Defining custom print methods for arrays and atomic vectors

Question

I would like to define a different print method for arrays, but I’m afraid I’m not understanding something about S3 dispach. My custom print method is called if I call print(x) explicitly, but is not called if I just type x at the console. However, if I define a custom S3 class, then the appropriate print method is called.

A similar thing happens if I try to define a method for print.numeric

Here is a minimal example:

print.array <- function(x, ...) cat("Hi!\n")

x <- array(1:8, c(2,2,2) )

print(x) # the print method defined above is called
# Hi!
x        # the print method defined above is NOT called

Does anyone have any insights into what is happening? What function is actually doing the printing when just x is evaluated at the console?

Answer 1

You need to define the S3 method in a NAMESPACE (see here) in an extension structure (package) as follows:

export(print.array)
S3method(print, array)

I suggest you use devtools to create your "package" (that can easily contain only your print.array function), you'll find some excellent resources here.