CODEWITHSUNDEEP
rerror-handlinglapply
Heisenberg
Heisenberg

Reputation: 8806

Error handling with lapply -- output the index of failed elements

Answer to question about error handling with lapply always return NA or NULL when an element fails, i.e.

myfun <- function(s) {
  tryCatch(doSomething(s), error = function(e) { return(NULL) }
}

However, this is not general enough since doSomething(s) may return NULL or NA itself. Therefore, ideally I want myfun written so that after lapply(mylist, myfun) I can somehow get all the indices of failed elements. How to do this?

Upvotes: 2

Views: 892

Answers (1)

Martin Morgan
Martin Morgan

Reputation: 46866

Catch and release the error by handing it with identity()

res = lapply(list(1, "two", 3), function(i) tryCatch({
    sqrt(i)
}, error=identity)

Check for errors

vapply(res, is, logical(1), "error")

Returning the error condition is often better the returning a 'magic' value like NA or NULL, unless the down-stream analysis works seamlessly with the value returned.

As a more advanced solution, create a more elaborate condition or extend the error class

my_condition = function(err)
    structure(list(message=conditionMessage(err),
                   original=err), class=c("my", "condition"))

and return that

res <- lapply(list(1, "two", 3), function(i) {
    tryCatch({
        sqrt(i)
    }, error=my_condition)
})

Upvotes: 5

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