Why is a length one vector initially at NAM(2)?

Question

I stumbled across this behavior:

x <- 1:5
> tracemem(x)
[1] \"<0x12145b7a8>\"
> \"names<-\"(x, letters[1:5])
a b c d e 
1 2 3 4 5 
> x
a         


        

Answer 1

Matthew Dowle asked the same question here, and Peter Dalgaard answered thusly:

This is tricky business... I'm not quite sure I'll get it right, but let's try

When you are assigning a constant, the value you assign is already part of the assignment expression, so if you want to modify it, you must duplicate. So NAMED==2 on z <- 1 is basically to prevent you from accidentally "changing the value of 1". If it weren't, then you could get bitten by code like for(i in 1:2) {z <- 1; if(i==1) z[1] <- 2}.

This may seem exotic, but really, the rationale is exactly the same as it is for incrementing NAM to 2 whenever doing an assignment of the form x <- y.

As discussed here, R supports a "call by value" illusion to avoid at least some unnecessary copying of objects. So, for instance, x <- y really just binds the symbol x to y's value. The danger of doing that without further precautions, though, is that subsequent modification of x would also modify y and any other symbols linked to y. R gets around this by marking y's value as "linked to" (by setting it's NAM=2) as soon as it is assigned (or even potentially assigned) to another symbol.

When you do x <- 1, the 1 is more or less just another y whose value is being linked to the symbol x by the assignment expression. It's just that the potential for mischief arising from subsequent modification of x's value (recalling that at this point, it's just a reference to the value of 1!) is awful to imagine. But, as always with assignments of one symbol to another, R sets NAM=2, and no modifications without actual copying are allowed.

The reason x <- 1:10 is different (as are x <- 1:1, x <- c(1), x <- seq(1), and even x <- -1) is that the RHS is actually a function call, and the result of that function call is what's being assigned to x. In these cases, the value of x is not just a reference to the value of some other symbol; modifying x won't potentially change the value of some other symbol, so there is no need to set NAM=2.