Assignment in R language

Question

I am wondering how assignment works in the R language.

Consider the following R shell session:

> x <- c(5, 6, 7)
> x[1] <- 10
> x
[1] 10 6 7
>

which I totally understand. The vector (5, 6, 7) is created and bound to the symbol 'x'. Later, 'x' is rebound to the new vector (10, 6, 7) because vectors are immutable data structures.

But what happens here:

> c(4, 5, 6)[1] <- 10
Error in c(4, 5, 6)[1] <- 10 :
  target of assignment expands to non-language object
>

or here:

> f <- function() c(4, 5, 6)
> f()[1] <- 10
Error in f()[1] <- 10 : invalid (NULL) left side of assignment
>

It seems to me that one can only assign values to named data structures (like 'x').

The reason why I am asking is because I try to implement the R language core and I am unsure how to deal with such assignments.

Thanks in advance

Answer 1

It seems to me that one can only assign values to named data structures (like 'x').

That's precisely what the documentation for ?"<-" says:

Description:

 Assign a value to a name.

x[1] <- 10 doesn't use the same function as x <- c(5, 6, 7). The former calls [<- while the latter calls <-.

Answer 2

As per @Owen's answer to this question, x[1] <- 10 is really doing two things. It is calling the [<- function, and it is assigning the result of that call to x.

So what you want to achieve your c(4, 5, 6)[1] <- 10 result is:

> `[<-`(c(4, 5, 6),1, 10)
[1] 10  5  6