Exceptions Debugging

Dealing with errors and exceptions

Exceptions

Creating

• don't use cat() or print(), except for print methods, or for optional debugging information.

• use message() to inform the user about something expected - I often do this when filling in important missing arguments that have a non-trivial computation or impact. Two examples are reshape2::melt package which informs the user what melt and id variables where used if not specific, and plyr::join which informs which variables where used to join the two tables.

• use warning() for unexpected problems that aren't show stoppers

• use stop() when the problem is so big you can't continue

Handling

• try
• tryCatch

Examples:

• capturing all messages or warnings produced by a function
• capturing user interrupts: Ctrl + C

Debugging

• traceback: show where error occurred
• browser: interact inside function environment. c, n, return, Q, where
• debug/undebug, debugonce: automatically inserts browser
• trace, untrace: automatically inserts any code
• recover, options(error = recover): automatic traceback + browser on error

If you're trying to track down where a warning occurs, it can be useful to turn it into an error with options(warn = 2). Turn back to default behaviour with options(warn = 0).

Don't forget that you can combine if statements with browser() to only debug when a certain situation occurs.