Write a function that can work as `labels(x) <- some_value`

Question

I want to write a function, labels, that works as follows:

x <- 1:6
labels(x)
# [1] 1 2 3 4 5 6

labels(x) <- 2:7
labels(x)
# [1] 2 3 4 5 6 7

labels(x)[1:2] <- 9:10
labels(x)
# [1] 9 10 4 5 6 7

How can I do so?

Answer 1

What you seem to want is to understand replacement functions. If we look at names, we also note that there is a names<- function as well, with the following definition:

> `names<-`
function (x, value)  .Primitive("names<-")

Which isn't very informative about what it actually does, but shows that you can write any function of the form foo<- that replaces some components of the object the function is applied to.

x <- 1:6
X <- matrix(1:9, ncol = 3)

Labels <- function(obj, ...) {
    UseMethod("Labels")
}

Labels.numeric <- function(obj, ...) {
    names(obj)
}

Labels.matrix <- function(obj, which = c("colnames","rownames"), ...) {
    if(missing(which))
        which <- "colnames"
    which <- match.arg(which)
    if(which == "colnames") {
        out <- colnames(obj)
    } else {
        out <- rownames(obj)
    }
    out
}

`Labels<-` <- function(obj, ..., value) {
    UseMethod("Labels<-")
}

`Labels<-.numeric` <- function(obj, ..., value) {
    names(obj) <- value
    obj
}

Which can be used as follows:

> x <- 1:6
> Labels(x)
NULL
> Labels(x) <- LETTERS[1:6]
> x
A B C D E F 
1 2 3 4 5 6

A matrix method, might be:

`Labels<-.matrix` <- function(obj, which = c("colnames","rownames"), ..., value) {
    if(missing(which))
            which <- "colnames"
        which <- match.arg(which)
        if(which == "colnames") {
            colnames(obj) <- value
        } else {
            rownames(obj) <- value
        }
        obj
}

Which is used as:

> Labels(X)
NULL
> Labels(X) <- letters[1:3]
> X
     a b c
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
> Labels(X, which = "rownames") <- LETTERS[24:26]
> X
  a b c
X 1 4 7
Y 2 5 8
Z 3 6 9

The trick is to remember that replacement functions are called with a value argument that takes the values from the right hand side of <-, so your function definition needs to have a value argument, and use this argument to set/change the labels.

Of course, all this can be done using names, colnames etc, but if you want to understand how this works then hopefully the above is of use?