Debugging lapply/sapply calls

Question

Code written using lapply and friends is usually easier on the eyes and more Rish than loops. I love lapply just as much as the next guy, but how do I debug it when things

Answer 1

Like geoffjentry said:

> sapply(x, function(x) {
  res <- tryCatch(1 / x,
                  error=function(e) {
                          cat("Failed on x = ", x, "\n", sep="") ## browser()
                          stop(e)
                        })
})

Also, your for loop could be rewritten to be much cleaner (possibly a little slower):

> y <- NULL
> for (xi in x)
    y <- c(y, 1 / xi)

Error in 1/xi : non-numeric argument to binary operator

For loops are slow in R, but unless you really need the speed I'd go with a simple iterative approach over a confusing list comprehension.

If I need to figure out some code on the fly, I'll always go:

sapply(x, function(x) {
  browser()
  ...
})

And write the code from inside the function so I see what I'm getting.

-- Dan