validating input to a function in R

Question

I would like to error trap an input value to ensure that the user is entering the correct choice. In this case there are five choices "ns", "dl", "sv", "asv", "cs". I would like to check the use input against these if none of these are present then return and error msg, if blank default to "ns" and send a message to the user. I tried scanning over a vector string but that did not work. Any suggestions are appreciated

   method = "ns"
   if(method != scan(c("ns", "dl", "sv", "asv" ))) {"Invalid Value"} else {method = method}  

Answer 1

You are probably looking for %in%, and you can use it along the lines of:

myFun <- function(input=NULL) {
  Check <- c("ns", "dl", "sv", "asv", "cs")
  if (is.null(input)) {
    message("No 'input' value defined. Using 'ns' by default")
    input <- "ns"
  } 
  if (!input %in% Check) stop("Invalid 'input' value")
  input
}

myFun()
# No 'input' value defined. Using 'ns' by default
# [1] "ns"
myFun("sv")
# [1] "sv"
myFun("vs")
# Error in myFun("vs") : Invalid 'input' value

---

Without knowing exactly what you want to do, you might also want to look into the switch function.

myFun2 <- function(input = NULL) {
  Check <- c("ns", "dl", "sv", "asv", "cs")
  if (is.null(input)) {
    message("No 'input' value defined. Using 'ns' by default")
    input <- "ns"
  } 
  switch(input,
         ns = "Whoo",
         dl = "Whee",
         sv = "Whaa",
         asv = "Whii",
         cs = "Whuu",
         stop("You did not say the magic word"))
}

myFun2()
# No 'input' value defined. Using 'ns' by default
# [1] "Whoo"
myFun2("sv")
# [1] "Whaa"
myFun2("sc")
# Error in myFun2("sc") : You did not say the magic word

---

Update: match.arg

By popular demand, here's a match.arg version of the above too, but note that you no longer get to put in a message about not using the magic word and instead have to settle with an automatically generated descriptive and helpful error message. That's no fun....

myFun3 <- function(input=NULL) {
  Check <- c("ns", "dl", "sv", "asv", "cs")
  if (is.null(input)) {
    message("No 'input' value defined. Using 'ns' by default")
    input <- "ns"
  } 
  input <- match.arg(input, Check)
  switch(input,
         ns = "Whoo",
         dl = "Whee",
         sv = "Whaa",
         asv = "Whii",
         cs = "Whuu")
}

myFun3()
# No 'input' value defined. Using 'ns' by default
# [1] "Whoo"
myFun3("sv")
# [1] "Whaa"
myFun3("sc")
# Error in match.arg(input, Check) : 
#   'arg' should be one of “ns”, “dl”, “sv”, “asv”, “cs”