r how to keep print method for custom class - r

Question

i have defined a method for printing a vector with the class test:
print.test <- function(x, ...) {
x <- formatC(
as.numeric(x),
format = "f",
big.mark = ".",
decimal.mark = ",",
digits = 1
)
x[x == "NA"] <- "-"
x[x == "NaN"] <- "-"
print.default(x)
}
which works fine for the following
a <- c(1000.11, 2000.22, 3000.33)
class(a) <- c("test", class(a))
print(a)
[1] "1.000,11" "2.000,22" "3.000,33"
this also works:
round(a)
[1] "1.000,0" "2.000,0" "3.000,0"
this does not:
median(a)
[1] 2000.22
class(median(a))
[1] "numeric"
now my question is: do i need to write a custom method for this class to use median e.g. and if so what would it look like or is there another way (as i simply would like this class to print the data in a certain format)?

Answer 1

The problem is that median.default returns an object of class numeric therefore autoprinting of the returned object does not call your custom print method.
The following will do so.
median.test <- function(x, na.rm = FALSE, ...){
y <- NextMethod(x, na.rm = na.rm, ...)
class(y) <- c("test", class(y))
y
}
median(a)
#[1] "2.000,2"
As for the handling of NA values, I will first define another method for a base R function. It is not strictly needed but save some code lines if objects of class test are used frequently.
c.test <- function(x, ...){
y <- NextMethod(x, ...)
class(y) <- c("test", class(y))
y
}
b <- c(a, NA)
class(b)
#[1] "test" "numeric"
median(b)
#[1] "-"
median(b, na.rm = TRUE)
#[1] "2.000,2"
EDIT.
The following defines a generic function wMedian, a default method and a method for objects of class "currency", as requested by the OP in a comment.
Note that there must be a method print.currency, which I don't redefine since it's exactly the same as print.test above. As for the other methods, I have made them simpler with the help of a new function, as.currency.
median.currency <- function(x, na.rm = FALSE, ...){
y <- NextMethod(x, na.rm = na.rm, ...)
as.currency(y)
}
c.currency <- function(x, ...){
y <- NextMethod(x, ...)
as.currency(y)
}
as.currency <- function(x){
class(x) <- c("currency", class(x))
x
}
wMedian <- function(x, ...) UseMethod("wMedian")
wMedian.default <- function(x, ...){
matrixStats::weightedMedian(x, ...)
}
wMedian.currency <- function(x, w = NULL, idxs = NULL, na.rm = FALSE, interpolate = is.null(ties), ties = NULL, ...) {
y <- NextMethod(x, w = w, idxs = idxs, na.rm = na.rm, interpolate = interpolate, ties = ties, ... )
as.currency(y)
}
set.seed(1)
x <- rnorm(10)
wMedian(x, w = (1:10)/10)
#[1] 0.4084684
wMedian(as.currency(x), w = (1:10)/10)
#[1] "0,4"