move up a frame, debug R environment

When debugging a function, I would like to move up to the parent frame and look at some variables there. How do I do this?

Here is a sample:

f <- function() {
   x <-1
   g(x+1)
}
g <- function(z) {
   y = z+2
   return(y)
}

I then debug both functions using debug("g") and debug("f"). When I end up in g at the Browser>, I would like to move back up to f to examine x.

Thanks

Solution

In R terminology, you are wanting to investigate the parent frame of g()'s evaluation environment (i.e. the environment in which g was called). The functions for doing that are documented in the help page for ?sys.parent.

Once your browser indicates that you are 'debugging in g(x + 1)', you can do the following. (Thanks to Joshua Ulrich for suggesting where to help locate ones position in the call stack .)

# Confirm that you are where you think you are
where
# where 1 at #3: g(x + 1)
# where 2: f()

# Get a reference to g()'s parent frame (an environment object)
pframe <- parent.frame()
pframe
# <environment: 0x019b9174>

# Examine the contents of the parent frame
ls(env=pframe)
# [1] "x"

# Get the value of 'x' in the parent frame
get("x", env = pframe)
# [1] 1

EDIT: To understand the collection of functions described in ?sys.parent, it's probably worth noting that parent.frame() is (basically) shorthand for sys.frame(sys.parent(1)). If you find yourself in an evaluation environment farther down a call stack (as revealed by where, for instance), you can reach into environments farther back up the call stack (say two steps up) by either parent.frame(2) or sys.frame(sys.parent(2)).