CODEWITHSUNDEEP
rexceptiontry-catchr-faq
Dd Pp
Dd Pp

Reputation: 5957

How to use the tryCatch() function?

I want to write code using tryCatch to deal with errors downloading data 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:

  1. When the URL is wrong, the output will be: "web URL is wrong, can't get".
  2. When the URL is wrong, the code does not stop, but continues to download until the end of the list of URLs?

Upvotes: 542

Views: 588938

Answers (6)

Rappster
Rappster

Reputation: 13070

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) {
    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")

            suppressWarnings(readLines(url))
            # The return value of `readLines()` is the actual value
            # that will be returned in case there is no condition
            # (e.g. warning or error).
        },
        error = function(cond) {
            message(paste("URL does not seem to exist:", url))
            message("Here's the original error message:")
            message(conditionMessage(cond))
            # Choose a return value in case of error
            NA
        },
        warning = function(cond) {
            message(paste("URL caused a warning:", url))
            message("Here's the original warning message:")
            message(conditionMessage(cond))
            # Choose a return value in case of warning
            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")
        }
    )
}

Using the code

> y <- lapply(urls, readUrl)
This is the 'try' part
Processed URL: http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html
Some other message at the end
This is the 'try' part
Processed URL: http://en.wikipedia.org/wiki/Xz
Some other message at the end
This is the 'try' part
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

Investigating the output

> head(y[[1]])
[1] "<!DOCTYPE html><html><head><title>R: Functions to Manipulate Connections (Files, URLs, ...)</title>"
[2] "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" />"
[3] "<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0, user-scalable=yes\" />"
[4] "<link rel=\"stylesheet\" href=\"https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.css\">"
[5] "<script type=\"text/javascript\">"
[6] "const macros = { \"\\\\R\": \"\\\\textsf{R}\", \"\\\\code\": \"\\\\texttt\"};"

> 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 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.

Upvotes: 933

Rahul
Rahul

Reputation: 2759

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 a 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: Using tryCatch for robust R scripts

Hope this is helpful.

Upvotes: 189

Gregor Thomas
Gregor Thomas

Reputation: 145745

The purrr package gives alternate functions that can be friendlier to set up than tryCatch. From the ?safely documentation, they are described as:

  • safely: wrapped function instead returns a list with components result and error. If an error occurred, error is an error object and result has a default value (otherwise). Else error is NULL.

  • quietly: wrapped function instead returns a list with components result, output, messages and warnings.

  • possibly: wrapped function uses a default value (otherwise) whenever an error occurs.

Note that these functions, unlike tryCatch(), are expected to wrap a function, not an expression, and they return a modified function. For OP's problem as stated, we would probably use possibly and wrap readLines directly to modify it.

url <- c(
    "http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html",
    "http://en.wikipedia.org/wiki/Xz", 
    "xxx")

library(purrr)
lapply(url, possibly(readLines, otherwise = "web URL is wrong, can't get"))
## with possibly, the error prints as a warning
## and the final value is the `otherwise` string

But also note that we could create a modified version of readLines, such as my_readLines <- possibly(readLines, otherwise = "web URL is wrong, can't get") that could be used in multiple places in our code.

I illustrate with possibly above, but we could easily imagine cases where we would want to use safely() (after which we could extract the result component from each list item, possibly skipping or otherwise handling items with a non-empty error component, perhaps even handling them differently based on the error), or quietly which also captures warnings and messages separately.

Upvotes: 5

Paul
Paul

Reputation: 4168

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.

Upvotes: 70

heretolearn
heretolearn

Reputation: 6545

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

Upvotes: 91

James
James

Reputation: 351

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.

  1. Set up tryCatch as part of a function. For example:

    irr2 <- function (x) {
  out <- tryCatch(irr(x), error = function(e) NULL)
  return(out)
}

  2. 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.

  3. Remember to create a sub-output (like my "out") and to return(out).

Upvotes: 33

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