Chapter 11 Error Handling

Error handling in R is functional. For beginners this might be a little bit hard to understand in the first glance. The template is as the followings:

result <- 
    tryCatch({
        # main expression block
        # last valuated expression will be returned in case of success
    }, warning=function(cond) {
        # warning handling block, wrapped in function
        # argument is a condition class auto-generated in case of warning
    }, error=function(cond) {
        # error handling block, wrapped in function
        # argument is a condition class auto-generated in case of error
    }, finally={
        # the finally block that always evaluated
    })

Except for the first argument, which is a closure (the expr argument) enclosed by {, all the other arguments are optional. That said, tryCatch prevents the execution of the expr from stopping due to any error if the argument error is supplied. And it prevents a warning being issued if the argument warning is supplied. The finally closure, if speficied, will be executed on any condition.

Consider this example:

tryCatch(1 + "1")

## Error in 1 + "1": non-numeric argument to binary operator

Error still occurs. This is because the error is NOT caught. To catch the error, one should write something like:

tryCatch({
  1 + "1"
}, error=function(cond) print("The error is caught!"))

## [1] "The error is caught!"

Now there is no error message, instead, the function specified in error argument is executed.

tryCatch({
  1 + "1"
  }, error=function(cond) print("The error is caught!"),
  finnaly = {
    print("This is printed whatsoever.")
  })

## [1] "This is printed whatsoever."
## [1] "The error is caught!"

The is also a simplied version of error handler: the try function. It is light-weighted but with less capability.