How can I check whether a function call results in a warning?

On the R-help mailing list (see http://tolstoy.newcastle.edu.au/R/help/04/06/0217.html), Luke Tierney wrote:

"If you want to write a function that computes a value and collects all warning you could do it like this:

withWarnings <- function(expr) {
    myWarnings <- NULL
    wHandler <- function(w) {
        myWarnings <<- c(myWarnings, list(w))
        invokeRestart("muffleWarning")
    }
    val <- withCallingHandlers(expr, warning = wHandler)
    list(value = val, warnings = myWarnings)
} 

If you want to use the try constructs, you can set the options for warn. See also ?options. Better is to use tryCatch() :

x <- function(i){
  if (i < 10) warning("A warning")
  i
}

tt <- tryCatch(x(5),error=function(e) e, warning=function(w) w)

tt2 <- tryCatch(x(15),error=function(e) e, warning=function(w) w)

tt
## <simpleWarning in x(5): A warning>

tt2
## [1] 15

if(is(tt,"warning")) print("KOOKOO")
## [1] "KOOKOO"

if(is(tt2,"warning")) print("KOOKOO")

To get both the result and the warning :

tryCatch(x(5),warning=function(w) return(list(x(5),w)))

## [[1]]
## [1] 5
## 
## [[2]]
## <simpleWarning in x(5): A warning>

Using try

op <- options(warn=2)

tt <- try(x())
ifelse(is(tt,"try-error"),"There was a warning or an error","OK")
options(op)