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:

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?

Solution

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/katex@0.15.3/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.