message("TESTING: Exception...") library("R.oo") oopts <- options(warn=1) ###################################################################### # 1. To catch a regular "error" exception thrown by e.g. stop(). ###################################################################### x <- NA y <- NA tryCatch({ x <- log(123) y <- log("a") }, error = function(ex) { print(ex) }) print(x) print(y) ###################################################################### # 2. Always run a "final" expression regardless or error or not. ###################################################################### filename <- tempfile("R.methodsS3.example") con <- file(filename) tryCatch({ open(con, "r") }, error = function(ex) { cat("Could not open ", filename, " for reading.\n", sep="") }, finally = { close(con) cat("The id of the connection is ", ifelse(is.null(con), "NULL", con), ".\n", sep="") }) ###################################################################### # 3. Creating your own Exception class ###################################################################### setConstructorS3("NegativeLogValueException", function( msg="Trying to calculate the logarithm of a negative value", value=NULL) { extend(Exception(msg=msg), "NegativeLogValueException", .value = value ) }) setMethodS3("as.character", "NegativeLogValueException", function(this, ...) { paste(as.character.Exception(this), ": ", getValue(this), sep="") }) setMethodS3("getValue", "NegativeLogValueException", function(this, ...) { this$.value }) mylog <- function(x, base=exp(1)) { if (x < 0) throw(NegativeLogValueException(value=x)) else log(x, base=base) } # Note that the order of the catch list is important: l <- NA x <- 123 tryCatch({ l <- mylog(x) }, NegativeLogValueException = function(ex) { cat(as.character(ex), "\n") }, "try-error" = function(ex) { cat("try-error: Could not calculate the logarithm of ", x, ".\n", sep="") }, error = function(ex) { cat("error: Could not calculate the logarithm of ", x, ".\n", sep="") }) cat("The logarithm of ", x, " is ", l, ".\n\n", sep="") options(oopts) message("TESTING: Exception...DONE")