R: error when printing character() in assert_that message

Question

I would like to assert some expressions involving certain variable that is potentially evaluated to character(). I would like to print an assertation message showing the value of the variable which can be character().

assertthat::assert_that(<expr containing variable [x]>, msg = sprintf("Test for x failed on value %s" , x))

## Variable [x] is a character variable that can be character() or other conventional characters like `"a"`, `"b"` or ``"c"``.

Below is a simplified example to show the error I obtained when the variable is evaluated to character():

assertthat::assert_that(FALSE, msg = sprintf("%s", character()))

gives me error

## > Error in stop(assertError(attr(res, "msg"))) : bad error message

However the sprintf itself works well

sprintf("%s", character())
## character(0)

sprintf("%s", character()) %>% typeof
## [1] "character"

What I have tried:

• Wrap %s with backticks assert_that(FALSE, msg = sprintf("`%s`", character()))
• Load the assertthat package first with library(assertthat) then call assert_that(...)
• Assign first character() to a variable and call that variable in assert_that which is actually more similar to my real life use case

  v <- character()
  assert_that(FALSE, msg = sprintf("`%s`", v))
  

• Assign the return value of sprintf call to a variable and then feed it into the call of assert_that

  v <- character()
  errmsg <- sprintf("`%s`", v)
  assert_that(FALSE, msg = errmsg)
  

All gave same error. It seems something inside assert_that affects.

What would be the reason of the error and how to fix it?

Answer 1

I think the problem here is that your sprintf statements results in a character with length 0:

length(sprintf("%s", character()))
#> [1] 0

This means there is not really a message for assert_that to print. So you have to ensure the length of the sprintf output has a length of 1:

require(assertthat)
#> Loading required package: assertthat
assert_that(F, msg=character())
#> Error in stop(assertError(attr(res, "msg"))) : bad error message
assert_that(F, msg=letters[1:2])
#> Error in stop(assertError(attr(res, "msg"))) : bad error message
assert_that(F, msg=letters[1])
#> Error: a