How do you print to stderr in R?

Question

How do you print to stderr in R?

This would especially useful for scripts written in Rscript.

Answer 1

Actually the following works for me:

write("prints to stderr", stderr())

write("prints to stdout", stdout())

Answer 2

Here's a more flexible version for debugging/verbose use in Rscript. Not only it prints to stderr as you ask, but it also allows you to pass variable number of arguments, types etc, like printf does.

v <- function(...) cat(sprintf(...), sep='', file=stderr())

Now one can do things like:

v("name: %s  age: %d\n", name, age)

etc.

Answer 3

Is it possible to configure the print function to print to stderr?

From Ripley himself:

No, but where standard output goes is controlled by sink(), so you can achieve the same effect. R internally has no idea what output comes from print() (which is not just one function but hundreds of methods).

Answer 4

message('for writing diagnostic info to standard error')

message is used for generating ‘simple’ diagnostic messages which are neither warnings nor errors, but nevertheless represented as conditions. Unlike warnings and errors, a final newline is regarded as part of the message, and is optional. The default handler sends the message to the stderr() connection.