Question

I am using stopifnot and I understand it just returns the first value that was not TRUE. I f that is some freaky dynamic expression someone who is not into the custom function cannot really make something out of that. So I would love to add a custom error message. Any suggestions?

Error: length(unique(nchar(check))) == 1 is not TRUE

Basically states that the elements of the vector check do not have the same length.
Is there a way of saying: Error: Elements of your input vector do not have the same length!?

Answer 1

Use stop and an if statement:

if(length(unique(nchar(check))) != 1) 
  stop("Error: Elements of your input vector do not have the same length!")

Just remember that stopifnot has the convenience of stating the negative, so your condition in the if needs to be the negation of your stop condition.

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This is what the error message looks like:

> check = c("x", "xx", "xxx")
> if(length(unique(nchar(check))) != 1) 
+   stop("Error: Elements of your input vector do not have the same length!")

Error in eval(expr, envir, enclos) : 
  Error: Elements of your input vector do not have the same length!