Why does setnames() affect copied tables?

Question

I want to create a copy of a data.table and change the name of a column in the new table. When I change the name of y$V1, the name of x$V1 also changes. Why is it so, and how do I avoid this behaviour?

Example:

x <- data.table(c(1,2,3),c(1,2,3))
y <- x
setnames(y, 'V1', 'new_name')
names(y) == names(x)

Answer 1

Because R implements simple reference counting, and generally only copies on modification and not on assignment. So y = x for any x and y would not copy anything, and no new objects will be created.

Combined with the fact that some data.table functions can modify the object without copying, like setnames, you get the effect you see.

Use copy as Frank mentioned to force an explicit copy.