Equivalent of "throw" in R

Beyond the base functions that Dirk mentions:

The R.oo package has additional exception handling functionality, including a throw() function which is very useful. You can catch exceptions with the usual try or trycatch functions:

> try(throw("Division by zero.")); print("It's ok!");
Error: [2009-10-22 10:24:07] Exception: Division by zero.
[1] "It's ok!"

You can read more about it here: http://www1.maths.lth.se/help/R/R.oo/

See help(tryCatch):

Conditions are signaled by 'signalCondition'. In addition, the
'stop' and 'warning' functions have been modified to also accept
condition arguments.

and later under 'See Also':

'stop' and 'warning' signal conditions, and 'try' is essentially a simplified version of 'tryCatch'.

so you probably want stop.

Simple example:

f <- function(a, b){ 

    if (a == 0){ 
            stop("error message")
    }
 }