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.)