Chapter 8 Exception handling
The exception handling facilities in R are provided through two mechanisms. Functions such as stop or warning can be called directly or options such as “warn” can be used to control the handling of problems.
8.1 stop
A call to stop halts the evaluation of the current expression, prints the message argument and returns execution to top-level.
8.2 warning
The function warning takes a single argument that is a character string. The behaviour of a call to warning depends on the value of the option “warn”. If “warn” is negative warnings are ignored. If it is zero, they are stored and printed after the top-level function has completed. If it is one, they are printed as they occur and if it is 2 (or larger) warnings are turned into errors.
If “warn” is zero (the default), a variable last.warning is created and the messages associated with each call to warning are stored, sequentially, in this vector. If there are fewer than 10 warnings they are printed after the function has finished evaluating. If there are more than 10 then a message indicating how many warnings occurred is printed. In either case last.warning contains the vector of messages, and warnings provides a way to access and print it.
8.3 on.exit
A function can insert a call to on.exit at any point in the body of a function. The effect of a call to on.exit is to store the value of the body so that it will be executed when the function exits. This allows the function to change some system parameters and to ensure that they are reset to appropriate values when the function is finished. The on.exit is guaranteed to be executed when the function exits either directly or as the result of a warning.
An error in the evaluation of the on.exit code causes an immediate jump to top-level without further processing of the on.exit code.
on.exit takes a single argument which is an expression to be evaluated when the function is exited.
8.4 Error options
There are a number of options variables that can be used to control how R handles errors and warnings. They are listed in the table below.
- ‘warn’
-
Controls the printing of warnings.
- ‘warning.expression’
-
Sets an expression that is to be evaluated when a warning occurs. The normal printing of warnings is suppressed if this option is set.
- ‘error’
-
Installs an expression that will be evaluated when an error occurs. The normal printing of error messages and warning messages precedes the evaluation of the expression.
Expressions installed by options(“error”) are evaluated before calls to on.exit are carried out.
One can use options(error = expression(q(“yes”))) to get R to quit when an error has been signalled. In this case an error will cause R to shut down and the global environment will be saved.