Undefined values: Difference between revisions

=={{header|R}}==

There are four cases to consider. To test whether a varaible has previously been defined, use <code>exists</code>.

<lang r>

exists("x")

</lang>

If you want to declare a variable with undefined contents, use <code>NULL</code>.

<lang r>

x <- NULL

</lang>

If you want to declare a variable with missing values, use <code>NA</code>.

<lang r>

y <- c(1, 4, 9, NA, 25)

z <- c("foo", NA, "baz")

</lang>

(Note that there are different types of <code>NA</code>, namely <code>NA_integer_</code>, <code>NA_real_</code>, <code>NA_character_</code>, <code>NA_complex_</code> and plain (logical) <code>NA</code>. In practice, you should hardly ever need to explicitly set which type of NA you are using, as it will be done automatically.)

Finally, you test for arguments that haven't been passed into a function with <code>missing</code>.

<lang r>

print_is_missing <- function(x)

{

print(missing(x))

}

print_is_missing() # TRUE

print_is_missing(123) # FALSE

</lang>