How to tell lapply to ignore an error and process the next thing in the list?

Question

I have an example function below that reads in a date as a string and returns it as a date object. If it reads a string that it cannot convert to a date, it returns an erro

Answer 1

One could try to keep it simple rather than to make it complicated:

• Use the vectorised date parsing

R> as.Date( c("2010-04-06", "foo", "2010-04-08") )
[1] "2010-04-06" NA           "2010-04-08"

You can trivially wrap na.omit() or whatever around it. Or find the index of NAs and extract accordingly from the initial vector, or use the complement of the NAs to find the parsed dates, or, or, or. It is all here already.

• You can make your testFunction() do something. Use the test there -- if the returned (parsed) date is NA, do something.

• Add a tryCatch() block or a try() to your date parsing.

The whole things is a little odd as you go from a one-type data structure (vector of chars) to something else, but you can't easily mix types unless you keep them in a list type. So maybe you need to rethink this.

Answer 2

Use a tryCatch expression around the function that can throw the error message:

testFunction <- function (date_in) {
  return(tryCatch(as.Date(date_in), error=function(e) NULL))
}

The nice thing about the tryCatch function is that you can decide what to do in the case of an error (in this case, return NULL).

> lapply(dates2, testFunction)
[[1]]
[1] "2010-04-06"

[[2]]
NULL

[[3]]
[1] "2010-04-08"

Answer 3

You can also accomplish this kind of task with the purrr helper functions map and possibly. For exampel

library(purrr)
map(dates2, possibly(testFunction, NA))

Here possibly will return NA (or whatever value you specified if an error occurs.

Answer 4

Assuming the testFunction() is not trivial and/or that one cannot alter it, it can be wrapped in a function of your own, with a tryCatch() block. For example:

> FaultTolerantTestFunction <- function(date_in) {
+    tryCatch({ret <- testFunction(date_in);}, error = function(e) {ret <<- NA});
+    ret
+ }
> FaultTolerantTestFunction('bozo')
[1] NA
> FaultTolerantTestFunction('2010-03-21')
[1] "2010-03-21"