Interrupt the execution of an expression and allow the inspection of
the environment where browser was called from.
browser(text = "", condition = NULL, expr = TRUE, skipCalls = 0L)a text string that can be retrieved once the browser is invoked.
a condition that can be retrieved once the browser is invoked.
An expression, which if it evaluates to TRUE the
debugger will invoked, otherwise control is returned directly.
how many previous calls to skip when reporting the calling context.
A call to browser can be included in the body of a function.
When reached, this causes a pause in the execution of the
current expression and allows access to the R interpreter.
The purpose of the text and condition arguments are to
allow helper programs (e.g., external debuggers) to insert specific
values here, so that the specific call to browser (perhaps its location
in a source file) can be identified and special processing can be
achieved. The values can be retrieved by calling browserText
and browserCondition.
The purpose of the expr argument is to allow for the illusion
of conditional debugging. It is an illusion, because execution is
always paused at the call to browser, but control is only passed
to the evaluator described below if expr evaluates to TRUE.
In most cases it is going to be more efficient to use an if
statement in the calling program, but in some cases using this argument
will be simpler.
The skipCalls argument should be used when the browser()
call is nested within another debugging function: it will look further
up the call stack to report its location.
At the browser prompt the user can enter commands or R expressions, followed by a newline. The commands are
cexit the browser and continue execution at the next statement.
contsynonym for c.
ffinish execution of the current loop or function
helpprint this list of commands
nevaluate the next statement, stepping over
function calls. For byte compiled functions interrupted by
browser calls, n is equivalent to c.
sevaluate the next statement, stepping into
function calls. Again, byte compiled functions make
s equivalent to c.
whereprint a stack trace of all active function calls.
rinvoke a "resume" restart if one is
available; interpreted as an R expression otherwise. Typically
"resume" restarts are established for continuing from user
interrupts.
Qexit the browser and the current evaluation and return to the top-level prompt.
Leading and trailing whitespace is ignored, except for an empty line.
Handling of empty lines depends on the "browserNLdisabled"
option; if it is TRUE, empty lines are ignored.
If not, an empty line is the same as n (or s, if it was used
most recently).
Anything else entered at the browser prompt is interpreted as an
R expression to be evaluated in the calling environment: in
particular typing an object name will cause the object to be printed,
and ls() lists the objects in the calling frame. (If you want
to look at an object with a name such as n, print it
explicitly.)
The number of lines printed for the deparsed call can be limited by
setting options(deparse.max.lines).
The browser prompt is of the form Browse[n]>: here
var{n} indicates the ‘browser level’. The browser can
be called when browsing (and often is when debug is in
use), and each recursive call increases the number. (The actual
number is the number of ‘contexts’ on the context stack: this
is usually 2 for the outer level of browsing and 1 when
examining dumps in debugger.)
This is a primitive function but does argument matching in the standard way.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.
debug, and
traceback for the stack on error.
browserText for how to retrieve the text and condition.