Using get() with replacement functions

Question

Can anyone explain to me why the following example occurs?

#Create simple dataframe
assign( \"df\" , data.frame( P = runif(5) , Q = runif(5) , R = runif(5) )         


        

Answer 1

To understand why this doesn't work, you need to understand what colnames<- does. Like every function in that looks like it's modifying an object, it's actually modifying a copy, so conceptually colnames(x) <- y gets expanded to:

copy <- x
colnames(copy) <- y
x <- copy

which can be written a little more compactly if you call the replacement operator in the usual way:

x <- `colnames<-`(x, y)

So your example becomes

get("x") <- `colnames<-`(get("x"), y)

The right side is valid R, but the command as a whole is not, because you can't assign something to the result of a function:

x <- 1
get("x") <- 2
# Error in get("x") <- 2 : 
#  target of assignment expands to non-language object