missingdetermines if an argument was provided a value when a function was called.
f <- function(a = 'default', b = 'default') {
if (missing(a)) cat("Argument a is missing") else cat(paste("Argument a =", a));
cat ("\n");
if (missing(b)) cat("Argument b is missing") else cat(paste("Argument b =", b));
cat("\n\n");
}
f(b = "foo");
#
# Argument a is missing
# Argument b = foo
f(a = "bar", b = "baz");
#
# Argument a = bar
# Argument b = baz
f(a = "default");
#
# Argument a = default
# Argument b is missing