move up a frame, debug R environment

Question

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

Answer 1

You can use recover (it is often used to debug code after an actual error, via options(error=utils::recover), but it can be called directly).

> f()
debugging in: g(x + 1)
debug at #1: {
    y = z + 2
    return(y)
}
Browse[2]> ls()
[1] "z"
Browse[2]> recover()

Enter a frame number, or 0 to exit   

1: f()
2: #3: g(x + 1)

Selection: 1
Called from: top level 
Browse[3]> ls()
[1] "x"
Browse[3]> x
[1] 1
Browse[3]> 

Answer 2

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)).

Answer 3

Just call on.exit(browser()) when browsing f's body and you'll return back to it after you're done with g()

See output copied from the console below :

> f <- function() {
+   x <-1
+   g(x+1)
+ }
> g <- function(z) {
+   y = z+2
+   return(y)
+ }
> debug("f")
> debug("g")
> f()
debugging in: f()
debug at #1: {
    x <- 1
    g(x + 1)
}
Browse[2]> on.exit(browser()) # browser() will be run in this environment just before we exit
Browse[2]> 
debug at #2: x <- 1
Browse[2]> 
debug at #3: g(x + 1)
Browse[2]> 
debugging in: g(x + 1)
debug at #1: {
    y = z + 2
    return(y)
}
Browse[3]> 
debug at #2: y = z + 2
Browse[3]> 
debug at #3: return(y)
Browse[3]> 
exiting from: g(x + 1)
Browse[2]> x # we're back to f so we can investigate x
[1] 1
Browse[2]> 
exiting from: f()
[1] 4