This question already has answers here:
How to write trycatch in R
(5 answers)
Closed 2 years ago.
I've tried multiple examples I found on here and elsewhere but it always fails to do what I want.
My R script is simply a configuration validation script, it checks to make sure the user has their environment setup correctly. So I want to try 5 different steps and report on the status of each so that the user can go and fix everything all at once. For example, my first step tests the connection to the database, if that fails I want to print a message and continue on to step 2. But it always halts execution when it errors out and so I only get to the failure and then no more.
I am running the script from command line using RScript.exe on W64. Here's my basic tryCatch, I tried it with the error function and without, it makes no difference, it always breaks on the error.
tryCatch(
expr = {
res<- dbGetQuery(con, sql)
print("SQL query results:")
print(res)
}
)
tryCatch catches nothing until you tell it what you want to catch. For instance,
tryCatch(
expr = {
res<- dbGetQuery(con, sql)
print("SQL query results:")
print(res)
},
error = function(e) NULL
)
will return NULL when there's an error. Without that line, it is catching nothing.
Side note: res's existence is not certain. Two scenarios:
res is never defined before now. If there's an error, res will not exist, and any code that follows it will fail with Error: object 'res' not found.
res was defined in a previous code block and is now likely unnecessary. However, if dbGetQuery fails then res will not have been redefined, so it is still present but not related to this query.
Both situations are problematic. I recommend something like:
res <- NULL
res <- tryCatch({
dbGetQuery(con, sql)
}, error = function(e) conditionMessage(e))
if (is.null(res)) {
print(paste("oops!", res))
} else {
print(res)
}
You're simply not handling your error when it occurs. Here's a reprex to show how to catch an error when it occurs:
function_that_always_fails <- function() stop("I failed")
exception_catcher <- function(e) "Carry on regardless"
tryCatch(expr = function_that_always_fails())
#> Error in function_that_always_fails(): I failed
tryCatch(expr = function_that_always_fails(),
error = exception_catcher)
#> [1] "Carry on regardless"
Or, if you wanted to run a bunch of tests in sequence like in your example, you could have something like:
function_that_sometimes_fails <- function(x)
{
if(x %% 2 == 0) stop("function ", x, " failed")
print(paste("test", x, "passed"))
}
exception_catcher <- function(e) print(paste(e$message, "but never mind"))
do_tests <- function() {
for(i in 1:5)
tryCatch(expr = function_that_sometimes_fails(i),
error = exception_catcher)
}
do_tests()
#> [1] "test 1 passed"
#> [1] "function 2 failed but never mind"
#> [1] "test 3 passed"
#> [1] "function 4 failed but never mind"
#> [1] "test 5 passed"
Created on 2020-08-17 by the reprex package (v0.3.0)
Related
I have a list of file to read. I wrote a loop, like this (it is only a example). However, some of these elements are missing and I got a message error interrupting the loop.I would like to skip the error and finish the loop, with the other elements of the list.
list<- c("ciao",
"miao",
"bau")
for (symbols in list){
a <- symbols
b <- as.Date(symbols)
c <- as.numeric(symbols)
d<- cbind(a,b,c)
write.csv(d)
}
As, far as I know, I could use the function try. I have already read some examples here, but they don't fit my need... or I don't know how to implement it.
Any idea to solve the problem?
Thank you
You can use tryCatch() for this and use the error = part to indicate what you want in the case of an error.
For example, in the below we can continue checking if the symbol == "bau" even after "miao" caused an error:
# error before it checks all args of list
list <- c("ciao", "miao", "bau")
for (symbols in list) {
if (symbols == "miao") stop("Er!")
print(symbols == "bau")
}
#> [1] FALSE
#> Error in eval(expr, envir, enclos): Er!
# now it will check all args even with error
for (symbols in list) {
tryCatch(
expr = {
if (symbols == "miao") stop("Er!")
print(symbols == "bau")
},
error = function(e) NA
)
}
#> [1] FALSE
#> [1] TRUE
Created on 2022-08-03 by the reprex package (v2.0.1)
I want to reattempt failing readLines fetches using tryCatch. This works as expected, as long as I don't wrap it inside a future.apply::future_lapply call for processing a list or vector.
The problem can be reproduced using this code:
read_lines_retrying <- function(url, attempts = 5, throttle = 5) {
result <- NA
while (is.na(result) && 0 < attempts) {
attempts <- attempts - 1
result <- tryCatch(
{
readLines(url)
},
error = function(cond) {
message("caught error:")
message(cond)
message("")
Sys.sleep(throttle)
return(NA)
}
)
}
if (is.na(result)) {
stop(paste("could not get URL ", url))
}
return(result)
}
urls <- c("http://nonexistant.nonexistant")
future.apply::future_lapply(urls, read_lines_retrying)
Of course, the code is meant to retry on transient readLines failures, while the example URL will always fail, but this way problem can be most easily seen. When using lapply instead of future.apply::future_lapply, it takes at least 5 seconds to complete because it waits 5 seconds after each of the 5 attempts. This in not the case with future.apply::future_lapply, demonstrating that the exception handling doesn't work.
What am I doing wrong, and how can I get tryCatch to work inside future.apply::future_lapply?
Author of futureverse here: This is an interesting problem.
Here's a minimal reproducible example:
boom <- function(x) {
tryCatch(stop("boom"), error = function(cond) {
message(1); message(cond); message(2)
})
}
y <- lapply(1L, FUN = boom)
## 1
## boom2
y <- future.apply::future_lapply(1L, FUN = boom)
## 1
## Error in doTryCatch(return(expr), name, parentenv, handler) : boom
We can even reproduce this with individual futures:
> y <- boom(1)
## 1
## boom2
> f <- future::future(boom(1))
> y <- future::value(f)
## 1
## Error in doTryCatch(return(expr), name, parentenv, handler) ## : boom
First of all, it turns out that it message(cond) that trigger this odd behavior. If you instead, for instance, use message(conditionMessage(cond)), it works fine.
UPDATE 2022-03-01: After asking about this on R-devel (thread 'message() and warning() circumvent calling handlers and signal the original class, e.g. an error' on 2022-03-01 (https://stat.ethz.ch/pipermail/r-devel/2022-March/081515.html)), I conclude that using message(e) where e is an error condition is incorrect and that one should use message(conditionMessage(e)).
Technical details below:
What happens is that message(cond) end up re-signalling the caught error (= cond). And, despite message() is muffling the error signal internally, it turns out that the future still detects it and takes it as a definite error.
I have a hunch what might be happening, but I cannot promise a quick solution. I'm now tracking this in https://github.com/HenrikBengtsson/future/issues/507. Until resolved, the workaround is: "avoid resignaling the error you just caught", i.e. don't call message(cond) or warning(cond) on an error condition.
Thanks a bunch for reporting this important issue.
PS. Please consider https://github.com/HenrikBengtsson/future/discussions for future discussions, because I'm only skimming StackOverflow occasionally.
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:
When the URL is wrong, the output will be: "web URL is wrong, can't get".
When the URL is wrong, the code does not stop, but continues to download until the end of the list of URLs?
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).
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).
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:
When the URL is wrong, the output will be: "web URL is wrong, can't get".
When the URL is wrong, the code does not stop, but continues to download until the end of the list of URLs?
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).
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).
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:
When the URL is wrong, the output will be: "web URL is wrong, can't get".
When the URL is wrong, the code does not stop, but continues to download until the end of the list of URLs?
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>"
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> 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).
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).