How does colnames function assign new column names?

Question

I've used this function so many times but only now thought 'why does it work?'. How can R's colnames() function assign new column names to a data frame? I mean I get how colnames(df) will return the column names of a data frame. But how can it also assign new ones?

aa <- mtcars
colnames(aa)
colnames(aa) <- LETTERS[1:ncol(aa)]
colnames(aa)
# ^ how can colnames function either return column names or assign new ones? It's just a function.

# but we can't change the number of columns this way:
ncol(aa)
ncol(aa) <- 10

As at now the colnames function is:

function (x, do.NULL = TRUE, prefix = "col") 
{
    if (is.data.frame(x) && do.NULL) 
        return(names(x))
    dn <- dimnames(x)
    if (!is.null(dn[[2L]])) 
        dn[[2L]]
    else {
        nc <- NCOL(x)
        if (do.NULL) 
            NULL
        else if (nc > 0L) 
            paste0(prefix, seq_len(nc))
        else character()
    }
}
<bytecode: 0x00000000091f1710>
<environment: namespace:base>

Q: I can't see how this is assigning new column names to data frame.

Answer 1

colnames on the left hand side of a <- is not the same function as on the right hand side. The former is called a replacement function and its name is colnames<-.

Displaying source

You can see its code by typing this at the R console:

`colnames<-`

The source displayed by that looks like this:

`colnames<-` <- function(x, value) { ...modify x...; x }

where the first argument x refers to the argument on the left hand side and the second argument, value, is the right hand side of the <-. They are both input to the replacement function and then R assigns the result of running the replacement function back to x.

Simple example of replacement function

For example, here is a simple replacement function:

# define simple replacement function
`add_n<-` <- function(x, value) x + value  

# test
my_number <- 4
add_n(my_number) <- 3
my_number
## [1] 7

More info

There is some discussion of replacement functions here: What are Replacement Functions in R?