#### 3.1.2 Missing Data

— Built-in Variable: NA

> Missing value.

— Mapping Function: isna (x)

> Return 1 for elements of x that are NA (missing) values and zero otherwise. For example,

>               is_NA ([13, Inf, NA, NaN])
>                    => [ 0, 0, 1, 0 ]

— Mapping Function: is_nan_or_na (x)

> Return 1 for elements of x that are NaN or NA (missing) values and zero otherwise. For example,

>               is_NAN_or_NA ([13, Inf, NA, NaN])
>                    => [ 0, 0, 1, 1 ]