How to start debugger only when condition is met

Question

Assume I have a function which uses a loop over integer i. Now something goes wrong and I assume the error happens when i=5. Now I can step through every single step (what I did up to now).

But now I read about the condition and text argument of browser and debug:

text a text string that can be retrieved when the browser is entered.
condition a condition that can be retrieved when the browser is entered.

Is it possible to use the arguments in a way it works as I want?

Here is an example. The debugger / browser should only start after i=5 is reached:

fun <- function(x, y, n) {
  result <- 0
  for (i in 1:n) {
    # browser(condition = (i == 5)) # does not work
    result <- result + i * ( x + y)
  }
  return(result)
}

x <- 2
y <- 3
n <- 10

# debug(fun, condition = (i == 5)) # does not work
debug(fun)
r <- fun(x, y, n)
print(r)

The solution

if (i == 5) { # inside loop of fun()
  browser()
}

is working, but I thougt there might be something better (No extra code inside the function)

Answer 1

You can use the argument expr in browser():

fun <- function(x, y, n) {
  result <- 0
  for (i in 1:n) {
    browser(expr = {i == 5})
    result <- result + i * ( x + y)
  }
  return(result)
}

It will then only open the environment where browser() was called from if the expression evaluates to TRUE.

If you want to use debug():

debug(fun, condition = i == 5)

and then call the function:

fun <- function(x, y, n) {
  result <- 0
  for (i in 1:n) {
    result <- result + i * ( x + y)
  }
  return(result)
}

fun(x, y, n)