base (version 3.3.1)

debug: Debug a Function


Set, unset or query the debugging flag on a function. The text and condition arguments are the same as those that can be supplied via a call to browser. They can be retrieved by the user once the browser has been entered, and provide a mechanism to allow users to identify which breakpoint has been activated.


debug(fun, text = "", condition = NULL) debugonce(fun, text = "", condition = NULL) undebug(fun) isdebugged(fun) debuggingState(on = NULL)


any interpreted R function.
a text string that can be retrieved when the browser is entered.
a condition that can be retrieved when the browser is entered.
logical; a call to the support function debuggingState returns TRUE if debugging is globally turned on, FALSE otherwise. An argument of one or the other of those values sets the state. If the debugging state is FALSE, none of the debugging actions will occur (but explicit browser calls in functions will continue to work).


When a function flagged for debugging is entered, normal execution is suspended and the body of function is executed one statement at a time. A new browser context is initiated for each step (and the previous one destroyed).

At the debug prompt the user can enter commands or R expressions, followed by a newline. The commands are described in the browser help topic.

To debug a function which is defined inside another function, single-step though to the end of its definition, and then call debug on its name.

If you want to debug a function not starting at the very beginning, use trace(..., at = *) or setBreakpoint.

Using debug is persistent, and unless debugging is turned off the debugger will be entered on every invocation (note that if the function is removed and replaced the debug state is not preserved). Use debugonce to enter the debugger only the next time the function is invoked.

In order to debug S4 methods (see Methods), you need to use trace, typically calling browser, e.g., as trace("plot", browser, exit = browser, signature = c("track", "missing"))

The number of lines printed for the deparsed call when a function is entered for debugging can be limited by setting options(deparse.max.lines).

When debugging is enabled on a byte compiled function then the interpreted version of the function will be used until debugging is disabled.

See Also

browser, trace; traceback to see the stack after an Error: ... message; recover for another debugging approach.