Restart an Expression

Description

restart performs a type of non-local return.

try is a user-friendly wrapper to run an expression that might fail.

Usage

restart(on = TRUE)
try(expr, first = TRUE)

Arguments

Details

When restart is called with on = TRUE the evaluator marks that function as a return point. Any errors or signals (such as control-C on Unix) cause control to return to the start of the function containing the call to restart. The most recently established function is always entered first.

Value

try returns the value of the expression if it succeeds, and an invisible object of class "try-error" containing the error message if it if fails. The normal error handling will print the same message unless options("show.error.messages") is false.

Note

The direct use of restart is likely to result in an infinite loop. Use try unless you are sure you know what you are doing.

See Also

options for setting error handlers and suppressing the printing of error messages; geterrmessage for retrieving the last error message.

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)

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