Find function arguments without default values

Question

How can I say with certainty that the arguments of a formal function have a default value outside the function?

In the example below, I would like to make sure that the test below cannot find when the argument has or does not have a default value:

myfun <- function(a, b=1, ...) {} formals(myfun) for (n in names(formals(myfun))) { if (is.name(formals(myfun)[[n]])) { cat(n, "has no default value\n") } else { cat(n, "has a default value:", formals(myfun)[[n]], "\n") } } 

is.name is the best test I came up with, but I would prefer to see that it is a name and that it is empty (or, even better, a more definitive test, for example, missing inside a function).

Answer 1

You can run an empty comparison string to check if each value is empty:

 myfun <- function(a, b=1, ...) {} formals(myfun) for (n in names(formals(myfun))) { if (formals(myfun)[[n]] == "") { cat(n, "has no default value\n") } else { cat(n, "has a default value:", formals(myfun)[[n]], "\n") } } 

a has no default
b has a default value: 1
... does not have a default value

UPDATED FOR A RARE, BUT POSSIBLE CASE OF GOOD PROCESSING "" AS DEFINED BY DEFAULT OF THE EMPTY LINE:

 myfun <- function(a, b=1, c="", ...) {} formals(myfun) for (n in names(formals(myfun))) { if (!nzchar(formals(myfun)[[n]]) & is.name(formals(myfun)[[n]])) { cat(n, "has no default value\n") } else { cat(n, "has a default value:", formals(myfun)[[n]], "\n") } } 

a has no default
b has a default value: 1
c has a default value:
... does not have a default value

ADDITIONAL EDIT TO SAVE AUTHORITY: actually display empty quotation marks as a result and define formals(myfun) instead of calling it again and again:

 myfun <- function(a, b=1, c="", ...) {} myfun_args <- formals(myfun) for (n in names(myfun_args)) { if (!nzchar(myfun_args[[n]]) & is.name(myfun_args[[n]])) { cat(n, "has no default value\n") } else { if (!nzchar(myfun_args[[n]])) cat(n, "has a default value:", dQuote(myfun_args[[n]]), "\n") else cat(n, "has a default value:", myfun_args[[n]], "\n") } } 

a has no default
b has a default value: 1
c has a default value: ""
... does not have a default value