Testing that a function receives a string

Question

I would like to build a function such as:

test <- function( fct = "default" ) {
  # here test that fct is a string
  # ...
}

in which it is tested that fct is a string of length one. Defining isSingleString() as:

isSingleString <- function(fct) { is.character(fct) & length(fct) == 1 }

seemed like a good idea. Thus:

test <- function( fct = "default" ) {
    # here test that fct is a string
    if (!isSingleString(fct)) stop ("error: not a string")
    # ...
    print("things are running smoothly!")
}

Using test("mean") and test("CI") works ok; test(mean) and and test(2) return the error as expected, but test(CI) crashes, saying that Error: object 'CI' not found.

What can I do to test the argument fct properly?

Without the quotes "", CI is treated as a symbol e.g. an object in your environment. If CI does not exist, you will get an error. By example, running CI <- 2; test(CI) and CI <- "CI"; test(CI) works. So for your function, do you want it to treat anything inside test() as literal? — L Tyrone
– L Tyrone, Commented Aug 13 at 1:28
I agree with LTyrone, it looks as if you are either (1) trying to operate in NSE using CI as a lazy-typing of "CI", I suggest that is rife with corner-cases and problems that are often not worth it in the end; or (2) a clear misunderstanding on our part for your intent to check not just the class and length of the argument, but if it exists at all. I think Edward's suggestion for tryCatch is appropriate in that second case, but frankly I think that level of resilience (a user supplying a non-existent symbol/name) may be over-engineering things. — r2evans
– r2evans, Commented Aug 13 at 4:07
"What can I do to test the argument fct properly?" Your solution tests the argument properly (although I would use stopifnot and, as mentioned already, &&). You may want to consider if NA_character_ is valid input. Why would you want to catch an object-not-found error only to throw a different error message? That error is very clear. — Roland
– Roland, Commented Aug 13 at 5:30
Rather than re-inventing the wheel, consider using the checkmate package. See here for details. In your case, something like assert_character(fct, len = 1). — Limey
– Limey, Commented Aug 13 at 5:50

Edward · Accepted Answer · 2025-08-13 02:50:05Z

2

You probably need to use tryCatch. See this post for some very good explanations on how to use this function.

isSingleString <- function(x) {

  tryCatch(
    {
      # Try to run this command
      is.character(x) && length(x) == 1 
    },
    error = function(e) {
      # Code to execute if an error occurs
      if (grepl("not found", e$message)) {
        message("Error: 'fct' was not found. Please ensure it is defined.")
        return(FALSE)
      } else {
        message("Error: There was an unexpected error.")
        return(FALSE)
      }
    },
    warning = function(w) {
      message("Warning: ", w$message)
    }
  )
}

> test(CI)
Error: 'fct' was not found. Please ensure it is defined.
[1] "error: not a string"

answered Aug 13 at 2:50

Edward

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Comments

Limey · Accepted Answer · 2025-08-13 07:42:44Z

2

Expanding on my comment...

library(checkmate)

test <- function(fct) {
  if (test_character(fct, len = 1)) {
    # Do something useful
    print("All OK")
  } else {
    print("Something's wrong")
  }
}
test("ok")
[1] "All OK"
test(c("bad", "worse"))
[1] "Something's wrong"
test(bad)
Error: object 'bad' not found

test(mean)
[1] "Something's wrong"
test(2)
[1] "Something's wrong"

Or, if you want to throw an error rather than print a message...

test1 <- function(fct) {
  assert_character(fct, len = 1)
  print("All OK")
}
test1("ok")
[1] "All OK"
test1(c("bad", "worse"))
Error in test1(c("bad", "worse")) : 
  Assertion on 'fct' failed: Must have length 1, but has length 2.

test1(bad)
Error: object 'bad' not found

test1(mean)
Error in test1(mean) : 
  Assertion on 'fct' failed: Must be of type 'character', not 'closure'.

test1(2)
Error in test1(2) : 
  Assertion on 'fct' failed: Must be of type 'character', not 'double'.

I agree with other commenters that attempting to handle a non-existent symbol passed to the function is probably not worthwhile. Throwing an error in that case seems completely appropriate to me: even if you handle the problem gracefully in test, the non-existent symbol may well cause other problems downstream.

edited Aug 13 at 7:42

answered Aug 13 at 7:32

Limey

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1 Comment

Denis Cousineau Aug 13 at 15:36

I did not know about checkmate. What a wonderful and useful library!

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