
}

And we can use it just as we would length(x) <- value

v <- 1:20
everyOther(v) <- 7
v
# [1]  1  7  3  7  5  7  7  7  9  7 11  7 13  7 15  7 17  7 19  7