Debugging unexpected errors in R -- how can I find where the error occurred?

Just to add to what @SachaEpskamp has already suggested, setting options(error=recover) and options(show.error.locations=TRUE) can be extremely helpful when debugging unfamiliar code. The first causes R to launch a debugging session on error, giving you the option to invoke the browser at any point in the call stack up to that error. The second option will tell R to include the source line number in the error.

You can use traceback() to locate where the last error occurred. Usually it will point you to a call you make in your function. Then I typically put browser() at that point, run the function again and see what is going wrong.

For example, here are two functions:

f2 <- function(x)
{
  if (x==1) "foo"
}

f <- function(x)
{
  f2(x)
}

Note that f2() assumes an argument of length 1. We can misuse f:

> f(NULL)
Error in if (x == 1) "foo" : argument is of length zero

Now we can use traceback() to locate what went wrong:

> traceback()
2: f2(x) at #3
1: f(NULL)

The number means how deep we are in the nested functions. So we see that f calls f2 and that gives an error at line 3. Pretty clear. We could reassign f with browser placed just before the f2 call now to check it's input. browser() simply allows you to stop executing a function and look around in its environment. Similar to debug and debugonce except that you don't have to execute every line up until the point you know something goes wrong.