How to write trycatch in R
tryCatch
has a slightly complex syntax structure. However, once we understand the 4 parts which constitute a complete tryCatch call as shown below, it becomes easy to remember:
expr: [Required] R code(s) to be evaluated
error : [Optional] What should run if an error occured while evaluating the codes in expr
warning : [Optional] What should run if a warning occured while evaluating the codes in expr
finally : [Optional] What should run just before quitting the tryCatch call, irrespective of if expr ran successfully, with an error, or with a warning
tryCatch(
expr = {
# Your code...
# goes here...
# ...
},
error = function(e){
# (Optional)
# Do this if an error is caught...
},
warning = function(w){
# (Optional)
# Do this if an warning is caught...
},
finally = {
# (Optional)
# Do this at the end before quitting the tryCatch structure...
}
)
Thus, a toy example, to calculate the log of a value might look like:
log_calculator <- function(x){
tryCatch(
expr = {
message(log(x))
message("Successfully executed the log(x) call.")
},
error = function(e){
message('Caught an error!')
print(e)
},
warning = function(w){
message('Caught an warning!')
print(w)
},
finally = {
message('All done, quitting.')
}
)
}
Now, running three cases:
A valid case
log_calculator(10)
# 2.30258509299405
# Successfully executed the log(x) call.
# All done, quitting.
A "warning" case
log_calculator(-10)
# Caught an warning!
# <simpleWarning in log(x): NaNs produced>
# All done, quitting.
An "error" case
log_calculator("log_me")
# Caught an error!
# <simpleError in log(x): non-numeric argument to mathematical function>
# All done, quitting.
I've written about some useful use-cases which I use regularly. Find more details here: https://rsangole.netlify.com/post/try-catch/
Hope this is helpful.
Well then: welcome to the R world ;-)
Here you go
Setting up the code
urls <- c(
"http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html",
"http://en.wikipedia.org/wiki/Xz",
"xxxxx"
)
readUrl <- function(url) {
out <- tryCatch(
{
# Just to highlight: if you want to use more than one
# R expression in the "try" part then you'll have to
# use curly brackets.
# 'tryCatch()' will return the last evaluated expression
# in case the "try" part was completed successfully
message("This is the 'try' part")
readLines(con=url, warn=FALSE)
# The return value of `readLines()` is the actual value
# that will be returned in case there is no condition
# (e.g. warning or error).
# You don't need to state the return value via `return()` as code
# in the "try" part is not wrapped inside a function (unlike that
# for the condition handlers for warnings and error below)
},
error=function(cond) {
message(paste("URL does not seem to exist:", url))
message("Here's the original error message:")
message(cond)
# Choose a return value in case of error
return(NA)
},
warning=function(cond) {
message(paste("URL caused a warning:", url))
message("Here's the original warning message:")
message(cond)
# Choose a return value in case of warning
return(NULL)
},
finally={
# NOTE:
# Here goes everything that should be executed at the end,
# regardless of success or error.
# If you want more than one expression to be executed, then you
# need to wrap them in curly brackets ({...}); otherwise you could
# just have written 'finally=<expression>'
message(paste("Processed URL:", url))
message("Some other message at the end")
}
)
return(out)
}
Applying the code
> y <- lapply(urls, readUrl)
Processed URL: http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html
Some other message at the end
Processed URL: http://en.wikipedia.org/wiki/Xz
Some other message at the end
URL does not seem to exist: xxxxx
Here's the original error message:
cannot open the connection
Processed URL: xxxxx
Some other message at the end
Warning message:
In file(con, "r") : cannot open file 'xxxxx': No such file or directory
Investigating the output
> head(y[[1]])
[1] "<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">"
[2] "<html><head><title>R: Functions to Manipulate Connections</title>"
[3] "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\">"
[4] "<link rel=\"stylesheet\" type=\"text/css\" href=\"R.css\">"
[5] "</head><body>"
[6] ""
> length(y)
[1] 3
> y[[3]]
[1] NA
Additional remarks
tryCatch
tryCatch
returns the value associated to executing expr
unless there's an error or a warning. In this case, specific return values (see return(NA)
above) can be specified by supplying a respective handler function (see arguments error
and warning
in ?tryCatch
). These can be functions that already exist, but you can also define them within tryCatch()
(as I did above).
The implications of choosing specific return values of the handler functions
As we've specified that NA
should be returned in case of error, the third element in y
is NA
. If we'd have chosen NULL
to be the return value, the length of y
would just have been 2
instead of 3
as lapply()
will simply "ignore" return values that are NULL
. Also note that if you don't specify an explicit return value via return()
, the handler functions will return NULL
(i.e. in case of an error or a warning condition).
"Undesired" warning message
As warn=FALSE
doesn't seem to have any effect, an alternative way to suppress the warning (which in this case isn't really of interest) is to use
suppressWarnings(readLines(con=url))
instead of
readLines(con=url, warn=FALSE)
Multiple expressions
Note that you can also place multiple expressions in the "actual expressions part" (argument expr
of tryCatch()
) if you wrap them in curly brackets (just like I illustrated in the finally
part).
R uses functions for implementing try-catch block:
The syntax somewhat looks like this:
result = tryCatch({
expr
}, warning = function(warning_condition) {
warning-handler-code
}, error = function(error_condition) {
error-handler-code
}, finally={
cleanup-code
})
In tryCatch() there are two ‘conditions’ that can be handled: ‘warnings’ and ‘errors’. The important thing to understand when writing each block of code is the state of execution and the scope. @source