Getting the state of variables after an error occurs in R

Question

Let's say I have just called a function, f, and an error occurred somewhere in the function. I just want to be able to check out the values of different variables right before the error occurred.

Suppose my gut tells me it's a small bug, so I'm too lazy to use debug(f) and too lazy to insert browser() into the part of the function where I think things are going wrong. And I'm far too lazy to start putting in print() statements.

Here's an example:

x <- 1:5
y <- x + rnorm(length(x),0,1)
f <- function(x,y) {
  y <- c(y,1)
  lm(y~x)
}

Calling f(x,y) we get the following error:

Error in model.frame.default(formula = y ~ x, drop.unused.levels = TRUE) : 
  variable lengths differ (found for 'x')

In this example, I want grab the state of the environment just before lm() is called; that way I can call x and y and see that their lengths are different. (This example may be too simple, but I hope it gets the idea across.)

Answer 1

As pointed out here, there's an easy way to do this, and I think this trick has the potential to change lives for the better.

First, call this:

options(error=recover)

Now when we call f(x,y) we will have an option to choose an environment to recover. Here I select option 1, which opens up a debugger and lets me play around with variables just before lm() is called.

> f(x,y)
Error in model.frame.default(formula = y ~ x, drop.unused.levels = TRUE) : 
  variable lengths differ (found for 'x')

Enter a frame number, or 0 to exit   

1: f(x, y)
2: lm(y ~ x)
3: eval(mf, parent.frame())
4: eval(expr, envir, enclos)
5: model.frame(formula = y ~ x, drop.unused.levels = TRUE)
6: model.frame.default(formula = y ~ x, drop.unused.levels = TRUE)

Selection: 1
Called from: eval(expr, envir, enclos)
Browse[1]> x
[1] 1 2 3 4 5
Browse[1]> y
[1] 1.6591197 0.5939368 4.3371049 4.4754027 5.9862130 1.0000000

Answer 2

You could also just use the debug() function:

> debug(f)
> f(x,y)
debugging in: f(x, y)
debug: {
    y <- c(y, 1)
    lm(y ~ x)
}
Browse[1]> 
debug: y <- c(y, 1)
Browse[1]> x
[1] 1 2 3 4 5
Browse[1]> y
[1] 2.146553 2.610003 2.869081 2.758753 4.433881

Answer 3

options(error=recover)

Probably answers the question best. However, I wanted to mention another handy debugging tool, traceback(). Calling this right after an error has occurred is often enough to pinpoint the bug.