Fill missing values in the data frame with the nearest available value (using the specified direction). Note that the frame may still contain missing values after call to this function (e.g. if the first value is not available and we attempt to fill series with previous values). This operation can only be used on ordered frames. ## Parameters - `frame` - An input data frame that is to be filled - `direction` - Specifies the direction used when searching for the nearest available value. `Backward` means that we want to look for the first value with a smaller key while `Forward` searches for the nearest greater key. [category:Missing values]

Fill missing values in the frame using the specified function. The specified function is called with all series and keys for which the frame does not contain value and the result of the call is used in place of the missing value. The operation is only applied to columns (series) that contain values of the same type as the return type of the provided filling function. The operation does not attempt to convert between numeric values (so a series containing `float` will not be converted to a series of `int`). ## Parameters - `frame` - An input data frame that is to be filled - `f` - A function that takes a series `Series<R, T>` together with a key `K` in the series and generates a value to be used in a place where the original series contains a missing value. [category:Missing values]

Fill missing values of a given type in the frame with a constant value. The operation is only applied to columns (series) that contain values of the same type as the provided filling value. The operation does not attempt to convert between numeric values (so a series containing `float` will not be converted to a series of `int`). ## Parameters - `frame` - An input data frame that is to be filled - `value` - A constant value that is used to fill all missing values [category:Missing values]