I want to write trycatch
code to deal with error in downloading from the web.
url <- c(
"http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html",
"http://en.wikipedia.org/wiki/Xz")
y <- mapply(readLines, con=url)
These two statements run successfully. Below, I create a non-exist web address:
url <- c("xxxxx", "http://en.wikipedia.org/wiki/Xz")
url[1]
does not exist. How does one write a trycatch
loop (function) so that:
This question is related to
r
exception
exception-handling
try-catch
r-faq
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.
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
Here goes a straightforward example:
# Do something, or tell me why it failed
my_update_function <- function(x){
tryCatch(
# This is what I want to do...
{
y = x * 2
return(y)
},
# ... but if an error occurs, tell me what happened:
error=function(error_message) {
message("This is my custom message.")
message("And below is the error message from R:")
message(error_message)
return(NA)
}
)
}
If you also want to capture a "warning", just add warning=
similar to the error=
part.
Since I just lost two days of my life trying to solve for tryCatch for an irr function, I thought I should share my wisdom (and what is missing). FYI - irr is an actual function from FinCal in this case where got errors in a few cases on a large data set.
Set up tryCatch as part of a function. For example:
irr2 <- function (x) {
out <- tryCatch(irr(x), error = function(e) NULL)
return(out)
}
For the error (or warning) to work, you actually need to create a function. I originally for error part just wrote error = return(NULL)
and ALL values came back null.
Remember to create a sub-output (like my "out") and to return(out)
.
Source: Stackoverflow.com