Try an Expression Allowing Error Recovery (original) (raw)

Description

try is a wrapper to run an expression that might fail and allow the user's code to handle error-recovery.

Usage

try(expr, silent = FALSE,
    outFile = getOption("try.outFile", default = stderr()))

Arguments

Details

try evaluates an expression and traps any errors that occur during the evaluation. If an error occurs then the error message is printed to the [stderr](../../base/help/stderr.html) connection unlessoptions("show.error.messages") is false or the call includes silent = TRUE. The error message is also stored in a buffer where it can be retrieved bygeterrmessage. (This should not be needed as the value returned in case of an error contains the error message.)

try is implemented using [tryCatch](../../base/help/tryCatch.html); for programming, instead of try(expr, silent = TRUE), something liketryCatch(expr, error = function(e) e) (or other simple error handler functions) may be more efficient and flexible.

It may be useful to set the default for outFile to[stdout](../../base/help/stdout.html)(), i.e.,

options(try.outFile = stdout())

instead of the default [stderr](../../base/help/stderr.html)(), notably when try() is used inside a [Sweave](../../utils/html/Sweave.html) code chunk and the error message should appear in the resulting document.

Value

The value of the expression if expr is evaluated without error: otherwise an invisible object inheriting from class "try-error"containing the error message with the error condition as the"condition" attribute.

Warning

Do not test

if (class(res) == "try-error"))

as if there is no error, the result might (now or in future) have a class of length > 1. Use if([inherits](../../base/help/inherits.html)(res, "try-error"))instead.

See Also

[options](../../base/help/options.html) for setting error handlers and suppressing the printing of error messages;[geterrmessage](../../base/help/geterrmessage.html) for retrieving the last error message. The underlying [tryCatch](../../base/help/tryCatch.html) provides more flexible means of catching and handling errors.

[assertCondition](../../tools/html/assertCondition.html) in package tools is related and useful for testing.

Examples

## this example will not work correctly in example(try), but
## it does work correctly if pasted in
options(show.error.messages = FALSE)
try(log("a"))
print(.Last.value)
options(show.error.messages = TRUE)

## alternatively,
print(try(log("a"), TRUE))

## run a simulation, keep only the results that worked.
set.seed(123)
x <- stats::rnorm(50)
doit <- function(x)
{
    x <- sample(x, replace = TRUE)
    if(length(unique(x)) > 30) mean(x)
    else stop("too few unique points")
}
## alternative 1
res <- lapply(1:100, function(i) try(doit(x), TRUE))
## alternative 2
## Not run: res <- vector("list", 100)
for(i in 1:100) res[[i]] <- try(doit(x), TRUE)
## End(Not run)
unlist(res[sapply(res, function(x) !inherits(x, "try-error"))])

[Package _base_ version 4.6.0 Index]