How can I check whether a function call results in a warning?

Question

In R, how can I determine whether a function call results in a warning?

That is, after calling the function I would like to know whether that instance of the call yi

Answer 1

2019 update

You can you use 'quietly' from the purrr package, which returns a list of output, result, warning and error. You can then extract each element by name. For instance, if you had a list, which you want to map a function over, and find the elements which returned a warning you could do

library(purrr)
library(lubridate)

datelist <- list(a = "12/12/2002", b = "12-12-2003", c = "24-03-2005")

# get all the everything
quiet_list <- map(datelist, quietly(mdy))

# find the elements which produced warnings
quiet_list %>% map("warnings") %>% keep(~ !is.null(.))

# or 
quiet_list %>% keep(~ length(.$warnings) != 0)

For this example it's quite trivial, but for a long list of dataframes where the NAs might be hard to spot, this is quite useful.