I would like to overwrite a not very helpful error message from a base R function and replace it with a custom error message. How could I achieve this?
To clarify, assume that I evaluate the following expression "a" + "b". Because I am trying to add two characters, R will complain and return "Error in "a" + "b" : non-numeric argument to binary operator".
Is there a way to catch this exact error message, and return a clearer error message, e.g. "You are trying to add two factors – this is not allowed"?
I suppose a starting point would be to work with the try function and grepl:
a <- try("a" + "a", silent = TRUE)
if(inherits(a, "try-error")){
cond <- sum(grepl("non-numeric argument to binary operator", deparse(a), fixed = TRUE)) > 0
if(cond){
stop("You are trying to add two characters. This is not allowed.")
}
}
But maybe there is a more ‘generic’ or ‘elegant’ way to do this?
>Solution :
You can check the class with inherits and then use the "condition" attribute as follows using grepl like what you suggest
a <- try("a" + "a", silent = TRUE)
if(inherits(a, "try-error") && grepl("non-numeric argument to binary operator$", attr(a, "condition")$message))
stop("You are trying to add two non-numbers")
#R> Error: You are trying to add two non-numbers
but many things could cause this error it seem. E.g.
a <- try(list() + list(), silent = TRUE)
if(inherits(a, "try-error") && grepl("non-numeric argument to binary operator$", attr(a, "condition")$message))
stop("You are trying to add two non-numbers")
#R> Error: You are trying to add two non-numbers
A better idea may be to check the arguments if possible. E.g. using stopifnot().